Next-Generation ThermalEnvironmental Barrier Coatings for Ceramic-Matrix
Composites
Dissertation
Presented in Partial Fulfillment of the Requirements of the Degree Doctor of Philosophy
in the
Graduate School of Brown University
By
Laura Ruth Turcer MS
Graduate Program Engineering
Brown University
2020
Dissertation Committee
Dr Nitin P Padture (Advisor)
Dr Reid F Cooper
Dr Brian W Sheldon
ii
copy Copyright 2020 by Laura R Turcer
iii
This dissertation by Laura R Turcer is accepted in its present form by the School of Engineering
as satisfying the dissertation requirement of Doctor of Philosophy
Date ________________________ _______________________________________
Nitin P Padture Advisor
Recommended to the Graduate Council
Date ________________________ _______________________________________
Reid F Cooper Reader
Date ________________________ _______________________________________
Brian W Sheldon Reader
Approved by the Graduate Council
Date ________________________ _______________________________________
Andrew G Campbell Dean of the Graduate
School
iv
CURRICULUM VITAE
2015 to presenthelliphelliphelliphelliphelliphelliphelliphelliphelliphellipGraduate Research Associate School of Engineering
Brown University
2017helliphelliphelliphelliphelliphelliphelliphelliphelliphellipMS Materials Science and Engineering School of Engineering
Brown University
2014helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBS Materials Science and Engineering
The Ohio State University
2010helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipDublin Scioto High School
1992helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBorn Youngstown Ohio
v
PUBLICATIONS
1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating
ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate
(CMAS) Glassrdquo Journal of Materials Research Invited Submitted
2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)
based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117
(2018) Invited Viewpoint Article
3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European
Ceramic Society 38 3905-3913 (2018)
4 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the
European Ceramic Society 38 3914-3924 (2018)
These authors contributed equally
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
REFERENCES
[1] NP Padture M Gell EH Jordan Thermal Barrier Coatings for Gas-Turbine Engine
Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609
[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
efficiency American Ceramic Society Bulletin 95 (nd) 7
[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for
Energy Conversion and Storage Elsevier 2020
[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
2005
[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
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[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
(1998) 621ndash691 httpsdoiorg101127ejm1040621
[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
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[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
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US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
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[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
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[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
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[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
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[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
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[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
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[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
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[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
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5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
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[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
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Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
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[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
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[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
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[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
httpsdoiorg101016S0065-2156(08)70164-9
[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
httpsdoiorg101016jtca200908002
[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
ii
copy Copyright 2020 by Laura R Turcer
iii
This dissertation by Laura R Turcer is accepted in its present form by the School of Engineering
as satisfying the dissertation requirement of Doctor of Philosophy
Date ________________________ _______________________________________
Nitin P Padture Advisor
Recommended to the Graduate Council
Date ________________________ _______________________________________
Reid F Cooper Reader
Date ________________________ _______________________________________
Brian W Sheldon Reader
Approved by the Graduate Council
Date ________________________ _______________________________________
Andrew G Campbell Dean of the Graduate
School
iv
CURRICULUM VITAE
2015 to presenthelliphelliphelliphelliphelliphelliphelliphelliphelliphellipGraduate Research Associate School of Engineering
Brown University
2017helliphelliphelliphelliphelliphelliphelliphelliphelliphellipMS Materials Science and Engineering School of Engineering
Brown University
2014helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBS Materials Science and Engineering
The Ohio State University
2010helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipDublin Scioto High School
1992helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBorn Youngstown Ohio
v
PUBLICATIONS
1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating
ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate
(CMAS) Glassrdquo Journal of Materials Research Invited Submitted
2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)
based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117
(2018) Invited Viewpoint Article
3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European
Ceramic Society 38 3905-3913 (2018)
4 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the
European Ceramic Society 38 3914-3924 (2018)
These authors contributed equally
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
REFERENCES
[1] NP Padture M Gell EH Jordan Thermal Barrier Coatings for Gas-Turbine Engine
Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609
[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
efficiency American Ceramic Society Bulletin 95 (nd) 7
[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for
Energy Conversion and Storage Elsevier 2020
[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
2005
[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
(1998) 621ndash691 httpsdoiorg101127ejm1040621
[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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137
[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
httpsdoiorg101016jactamat201009013
[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
Geophysical Research Letters 7 (1980) 949ndash952
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[82] WH Chesner User guidelines for waste and by-product materials in pavement construction
US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
httpsdoiorg101016jsurfcoat201009008
[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
2914 httpsdoiorg101016jjeurceramsoc201503032
[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
concept Journal of Non-Crystalline Solids 21 (1976) 373ndash410
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
httpsdoiorg101021jp040330w
[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
httpswwwingentaconnectcomcontentsgtpcg20050000004600000001art00001
(accessed February 25 2020)
[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
httpsdoiorg101016jphysb200611015
[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
httpsdoiorg102298JMMB1001041G
[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-
5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
Articles (2011)
[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic
Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
Metallkunde v92 1083-1097 (2001) 92 (2001)
[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
httpsdoiorg101016jwear200812018
[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
httpsdoiorg101016jactamat201608077
[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
httpsdoiorg101016S0065-2156(08)70164-9
[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
httpsdoiorg101016jtca200908002
[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
iii
This dissertation by Laura R Turcer is accepted in its present form by the School of Engineering
as satisfying the dissertation requirement of Doctor of Philosophy
Date ________________________ _______________________________________
Nitin P Padture Advisor
Recommended to the Graduate Council
Date ________________________ _______________________________________
Reid F Cooper Reader
Date ________________________ _______________________________________
Brian W Sheldon Reader
Approved by the Graduate Council
Date ________________________ _______________________________________
Andrew G Campbell Dean of the Graduate
School
iv
CURRICULUM VITAE
2015 to presenthelliphelliphelliphelliphelliphelliphelliphelliphelliphellipGraduate Research Associate School of Engineering
Brown University
2017helliphelliphelliphelliphelliphelliphelliphelliphelliphellipMS Materials Science and Engineering School of Engineering
Brown University
2014helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBS Materials Science and Engineering
The Ohio State University
2010helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipDublin Scioto High School
1992helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBorn Youngstown Ohio
v
PUBLICATIONS
1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating
ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate
(CMAS) Glassrdquo Journal of Materials Research Invited Submitted
2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)
based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117
(2018) Invited Viewpoint Article
3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European
Ceramic Society 38 3905-3913 (2018)
4 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the
European Ceramic Society 38 3914-3924 (2018)
These authors contributed equally
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
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httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
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[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
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[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
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[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
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[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
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[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
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httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
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2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
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29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
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[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
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[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
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[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
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[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
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[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
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[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
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[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
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[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
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httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
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[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
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[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
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[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
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[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
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[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
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[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
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Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
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aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
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the minimum level to initiate damage (2010)
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[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
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[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
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[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
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[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
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[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
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[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
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barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
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[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
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Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
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[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
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[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
Geophysical Research Letters 7 (1980) 949ndash952
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US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
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[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
httpsdoiorg101016jsurfcoat201009008
[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
httpsdoiorg101111j1744-7402200902373x
[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
2914 httpsdoiorg101016jjeurceramsoc201503032
[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
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[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
concept Journal of Non-Crystalline Solids 21 (1976) 373ndash410
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
httpsdoiorg101021jp040330w
[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
httpswwwingentaconnectcomcontentsgtpcg20050000004600000001art00001
(accessed February 25 2020)
[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
httpsdoiorg102298JMMB1001041G
[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-
5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
Articles (2011)
[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic
Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
Metallkunde v92 1083-1097 (2001) 92 (2001)
[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
httpsdoiorg101016jwear200812018
[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
httpsdoiorg101016jactamat201608077
[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
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[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
httpsdoiorg101016jtca200908002
[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
iv
CURRICULUM VITAE
2015 to presenthelliphelliphelliphelliphelliphelliphelliphelliphelliphellipGraduate Research Associate School of Engineering
Brown University
2017helliphelliphelliphelliphelliphelliphelliphelliphelliphellipMS Materials Science and Engineering School of Engineering
Brown University
2014helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBS Materials Science and Engineering
The Ohio State University
2010helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipDublin Scioto High School
1992helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBorn Youngstown Ohio
v
PUBLICATIONS
1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating
ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate
(CMAS) Glassrdquo Journal of Materials Research Invited Submitted
2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)
based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117
(2018) Invited Viewpoint Article
3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European
Ceramic Society 38 3905-3913 (2018)
4 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the
European Ceramic Society 38 3914-3924 (2018)
These authors contributed equally
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
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httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
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[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
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[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
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[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
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[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
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[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
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[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
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[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
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2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
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[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
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[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
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[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
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[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
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[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
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[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
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[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
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[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
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[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
httpsdoiorg101111jace15978
[52] NS Jacobson Silica Activity Measurements in the Y 2 O 3 -SiO 2 System and Applications
to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
httpsdoiorg101016jceramint201706167
[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
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[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
Society 88 (2005) 3483ndash3488 httpsdoiorg101111j1551-2916200500640x
137
[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
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[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
httpsdoiorg101016jactamat201009013
[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
Geophysical Research Letters 7 (1980) 949ndash952
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[82] WH Chesner User guidelines for waste and by-product materials in pavement construction
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Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
httpsdoiorg101016jsurfcoat201009008
[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
httpsdoiorg101111j1744-7402200902373x
[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
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[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
concept Journal of Non-Crystalline Solids 21 (1976) 373ndash410
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
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[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
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(accessed February 25 2020)
[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
httpsdoiorg102298JMMB1001041G
[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-
5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
Articles (2011)
[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic
Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
Metallkunde v92 1083-1097 (2001) 92 (2001)
[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
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[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
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[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
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[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
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[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
v
PUBLICATIONS
1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating
ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate
(CMAS) Glassrdquo Journal of Materials Research Invited Submitted
2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)
based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117
(2018) Invited Viewpoint Article
3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European
Ceramic Society 38 3905-3913 (2018)
4 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-
Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the
European Ceramic Society 38 3914-3924 (2018)
These authors contributed equally
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
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[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
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[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
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[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
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[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
httpsdoiorg101111jace15978
[52] NS Jacobson Silica Activity Measurements in the Y 2 O 3 -SiO 2 System and Applications
to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
httpsdoiorg101016jceramint201706167
[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
(1998) 621ndash691 httpsdoiorg101127ejm1040621
[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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137
[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
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[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
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US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
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[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
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[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
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[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
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[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
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[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
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[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
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[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
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[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
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[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
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[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
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[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
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Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
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Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
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[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
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[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
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[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
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143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
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[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
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[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
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[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
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[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
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[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
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[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
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[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
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[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
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[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
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[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
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[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
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[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
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[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
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[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
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[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
vi
DEDICATION
Dedicated to my family
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
REFERENCES
[1] NP Padture M Gell EH Jordan Thermal Barrier Coatings for Gas-Turbine Engine
Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609
[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
efficiency American Ceramic Society Bulletin 95 (nd) 7
[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for
Energy Conversion and Storage Elsevier 2020
[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
2005
[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
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[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
(1998) 621ndash691 httpsdoiorg101127ejm1040621
[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
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[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
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US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
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[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
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[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
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[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
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[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
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[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
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[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
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[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
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5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
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[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
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Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
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[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
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[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
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[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
httpsdoiorg101016S0065-2156(08)70164-9
[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
httpsdoiorg101016jtca200908002
[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
vii
ACKNOWLEDGEMENTS
I would like to thank Professor Nitin Padture my advisor for his support and supervision
His mentorship has helped me grow as a researcher and as an individual I really appreciate how
much he cares about his graduate students He not only focuses on supporting my research goals
but has supported me through my experimentsrsquo successes and failures papers and presentations
Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our
discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos
support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so
grateful I was able to take their classes and that they made time for my defense
My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)
Krause When I first started at Brown University she was concluding work on her PhD Mandie
mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing
FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped
me select classes to achieve my research goals Overall Mandie made my transition into grad
school a smooth one Hector Garces was also very helpful as I began graduate work He taught me
ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I
would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati
Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr
Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos
Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks
to Qizhong Wang who helped me talk through problems and checked my math I would like to
thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
REFERENCES
[1] NP Padture M Gell EH Jordan Thermal Barrier Coatings for Gas-Turbine Engine
Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609
[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
efficiency American Ceramic Society Bulletin 95 (nd) 7
[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for
Energy Conversion and Storage Elsevier 2020
[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
2005
[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
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[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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137
[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
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[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
Geophysical Research Letters 7 (1980) 949ndash952
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[82] WH Chesner User guidelines for waste and by-product materials in pavement construction
US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
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[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
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[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
concept Journal of Non-Crystalline Solids 21 (1976) 373ndash410
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
httpsdoiorg101021jp040330w
[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
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(accessed February 25 2020)
[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
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[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
httpsdoiorg102298JMMB1001041G
[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-
5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
Articles (2011)
[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic
Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
Metallkunde v92 1083-1097 (2001) 92 (2001)
[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
httpsdoiorg101016jwear200812018
[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
oxides and silicate melts Acta Materialia 120 (2016) 302ndash314
httpsdoiorg101016jactamat201608077
[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and
Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16
[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-
Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-
010-9672-1
[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates
of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103
[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth
Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305
httpsdoiorg101111jace12388
[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-
Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81
(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x
[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of
environmental barrier coatings for crack resistance Journal of the European Ceramic Society
34 (2014) 2005ndash2012 httpsdoiorg101016jjeurceramsoc201312049
[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and
Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000
144
[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare
Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837
httpsdoiorg101007s11666-017-0635-5
[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of
γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823
httpsdoiorg101016jjeurceramsoc201604022
[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia
and silica Materials Research Bulletin 42 (2007) 197ndash202
httpsdoiorg101016jmaterresbull200606013
[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
system Journal of Solid State Chemistry 180 (2007) 1436ndash1445
httpsdoiorg101016jjssc200611029
[145] S Suresh Fatigue of Materials Cambridge Core (1998)
httpsdoiorg101017CBO9780511806575
[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered
Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev
Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000
[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation
type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5
[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt
and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74
(1991) 2495ndash2505 httpsdoiorg101111j1151-29161991tb06791x
[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression
in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic
Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x
[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus
for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system
Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2
[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW
Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191
httpsdoiorg101016S0065-2156(08)70164-9
[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7
(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-
alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39
(2019) 4245ndash4254 httpsdoiorg101016jjeurceramsoc201905036
145
[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates
and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635
httpsdoiorg101016jjssc200602019
[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
Aircraft Engine Applications International Journal of Applied Ceramic Technology 1
(2004) 291ndash301 httpsdoiorg101111j1744-74022004tb00181x
[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials
Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214
[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in
Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010
[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings
for Si-based ceramics in corrosive environments International Journal of Refractory Metals
and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013
[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay
Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7
Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308
[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7
and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102
httpsdoiorg10108001411590412331282814
[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth
disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European
Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013
[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high
temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299
httpsdoiorg101016jjeurceramsoc200504029
[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the
Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash
469 httpsdoiorg101016jjpcs200612025
[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule
for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13
httpsdoiorg101016jtca200908002
[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed
Pergamon Oxford UK 1993
[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments Journal of Materials
Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564
146
[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK
1969
[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal
conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17
(2002) 3193ndash3200 httpsdoiorg101557JMR20020462
[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of
low thermal conductivity materials A review Journal of the European Ceramic Society 37
(2017) 1ndash13 httpsdoiorg101016jjeurceramsoc201607036
[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960
[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier
coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74
httpsdoiorg101016S0257-8972(02)00593-5
[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement
of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid
solution Acta Materialia 144 (2018) 292ndash304
httpsdoiorg101016jactamat201710064
[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating
Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035
httpsdoiorg101111j1151-29162002tb00574x
[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y
Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy
Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303
httpsdoiorg101002adem200300567
[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S
Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8
httpsdoiorg101038ncomms9485
[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural
evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal
of the American Ceramic Society 102 (2019) 2228ndash2237
httpsdoiorg101111jace16075
[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C
Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth
oxides Materials Research Letters 5 (2017) 102ndash109
httpsdoiorg1010802166383120161220433
[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K
Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials
147
and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10
httpsdoiorg101038srep37946
[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS
Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy
descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-
07160-7
[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-
Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
httpsdoiorg101038s41598-018-26827-1
[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui
(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
Journal of the American Ceramic Society 101 (2018) 4486ndash4491
httpsdoiorg101111jace15779
[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-
Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly
Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033
httpsdoiorg101021acsinorgchem8b02379
[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive
spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-
0319-3
[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo
A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
337ndash343 httpsdoiorg101016jjmat201903002
[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)
httpsdoiorg101038s41578-019-0170-8
[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier
coating for the ceramic matrix composites Journal of the European Ceramic Society 39
(2019) 2574ndash2579 httpsdoiorg101016jjeurceramsoc201902022
[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy
(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
Materials Science amp Technology 36 (2020) 134ndash139
httpsdoiorg101016jjmst201907022
[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in
High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY
2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)
148
[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical
and mechanical properties of sintered mullite ceramic as an environmental barrier coating
material Ceramics International 42 (2016) 15203ndash15208
httpsdoiorg101016jceramint201606149
[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates
Ceramics International 45 (2019) 24957ndash24964
httpsdoiorg101016jceramint201903157
viii
Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their
support and friendship
I would like to thank Tony McCormick for his help He taught me how to use the
characterization tools necessary for most of this work and was always friendly and willing to help
I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to
thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy
McElroy and Diane Felber for their friendly assistance and administrative expertise Although my
defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons
and Paul Waltz for their assistance navigating arrangements and helping me find a room for my
defense
All of this work would not have been completed without the contributions of Professor
Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook
University I am grateful for their collaboration and ability to produce APS coatings Thanks to
Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor
Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks
to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster
and Mackenzie Ridley for their help with water vapor corrosion studies
Last but not least I would like to thank my family and friends for their support and love
A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth
(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my
thesis They sent care packages and listened to my whining
ix
TABLE OF CONTENTS
TITLE PAGE i
COPYRIGHT PAGE ii
SIGNATURE PAGE iii
CURRICULUM VITAE iv
PUBLICATIONS v
DEDICATION vi
ACKNOWLEDGEMENTS vii
TABLE OF CONTENTS ix
TABLE OF TABLES xiii
TABLE OF FIGURES xv
CHAPTER 1 INTRODUCTION 1
11 Gas-Turbine Engine Materials 1
12 Environmental Barrier Coatings 3
121 EBC Requirements 4
122 EBC Materials and Processing 5
123 EBC Failure 7
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8
131 CMAS Induced Failure 10
132 Approaches for CMAS Mitigation 12
14 Approach 13
141 Materials SelectionOptical Basicity 13
142 Objectives 16
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS 18
21 Introduction 18
22 Experimental Procedure 19
221 Processing 19
222 CMAS interactions 20
223 Characterization 21
23 Results 22
231 Polycrystalline Pellets 22
x
232 YAlO3-CMAS Interactions 24
233 Y2Si2O7-CMAS Interactions 30
24 Discussion 34
25 Summary 36
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS 38
31 Introduction 38
32 Experimental Procedure 40
321 Processing 40
322 CMAS Interactions 41
323 Characterization 41
33 Results 42
331 Polycrystalline Pellets 42
332 Yb2Si2O7-CMAs Interactions 44
333 Sc2Si2O7-CMAS Interactions 51
334 Lu2Si2O7-CMAS Interactions 55
34 Discussion 60
35 Summary 65
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67
41 Introduction 67
42 Experimental Procedures 69
421 Powders 69
422 CMAS Interaction 70
423 Characterization 70
43 Results 71
431 Powder and Polycrystalline Pellets 71
432 NAVAIR CMAS Interactions 75
433 NASA CMAS Interactions 78
434 Icelandic Volcanic Ash CMAS Interactions 80
44 Discussion 82
45 Summary 84
xi
CHAPTER 5 THERMAL CONDUCTIVITY 85
51 Introduction 85
511 Coefficient of Thermal Expansion 86
512 Phase Stability 87
513 Solid solutions 88
52 Calculated Thermal Conductivity of Binary Solid-Solutions 89
521 Experimental Procedure 89
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90
523 Thermal Conductivity Calculations for Binary Solid-Solutions 91
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96
531 Experimental Procedure 96
532 Comparison of Experimental and Calculated Thermal Conductivity 97
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100
541 Introduction to High-Entropy Ceramics 100
542 Experimental Procedure 101
543 Solid Solution Confirmation 103
544 Experimental Thermal Conductivity Results 106
55 Summary 107
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109
61 Introduction 109
62 Experimental Procedures 111
621 Air Plasma Sprayed Coatings 111
622 Heat Treatments 111
623 CMAS Interactions 111
624 Characterization 112
63 Results 113
631 As-sprayed and Heat-Treated Coatings 113
632 NAVAIR CMAS Interactions 117
64 Discussion 122
65 Future Work 124
66 Summary 124
xii
CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126
71 Summary and Conclusions 126
72 Future Work 129
REFERENCES 132
xiii
TABLE OF TABLES
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78] 15
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included 25
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included 46
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h 49
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each 69
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included 75
xiv
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included 78
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h 80
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h 82
Table 18 Properties and parameters for pure β-RE-pyrosilicates 93
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
96
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations 99
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet 106
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 119
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h 122
xv
TABLE OF FIGURES
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1] 1
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings 2
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12] 4
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51] 8
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88] 9
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36] 11
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present) 23
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7 23
xvi
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively 26
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected 28
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5 29
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3 30
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6 31
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected 32
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7 33
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
34
xvii
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7 42
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7 43
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7 44
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB 45
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB 48
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
49
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9 49
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting 50
xviii
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
51
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10 52
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB 53
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11 54
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12 56
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet 59
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59
xix
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer 61
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38] 68
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns 72
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C) 74
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively 77
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
xx
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16) 79
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17) 81
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept 85
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1 94
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1 97
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-
(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone
xxi
axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing
grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The
circled regions are where EDS elemental compositions were obtained and can be found in Table
21 105
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1 107
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 113
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating 114
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase 115
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present 116
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps 118
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
xxii
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23 119
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps 120
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24 121
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively 130
1
CHAPTER 1 INTRODUCTION
11 Gas-Turbine Engine Materials
The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components
in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to
operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a
TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal
gradient through the TBC layers This has resulted in outstanding power and efficiency gains in
gas-turbine engines used for aircraft propulsion and land-based power generation
Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal
gradient through the TBC layers From Ref [1]
TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC
thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to
the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even
2
higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure
2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs
and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-
turbine engine However TBC developments have outpaced those of the Ni-based superalloys
which has led to more aggressive cooling requirements Unfortunately this results in an increase
of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet
temperature [46]
Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from
Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate
(CMAS) deposits melt interact and degrade coatings
3
Therefore hot-section materials with inherently higher temperature capabilities are
needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix
are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have
already replaced some Ni-based superalloy hot-section stationary components in gas-turbine
engines that are in-service commercially both for aircraft propulsion and power generation
12 Environmental Barrier Coatings
CMCs for gas-turbine applications both aerospace and power generation are primarily
SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage
tolerant resistant to thermal shock and impact and display better resistance to high temperatures
and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature
capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]
Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-
gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC
forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a
schematic illustration in Figure 3A
119878119894119862 + 3
21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)
However in the gas-turbine engine combustion environment ~ 10 water vapor is also present
This leads to the volatilization of the SiO2 layer and active recession of the base layer according
to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B
1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)
4
Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)
volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-
based CMC material [12]
Therefore SiC-based CMCs need to be protected by ceramic environmental barrier
coatings (EBCs) [47131617]
121 EBC Requirements
Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active
oxidation and recession there are many other requirements on EBCs EBCs should have low
permeability of oxygen and water vapor Therefore they should also be dense and crack-free to
prevent recession of the SiC-based CMC Consequently they must have a good coefficient of
thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica
activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will
be operating at temperatures around 1500 degC so they should have high-temperature capability
phase stability and robust mechanical properties They need to have chemical compatibility with
the bond-coat material And lastly they must be resistant to molten calcia-magnesia-
aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13
A B
5
122 EBC Materials and Processing
In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium
strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite
(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC
architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited
by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite
layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)
which adds additional protection to the CMC [131718] Mullite was promising due to its low
CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability
in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding
a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in
the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide
adequate high-pressure protection at temperatures below 1300 degC [18]
Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)
CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]
The next generation EBCs or Gen II to VI were developed for higher temperature
applications These are based on rare earth (RE) silicates with several variations such as the
A B
6
additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates
(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7
[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high
melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have
a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail
mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase
stability as it does not experience a phase transition up to 1700 degC [3738]
Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)
[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance
and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)
on top of the EBC system described called TEBC have also been studied The TBC has a lower
thermal conductivity to help with high temperatures experienced in a gas-turbine engine However
the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems
is large which means a graded CTE interlayer is needed between the two coatings to alleviate
stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen
in Figure 4B
EBC deposition is still a significant challenge [3940] Conventional air plasma spray
(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have
performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC
so that the coating can crystalize during spraying [1733364243] but this is difficult in a
manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC
coatings [41] however crystallization has a significant volume change which leads to porous
coatings and undesirable phases can form during crystallization Other methods being studied are
7
plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying
(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]
chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle
application [50]
123 EBC Failure
EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine
engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream
that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm
[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms
EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water
vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a
non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC
which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam
oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond
coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat
becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-
fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)
of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])
and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build
up and once a critical thickness is reached spallation of the EBC occurs [5158]
EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and
degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section
8
13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion
(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes
significantly [51] And in the case of rotating parts they will need to carry loads that may cause
creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the
components which can be several 10000s of hours of operation [9]
Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)
Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)
Foreign object damage [51]
13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits
As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage
mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash
2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)
A
B
C
D
E
F
9
deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash
656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in
the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows
the composition of various CMASs including mineral sources like volcanic ash deposits found in
engines and synthetic CMASs used in laboratory experiments The compositional differences lead
to differences in the melt temperature viscosity and wetting of the CMAS which all play a role
in how the CMAS will interact with EBCs
Figure 6 Compositions of major components of three different classes of CMAS (mineral sources
engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the
x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from
References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand
[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]
DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]
ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek
[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun
[7086] Kraumlmer [65] Wu [87] and Rai [88]
10
131 CMAS Induced Failure
The most prevalent failure mode in EBCs is caused by the CTE mismatch between the
CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs
have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through
cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al
[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and
Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused
cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al
[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5
and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h
and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the
apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they
seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have
interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks
form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of
the coating due to crack propagation and coalescence [64]
If spallation occurs the base materials are exposed and silica volatilization will proceed
If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water
vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a
TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-
coat has a direct path for oxygen and water vapor so localized silica volatilization can occur
leading to premature spallation of the coatings
11
Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat
EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional
SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter
streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]
Another CMAS-induced failure mechanism observed in EBCs has been the formation of a
reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao
et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been
incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that
apatite formation in Y-containing materials is faster than ytterbium silicates [2427]
Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing
Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)
100 h and (B) 200 h [36]
A B ndash 4 h
C ndash 24 h
A ndash 100 h
B ndash 200 h
12
132 Approaches for CMAS Mitigation
CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for
CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness
and have additional reaction products form [3336] The CTE of potential reaction products are
unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can
be found in 131) An example of a reaction product with a mismatched CTE can be seen in
Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical
basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical
reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not
react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a
major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC
in CMAS was reached
Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC
system Significant amount of research has gone into improving TBC resistance to CMAS
Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop
CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or
viscosity upon dissolution [909293] However once consumed CMAS can then attack the
coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by
the formation of a reaction-crystallization product have been shown to provide better protection
[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs
are porous However TBCs are not the focus of this study
13
14 Approach
First the concept of optical basicity (OB Λ) was used as a first order screening for potential
EBCs (see Section 141 for more details) Then the selected materials were made through powder
processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic
pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see
Section 142 for more details)
141 Materials SelectionOptical Basicity
As a first order screening optical basicity (OB Λ) was used to determine potential EBC
materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS
is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with
unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical
activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which
defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is
defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the
polarizability of the metal cation [9596]
Cations with high polarizability draw the electrons away from the oxygen which does not
allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value
On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other
cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for
EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron
spectroscopy [97] and mathematical relationships between refractivity and electronegativity
[100ndash102] have been used to measure or estimate the OBs for single cation oxides
14
Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]
Single Cation Oxide Λ Ref
CaO 100 [103]
MgO 078 [103]
Al2O3 060 [103104]
SiO2 048 [103]
Gd2O3 118 [105]
Y2O3 100 [100]
Yb2O3 094 [105]
La2O3 118 [105]
Sc2O3 089 [100]
Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040
Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation
oxides can be calculated using the equation below
Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)
where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are
the fraction of oxygen ions each single cation oxide donates Although this model was used to
determine the chemical reactivity of glasses it has also been used to access crystalline materials
as well [104107] However for crystalline materials coordination states need to be considered
OB values change based on the coordination number (CN) in glasses with an intermediate oxide
Al2O3 [104]
The difference in OB values of products in a reaction tend to be less than that of the
reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]
Therefore the reactivity is proportional to the change in OB
119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)
This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses
[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to
15
explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found
that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al
[78] showed that OB difference calculations are a quantitative chemical basis for screening
CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high
reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which
will stop the progression of CMAS into the base material The OBs of a wide range of CMAS
compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which
is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]
Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS
(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS
sand with an OB of 064 and the chosen EBC ceramics
Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested
with CMASs Based off Ref [78]
Multi-Cation Oxide Ref Λ ΔΛ wrt Sand
(Λ = 064)
Gd4Al2O9 [112] 099 035
Y4Al2O9 [112] 087 023
GdAlO3 [112] 079 015
LaAlO3 [112] 079 015
Y2SiO5 [69113] 079 015
Yb2SiO5 [114] 076 012
YAlO3 [115] 070 006
Y2Si2O7 [2569] 070 006
Yb2Si2O7 [25114] 068 004
Sc2Si2O7 [25] 066 002
Lu2Si2O7 [25] 066 002
Yb18Y02Si2O7 -- 069 005
Yb1Y1Si2O7 -- 068 004
Based off Krause et al [78] For Al3+ CN = 4 CN = 6
16
As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to
have small OB difference with CMAS glass There have been a few experiments conducted with
these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic
study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be
done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and
Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a
potential EBC ceramic [115]
142 Objectives
This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS
interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC
ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the
overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free
EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with
CMAS are seen in Chapter 2 and 3 respectively [116117]
Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics
of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7
and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to
explore their high temperature interactions with CMAS In this section three different CMAS
compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to
determine how different compositions change the interaction with the same EBC ceramics The
17
thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity
thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]
After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a
few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at
Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the
focus will be on the coating interactions with CMAS and understanding the effect of the APS
coating microstructure (ie grain size porosity and splat boundaries)
18
CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST
ATTACK BY MOLTEN CMAS
This chapter was reproduced from a previously published article LR Turcer AR Krause
HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance
against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-
Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]
21 Introduction
Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-
Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-
resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a
rough screening criterion based on purely chemical considerations and that the actual reactivity
will depend on various other factors including the nature of the cations in the EBC ceramics and
the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense
polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The
overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these
interactions and to use this understanding to guide the design and development of CMAS-resistant
EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show
distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7
Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-
boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no
reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient
and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in
this chapter and the latter group is presented in the next chapter
19
YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal
structure [120] with no phase transformation from room temperature up to its congruent melting
point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa
[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared
to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for
EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC
ceramic [115] but there has been no significant research reported in the open literature on this
ceramic in the context of EBCs
In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-
temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7
monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC
[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of
396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a
challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match
with SiC and it is also relatively lightweight
22 Experimental Procedure
221 Processing
The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3
(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)
LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature
required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in
ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder
20
mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM
Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The
reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to
remove the LiCl from the mixture The YAlO3 powder was then dried and crushed
The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts
of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
respectively [127] This mixture was then ball-milled and dried using the same procedure
described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC
in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an
additional 24 h dried and crushed
The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and
densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in
an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a
low-speed diamond saw and the cross-sections were polished to a 1-μm finish
222 CMAS interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and it is close to the composition of the AFRL-03
21
standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a
procedure described elsewhere [7086] CMAS interaction studies were performed by applying the
CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3
and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet
with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for
different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were
then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish
In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7
powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described
in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the
box furnace for 1500 degC in air for 24 h and crushed into fine powders
223 Characterization
The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also
characterized using XRD The phases present in the reaction products were identified using the
PDF2 database
The densities of the as-SPSed pellets were measured using the Archimedes principle with
distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were
thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)
The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a
scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600
FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems
22
(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to
measure the average grain sizes (linear-intercept method) of the as-SPSed pellets
Transmission electron microscopy (TEM) specimens from specific locations within the
polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam
(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then
examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca
Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area
electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were
recorded and indexed using standard procedures
23 Results
231 Polycrystalline Pellets
Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet
respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8
μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or
YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have
YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during
processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-
powder processing
23
Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed
XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are
present)
Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7
pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size
is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-
pure γ-Y2Si2O7
Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure γ-Y2Si2O7
A B
B A
24
232 YAlO3-CMAS Interactions
Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between
the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding
EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to
have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)
The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si
and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some
Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer
although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains
lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)
phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich
composition
Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at
1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 3 The dashed
boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB
A B
Figure 12A
Figure 12B
25
The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at
1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists
of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure
12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer
(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet
(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and
10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)
Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The
ideal compositions of the three main phases and CMAS are also included
Region Y Al Ca Si Mg Phase
1 18 23 23 31 5 CMAS Glass
2 47 2 15 36 - Y-Ca-Si Apatite (ss)
3 34 45 8 11 2 Y-Al-Ca YAG (ss)
4 54 46 - - - Y-rich YAP (Base)
5 50 1 13 36 - Y-Ca-Si Apatite (ss)
6 36 43 7 12 2 Y-Al-Ca YAG (ss)
7 46 43 11 - - Y-Al-Ca YAG (ss)
8 55 45 - - - Y-rich YAP (Base)
9 55 45 - - - Y-rich YAG (Base)
10 46 54 - - - Y-rich YAG (Base)
11 45 55 - - - Y-rich YAP (Base)
Ideal Compositions
500 500 - - - YAlO3 (YAP)
500 - - 500 - γ-Y2Si2O7
500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite
375 625 - - - Y3Al5O12 (YAG)
- 79 376 495 50 Original CMAS Glass
Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in
Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The
indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While
26
region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG
with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent
Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from
regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)
near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their
elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP
from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo
respectively
Table 4 Average EDS elemental composition (at cation basis) from the regions indicated in the
TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 46 - 12 42 - Y-Ca-Si Apatite (ss)
2 27 53 7 11 2 Y-Al-Ca YAG (ss)
3 33 61 4 - 2 Y-Al-Ca YAG (ss)
4 33 62 3 - 2 Y-Al-Ca YAG (ss)
5 30 62 3 - 2 Y-Al-Ca YAG (ss)
6 31 63 6 - - Y-Al-Ca YAG (ss)
7 32 63 5 - - Y-Al-Ca YAG (ss)
B
A
27
Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-
layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet
showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-
Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-
magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca
and Si elemental EDS maps respectively
28
Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14
were collected
A
Figure 13B
B
C
D
Figure 14A
Figure 14B
29
The chemical composition of the different regions in the higher-magnification SEM images
in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given
in Table 5 From these results the remnants of the three reaction layers can be seen with the top
Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)
and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between
the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass
has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found
Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 5
Table 5 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 51 - 13 36 - Y-Ca-Si Apatite (ss)
2 50 11 16 23 - Y-Ca-Si Apatite (ss)
3 37 48 5 9 1 Y-Al-Ca YAG (ss)
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
5 37 48 5 9 1 Y-Al-Ca YAG (ss)
6 53 47 - - - Y-rich YAP (Base)
B A
30
Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at
1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG
phases along with some unreacted YAlO3 and YAM phases
Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9
(YAM) in addition to unreacted YAlO3
233 Y2Si2O7-CMAS Interactions
Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7
EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked
regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a
depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3
case under the same conditions The reaction zone has two layers The top layer contains only
needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant
amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer
(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets
31
Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at
1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions
were measured by EDS and they are reported in Table 6
Table 6 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Y Al Ca Si Mg Phase
1 8 8 19 61 4 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 9 6 16 65 4 CMAS Glass
4 49 13 16 22 - Y-Ca-Si Apatite (ss)
Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after
CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are
observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer
thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared
with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200
μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)
layer
32
Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)
low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The
dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18
were collected
A B
C
D
Figure 17B
Figure 18A
Figure 18B
33
Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone
at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table
7) appear to be very similar to the ones in the 1-h case (Table 6)
Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7
pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure
17B (A) near-top and (B) near-bottom The circled numbers correspond to locations where
elemental compositions were obtained using EDS and they are reported in Table 7
Table 7 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Y Al Ca Si Mg Phase
1 8 7 14 68 3 CMAS Glass
2 51 - 12 37 - Y-Ca-Si Apatite (ss)
3 6 8 14 68 4 CMAS Glass
4 51 - 12 37 - Y-Ca-Si Apatite (ss)
Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at
1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some
unreacted γ-Y2Si2O7
A B
34
Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at
1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7
24 Discussion
The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC
ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar
resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-
crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and
the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening
criterion
In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It
appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)
Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]
and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of
compositions This typically occurs by the dissolution of the ceramic in the CMAS glass
supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This
35
same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the
CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS
glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to
be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)
formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also
known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral
sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which
could be the result of no access to the CMAS glass In this context YAG (ss) is known to have
appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect
reaction [135]
2119862119886119874 2119862119886119884prime + 119881119874
∙∙ (Equation 5)
Rapid reaction with the CMAS and the formation of a relatively thin protective reaction
layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of
YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint
of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-
capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the
moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-
induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat
between the CMC and the YAlO3 EBC
γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction
is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-
of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant
amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily
36
because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of
reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be
similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG
(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-
Y2Si2O7 This is consistent with what has been observed by others [2569]
While the CTE match with SiC is very good and it is relatively lightweight the formation
of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less
effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7
EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable
polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its
silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an
attractive candidate ceramic for EBCs
25 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions between
two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS
glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS
they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it
comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)
grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)
blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only
Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the
reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail
37
and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in
the YAlO3 and γ-Y2Si2O7 EBC ceramics
38
CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY
MOLTEN CMAS
This chapter was modified from previously published articles along with unpublished data
LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)
glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-
3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal
environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo
Scripta Materialia 154 111-117 (2018) [119]
31 Introduction
In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-
Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-
Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive
reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast
the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary
penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage
β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]
Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions
between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash
3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7
(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al
[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly
the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed
(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that
are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the
results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at
39
1500 degC However the density of the pellets is not reported and their microstructures appear to
be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and
CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable
quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally
Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-
beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their
experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC
making these interactions indirect and strongly influenced by the TBC or the TEBC [35]
β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]
Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one
report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu
et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor
environment They showed penetration of the molten CMAS in the porous pellet and some
reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet
precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS
β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1
[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]
is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and
CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction
between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous
Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of
the results [25]
40
Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo
CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical
mechanisms of these interactions and to use this understanding to guide the design and
development of future CMAS-resistant EBCs
32 Experimental Procedure
321 Processing
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY)
The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of
Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers
Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment
Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls
media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried
powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box
furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7
powder were then ball-milled for an additional 24 h and dried
The powders were then densified into 20 mm diameter polycrystalline pellets using spark
plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More
details can be found in Section 221
41
In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled
for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)
followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were
pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered
pellets was sim25 mm
The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic
polishing techniques for CMAS-interaction testing Some pellets were cut through the center using
a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some
instances the polished cross-sections were etched using dilute HF for 10 min
322 CMAS Interactions
CMAS interaction experiments were preformed like the CMAS interaction with Y-
containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52
MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-
Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The
specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10
degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone
CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were
mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into
fine powders Please see Section 222 for more details
323 Characterization
The characterization for these experiments is similar to the Y-containing EBC ceramics
found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
42
was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7
powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets
were measured using the Archimedes principle (immersion medium = distilled water)
Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were
used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted
pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to
observe specific locations within the cross-sections of the CMAS-interacted pellets These samples
were prepared using focused ion beam and in-situ lift-out
33 Results
331 Polycrystalline Pellets
Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7
pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is
sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7
Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Yb2Si2O7
A B
43
Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7
pellet respectively The density of the pellet is 334 Mgm-3 (99) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7
Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed
XRD pattern showing phase-pure β-Sc2Si2O7
Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7
pellet respectively The density of the pellet is 615 Mgm-3 (98) and the average grain size is
sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7
B A
44
Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)
indexed XRD pattern showing phase-pure β-Lu2Si2O7
332 Yb2Si2O7-CMAs Interactions
Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with
CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-
Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are
higher magnification SEM image and corresponding Ca elemental EDS map respectively of the
interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The
amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed
for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher
solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si
apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The
distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C
which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)
formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in
Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is
A B
45
base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the
Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics
[116]
Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at
(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed
box in (A) indicates the region from where higher-magnification SEM image in (B) was collected
The circled numbers correspond to locations where elemental compositions were obtained using
EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where
the TEM specimens were extracted using the FIB
A
B C
Figure 23B
Figure 24A
Figure 24B
46
Table 8 Average EDS elemental composition (at cation basis) from the regions indicated in the
SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The
ideal compositions of the two main phases and the CMAS are also included
Region Yb Al Ca Si Mg Phase
1 8 5 27 57 3 CMAS Glass
2 47 - 13 41 - Yb-Ca-Si Apatite (ss)
3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)
4 46 - - 54 - β-Yb2Si2O7 (Base)
Ideal Compositions
500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite
500 - - 500 - β-Yb2Si2O7 (Base)
- 79 376 495 50 Original CMAS Glass
Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7
pellet (1500 degC 1 h) from regions within the interaction zone similar to those indicated in Figure
23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass
are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively
Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo
cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)
phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in
Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no
CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the
pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure
A B
47
27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between
β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-
crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated
throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The
higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are
wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This
indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling
48
Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)
(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (D) were collected The circled numbers
in (B) correspond to locations where elemental compositions were obtained using EDS and they
are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen
was extracted using the FIB
A B
C
D
Figure 25B
Figure 25D
Figure 27
49
Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in
SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with
CMAS at 1500 degC for 24 h
Region Yb Al Ca Si Mg Phase
1 46 - 12 42 - Yb-Ca-Si Apatite (ss)
2 46 - - 54 - β-Yb2Si2O7 (Base)
3 10 11 21 53 5 CMAS Glass
Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7
Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions
within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS
glass are marked The circled number corresponds to a location where elemental composition was
obtained using EDS and it is reported in Table 9
50
Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2
mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant
damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is
the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the
interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h
(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in
Figure 28E
Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have
interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets
in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows
is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the
micrographs is epoxy from the sample mounting
Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after
interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the
A
B
C
D
E
51
pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large
amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom
Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm
thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region
333 Sc2Si2O7-CMAS Interactions
Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental
EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1
h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-
Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si
apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-
crystallized Yb-Ca-Si apatite (ss) is found
A B
52
Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)
and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations
where elemental compositions were obtained using EDS and they are reported in Table 10
Table 10 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Sc Al Ca Si Mg Phase
1 9 6 31 50 4 CMAS Glass
2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)
4 51 - - 49 - β-Sc2Si2O7 (Base)
After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no
CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in
β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B
and 31C)
A B
53
Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions
from where higher-magnification SEM images in (B) and (C) were collected and the region from
where the TEM specimen was extracted using the FIB
A B
C
Figure 31B
Figure 31C
Figure 32A
54
TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the
only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet
(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to
have penetrated throughout the pellet Once again this is most likely via the grain-boundary
network as the β-Sc2Si2O7 pellet is also fully dense
Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)
from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP
is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from
region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)
Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in
(B) correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 11
Figure 32B
A
A
B
C
55
Table 11 Average EDS elemental composition (at cation basis) from the regions indicated in
the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h
Region Sc Al Ca Si Mg Phase
1 11 12 13 62 2 CMAS Glass
2 47 - - 53 - β-Sc2Si2O7 (Base)
Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at
1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7
334 Lu2Si2O7-CMAS Interactions
Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in
the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness
in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional
SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps
respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC
Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen
in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No
56
Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with
the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found
Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at
(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher
magnification The dashed boxes in (A) indicate regions from where higher-magnification images
in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed
boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 12
A
B
D
C
E
F G
Figure 34C Figure 34B
Figure 34D
Figure 34F
57
Table 12 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h
Region Lu Al Ca Si Mg Phase
1 55 - - 45 - β-Lu2Si2O7
2 55 - - 45 - β-Lu2Si2O7
3 11 7 24 55 3 CMAS Glass
4 10 7 26 54 3 CMAS Glass
5 6 9 32 50 4 CMAS Glass
6 16 9 24 49 3 CMAS Glass
7 55 - - 45 - β-Lu2Si2O7
8 55 - - 45 - β-Lu2Si2O7
After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure
35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change
in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can
also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)
Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone
(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of
the pellet) respectively
Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD
(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and
β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most
likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense
58
Figure 35 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the
region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)
A
B
C
Figure 35B
59
Figure 36 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 24 h) at
(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the
CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction
zone close to the edge of the pellet
Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated
at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7
A
B C
60
34 Discussion
In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the
reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7
and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than
those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and
provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead
of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense
β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical
equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries
in ceramics is reduction in the total energy of the system due to the formation of two glassceramic
interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]
120574119866119861 gt 2120574119868 (Equation 6)
where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo
of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike
porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within
minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process
Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This
results in a dilatation gradient where the top region wants to expand compared to the bottom
unaffected region as depicted schematically in Figure 38A But the constraint provided by the
unpenetrated (undilated) base material creates effective compression in the top dilated layer This
compression is likely to build up as the top dilated layer thickens albeit some relaxation due to
creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg
3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears
61
to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films
(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking
occurred at high temperature as hypothesized and not during cooling to room temperature
Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain
boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the
top dilated layer
It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC
ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly
inadequate creep relaxation of the built-up compressive strain While full understanding of this
phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking
damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic
powders before sintering This CMAS glass is expected to segregate at grain boundaries in the
sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)
facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby
reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the
compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder
before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding
A
B
62
Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched
cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure
(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in
Figure 39B
Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-
Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map
Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with
CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found
Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no
lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding
Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp
narrow) during cooling due to CTE mismatch at the surface
A B
63
Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet
that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the
CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked
by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region
marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map
A
B C
D
Figure 40B
Figure 40C
64
These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo
cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-
Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive
needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic
significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS
glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very
attractive
In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability
are advantages However its high silica activity is a disadvantage Also APS deposition of phase-
pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace
during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good
CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and
high temperature capabilities But the high silica activity and high cost are disadvantages for both
β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure
β-Sc2Si2O7 and β-Lu2Si2O7 are not known
Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets
in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate
with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated
(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to
be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the
mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that
remains to be demonstrated
65
35 Summary
Here we have systematically studied the high-temperature (1500 degC) interactions of three
promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a
CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction
is found between the Y-free EBC ceramics and the CMAS
Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic
pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)
Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7
A B
C D
66
In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si
apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7
Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and
β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage
comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient
caused by the slow penetration of the CMAS glass into the grain boundaries Based on this
understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully
demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering
The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the
CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration
thereby avoiding the dilatation-gradient
67
CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER
COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN
CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter was modified from a submitted (February 20 2020) article LR Turcer and
NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for
resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of
Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier
coatings for gas turbine engines (2020)
41 Introduction
In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a
CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the
progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In
contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the
CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This
raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by
forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of
substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to
be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will
be discussed further in Chapter 5
In this context although there have been several studies focused on the interactions
between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about
CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of
several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and
remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows
several polymorphic transformations in that temperature range In this context it has been shown
68
that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)
[38153]
Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn
and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the
Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions
chosen in this chapter Adapted from Ref [38]
Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC
ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and
Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval
Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics
and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash
(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in
Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7
(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the
chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to
guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs
A B
69
Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for
each
Phase CaO MgO AlO15 SiO2 CaSi
NAVAIR CMAS [116117128] 376 50 79 495 076
NASA CMAS [61] 266 50 79 605 044
Icelandic Volcanic Ash [71] 79 50 79 792 010
42 Experimental Procedures
421 Powders
Experimental procedures for making γ-Y2Si2O7 powder have already been reported and
can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from
Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7
solid-solution powders were prepared in-house by combining stoichiometric amounts of β-
Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same
procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination
at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled
for an additional 24 h and dried
These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7
Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly
pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa
applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100
degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for
CMAS-interaction testing
70
422 CMAS Interaction
Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)
NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these
CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi
ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is
from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of
the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]
compositions are based on literature where the CaSi ratio is changed while maintaining the same
amounts of MgO and AlO15
Powders of the CMAS glasses of these compositions were prepared using a procedure
described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS
powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were
then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box
furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted
pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-
μm finish
423 Characterization
The characterization for these experiments is similar to the EBC ceramics found in
Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)
was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-
71
treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle
(immersion medium = distilled water)
Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy
(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets
Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the
β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-
out
43 Results
431 Powder and Polycrystalline Pellets
Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and
Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of
Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the
Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14
μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the
average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and
Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with
increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates
single-phase (β) solid solutions
72
Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM
images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD
pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher
resolution XRD patterns
73
Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with
Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS
composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure
44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within
the grains Figure 44C is another high-magnification bright-field TEM image showing no phase
contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)
along the line marked L-R The YYb ratios along the entire line are within the EDS detection
limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus
the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table
14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-
solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to
form a complete solid-solution without phase separation as well
74
Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher
magnification bright-field TEM image of the region marked in (A) The circled numbers
correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)
High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along
L-R in (C)
Figure 44B
75
Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition
is also included
Region Yb Y Si
1 30 25 45
2 30 23 47
3 amp 4 28 23 49
Ideal Composition
25 25 50
432 NAVAIR CMAS Interactions
Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the
NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and
Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with
NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In
contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks
(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these
extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the
Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in
Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the
corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)
to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for
reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to
arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be
found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the
Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With
76
the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner
similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich
zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si
apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite
layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in
Figure 45C
77
Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7
and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS
elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and
[116] respectively
Figure 45E Figure 45G
78
Table 15 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7
respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions
are also included
Region Yb Y Ca Mg Al Si Phase
1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite
3 amp 4 4 1 28 4 8 55 CMAS Glass
5 41 4 - - - 55 Yb18Y02Si2O7
6 3 1 28 5 8 55 CMAS Glass
7 amp 8 39 5 - - - 56 Yb18Y02Si2O7
9 20 20 13 - - 47 Y-Y-Ca-Si Apatite
10 amp 11 4 4 22 3 5 62 CMAS Glass
12 4 3 21 3 5 64 CMAS Glass
13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite
14 2 3 24 4 6 61 CMAS Glass
15 amp 16 23 18 - - - 59 Yb1Y1Si2O7
Ideal Compositions
45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite
25 25 125 - - 375 Yb4Y4Ca2(SiO4)6O2 Apatite
45 5 - - - 50 Yb18Y02Si2O7
25 25 - - - 50 Yb1Y1Si2O7
433 NASA CMAS Interactions
Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =
044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show
lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca
elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure
46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are
absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C
46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA
CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-
Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS
79
(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm
thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in
Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after
NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample
as compared to the middle of the sample (toward the bottom of the micrograph)
Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC
ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes
indicate from where the corresponding higher-magnification SEM images are collected (E)
Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca
elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled
numbers in (E) through (G) correspond to regions from where EDS elemental compositions are
obtained (see Table 16)
Figure 46E Figure 46F
Figure 46G
Figure 46H
80
Table 16 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500
˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 44 - - - - 56 Yb2Si2O7
2 18 - 15 3 3 61 CMAS Glass
3 25 - 10 3 1 61 CMAS Glass
4 44 - - - - 56 Yb2Si2O7
5 40 4 - - - 56 Yb18Y02Si2O7
6 3 1 26 4 6 60 CMAS Glass
7 40 4 - - - 56 Yb18Y02Si2O7
8 5 1 23 3 6 63 CMAS Glass
9 23 18 - - - 59 Yb1Y1Si2O7
10 3 2 24 4 6 61 CMAS Glass
11 22 18 - - - 59 Yb1Y1Si2O7
12 3 2 24 4 5 62 CMAS Glass
13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite
15 - 15 15 4 6 60 CMAS Glass
16 - 45 - - - 55 Y2Si2O7
Includes signal from surrounding material
434 Icelandic Volcanic Ash CMAS Interactions
Figures 47A 47B 47C and 47D are cross-sectional SEM micrographs of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA
CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images
and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively
This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2
(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite
formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed
in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets
reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical
compositions are reported in Table 17 (regions 3 7 and 10)
81
Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics
(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from
where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)
Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)
Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)
through (G) correspond to regions from where EDS elemental compositions are obtained (see
Table 17)
Figure 47E Figure 47F
Figure 47G Figure 47H
82
Table 17 Average EDS elemental composition (at cation basis) from the regions numbered in
the SEM micrographs in Figures 47E 47F 47G and 47H of interactions of Yb2Si2O7
Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic
Ash CMAS at 1500 ˚C for 24 h
Region Yb Y Ca Mg Al Si Phase
1 - - - - - 100 SiO2
2 4 - 17 7 11 61 CMAS Glass
3 36 - 2 - - 62 Re-precipitated Yb2Si2O7
4 44 - - - - 56 Yb2Si2O7
5 3 1 16 7 12 61 CMAS Glass
6 - - - - - 100 SiO2
7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7
8 38 5 - - - 57 Yb18Y02Si2O7
9 2 3 17 7 11 60 CMAS Glass
10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7
11 - - - - - 100 SiO2
12 17 25 - - - 58 Yb1Y1Si2O7
13 - - - - - 100 SiO2
14 - 5 12 5 10 68 CMAS Glass
15 amp 16 - 45 - - - 55 Y2Si2O7
44 Discussion
The results from this study show systematically that the CaSi ratio in the CMAS can
influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends
critically on the x value First consider the propensity for the formation of the apatite reaction
product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic
radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite
formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =
076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest
propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand
formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores
andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this
reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the
83
EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC
ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite
formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely
eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to
penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving
force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this
study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to
reduce apatite formation
As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for
apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA
CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-
boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could
be related to the relatively facile grain-boundary penetration of NASA CMAS which may
preclude the formation of a dilatation gradient
With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite
formation decreases further The amount of molten CMAS that can react or interact with the pellets
decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi
ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining
CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite
formation and the suppression of lsquoblisterrsquo cracks
This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics
clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by
84
reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining
polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs
45 Summary
Here a systematic study of the high-temperature (1500 degC) interactions between promising
dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7
and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash
(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase
NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive
reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in
Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not
show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case
of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show
lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7
Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of
Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the
NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and
pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio
does not show apatite formation in any of the EBC ceramics studied There is some crystallization
of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC
ceramics This study highlights the interplay between the CMAS and the EBC ceramic
compositions in determining the nature of the high-temperature interaction and suggests a way to
tune that interaction in rare-earth pyrosilicate solid-solutions
85
CHAPTER 5 THERMAL CONDUCTIVITY
This chapter was modified from a previously published article along with unpublished data
that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional
thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution
ceramicsrdquo Scripta Materialia 154 111-117 (2018)
51 Introduction
EBC-coated CMC components need to be attached to the lower-temperature metallic
hardware within the engine which invariably results in temperature gradients It is therefore
imperative that EBCs have enhanced thermal-insulation properties There is also an increasing
demand for thermal protection of CMCs for even higher temperature applications [41335154]
Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile
structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an
additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700
degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]
Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic
illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC
concept
The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as
a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are
typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC
A B
C
86
top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching
1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases
the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or
gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch
problems to some extent However the options of available high-temperature materials for this
additional layer or gradation that satisfy the various onerous requirements is vanishingly small
intermediate CTE high-temperature capability phase stability chemical compatibility with both
TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching
1700 degC deleterious reactions between the different layers and homogenization of any gradations
are inevitable over time Also any additional interfaces can become sources of failure during in-
service thermal cyclingexcursions
In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace
the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material
that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC
concept Thus the four most important properties among several other requirements this single
material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)
inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This
chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may
satisfy these key requirements for TEBC applications
511 Coefficient of Thermal Expansion
As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC
application as they have good CTE match with SiC Figure 49A shows the measured average CTEs
87
of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er
Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC
[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the
monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the
polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by
Felsche [37])
Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from
Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of
the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]
512 Phase Stability
While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application
some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC
Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE
= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC
[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7
have a good CTE match with SiC they may not be suitable for EBC application as both undergo
phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application
88
as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation
δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic
radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860
degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7
β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-
pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically
stabilized at lower temperatures Also the volume change associated with some of the
polymorphic transformations can be small making them relatively benign for high-temperature
structural applications but the CTEs of the product phases may be undesirable (Figure 49A))
513 Solid solutions
Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-
Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While
Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with
Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows
βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to
1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since
these solid-solutions are isomorphous without any low-melting eutectics they are expected to have
higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates
considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or
Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown
to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-
Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is
likely that they also form complete solid-solutions considering that these RE-pyrosilicates are
89
isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)
Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the
list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate
solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt
17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these
solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be
depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next
section
52 Calculated Thermal Conductivity of Binary Solid-Solutions
521 Experimental Procedure
In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)
II Si2O7)
experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal
conductivity and Youngrsquos modulus
Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and
β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent
to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)
measurements They machined the pellets to fit their testing apparatus and followed the ASTM
E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash
diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities
at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides
[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the
specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]
90
The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is
the measured room-temperature density
The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains
using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used
to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-
displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine
indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum
of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of
monoclinic β-Sc2Si2O7
522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity
Among the four β-RE-pyrosilicates considered here the high temperature thermal
conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured
experimentally However the pellets used were not completely dense and instead thermal
conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar
to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are
plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of
the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1
(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been
reported in the open literature In order to address this paucity the thermal conductivities of a fully
dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured
These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal
conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates
91
Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets
as a function of temperature The data for Lu2Si2O7 is from Ref [142]
523 Thermal Conductivity Calculations for Binary Solid-Solutions
None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been
reported in literature In this context there is a tantalizing possibility of obtaining even lower
thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute
point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)
contrast between the host and the solute RE-ions is large To that end analytical calculations have
been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six
systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and
LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71
92
The thermal conductivity of a solid-solution in relation with its pure host material as a
function of temperature is given by [166]
119896119904119904 = 119896119875119906119903119890 (120596119900
120596119872) tanminus1 (
120596119872
120596119900) (Equation 7)
where
(
120596119900
120596119872)
2
= 119891(119879) (41205951205742119898119896119861
31205871205831198863) 119879 [119888 (
Δ119872
119872)
2
]
minus1
(Equation 8)
Here ωo is the phonon frequency at which the mean free paths due to point-defect
scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the
maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m
is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here
a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros
number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the
shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant
c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a
simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an
empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity
in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is
given empirically by [167]
119891(119879) =
300 times 119896119875119906119903119890|300
119879 times 119896119875119906119903119890|119879 (Equation 9)
Using the available values for all the parameters (listed in Table 18) [34125138142143]
the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51
Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was
confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a
93
dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)
However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual
grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are
included on the y-axes in Figure 51
Table 18 Properties and parameters for pure β-RE-pyrosilicates
β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7
ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect
v 031para 032 031 032
Ave μ (GPa) 77 65 62 68
Ave E (GPa) 202 170 162 178
a3 (x 10-29 m2) 115 133 127 127
m () 11 11 11 11
γ 3373para 3491 3477 3487
v (mmiddots-1) 4762 4067 3180 3322
Min E (GPa) 153 102 102 114
MW (gmiddotmol-1) 2582 3460 5142 5182
kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are
from Ref [142]
94
Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions
at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7
(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the
pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes
The dashed lines represent 1 Wmiddotm-1middotK-1
95
As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-
x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying
Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show
the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next
to each other in the periodic table and both have high Z All but the last two of the dense solid-
solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This
is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good
candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions
have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs
eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective
In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free
path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-
pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal
conductivityrsquo (kMin) is estimated using the following relation [170]
119896119872119894119899 rarr 087119896119861119873119860
23 119898231205881611986412
(119872119882)23 (Equation 10)
where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties
(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but
are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest
thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are
listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-
solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is
derived based on approximations and provides a rough estimate for the lsquominimum thermal
conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-
96
1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or
ternary) ceramics
Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the
calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10
x
ρ
(Mgmiddotm-3)
Min E
(Gpa)
MW
(gmiddotmol-1)
kMin
(Wmiddotm-1middotK-1)
YxYb(2-x)Si2O7 104 500 102 4266 099
YxLu(2-x)Si2O7 079 534 109 4505 100
YxSc(2-x)Si2O7 172 388 109 3337 107
YbxSc(2-x)Si2O7 134 523 119 4294 115
LuxSc(2-x)Si2O7 167 578 120 4756 102
LuxYb(2-x)Si2O7 200 625 114 5181 099
53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity
531 Experimental Procedure
Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-
Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity
They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal
diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process
please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT
HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were
measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific
heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat
capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity
(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-
temperature density
97
Other experimental data including density Youngrsquos modulus etc were obtained by using
rule-of-mixture calculations
532 Comparison of Experimental and Calculated Thermal Conductivity
Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-
02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is
the lowest For the rest of the thermal conductivity measurements the solid-solutions
Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure
components Yb2Si2O7 and Y2Si2O7
Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line
represents 1 Wmiddotm-1middotK-1
98
To more easily compare this data the experimental data points are plotted against the
calculated values from Section 523 which can be seen in Figure 53 The experimental data does
not have as significant a decrease in thermal conductivity as expected from the analytical
calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity
lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From
600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures
estimate
Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600
800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities
which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1
99
Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and
rule-of-mixture calculations
Temperature
(degC)
Thermal Conductivities (Wmiddotm-1middotK-1)
Yb18Y02Si2O7 Yb1Y1Si2O7
Experimental Rule-of-Mixture Experimental Rule-of-Mixture
27 420 507 361 447
200 351 405 302 342
400 304 335 264 276
600 263 280 231 229
800 247 258 216 210
1000 247 252 212 209
Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-
solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to
1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal
conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However
they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =
017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of
Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing
atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an
oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by
alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa
= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)
It is known that point defects such as oxygen vacancies are potent phonon scatterers in
RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-
pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast
cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874
∙∙ This effect could be
further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA
oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria
100
studies in these systems have not been reported in the open literature and therefore the relative
solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor
glasses in such multicomponent silicate systems which may limit their utility in high-temperature
TEBC applications
Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be
to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed
further in the following section
54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution
541 Introduction to High-Entropy Ceramics
High-entropy alloys were first studied in 2004 [173] These were made by mixing
equimolar amounts of metallic elements which creates a disordered solid-solution This increases
the entropy of the system which causes a decrease in the energy of the system Since then many
studies have focused on high-entropy ceramic materials to enhance certain properties High-
entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and
silicides [183184] have all been studied They have demonstrated phase stability and have been
shown to have adjustable and enhanced properties [185]
In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first
studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which
was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied
the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5
compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed
solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy
101
RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms
a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-
entropyrsquo nature of this system has not been confirmed
For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-
solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-
entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-
solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =
71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the
thermal conductivity will decrease The overall goal is to provide insights into the thermal
conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this
understanding to guide the design and development of future low thermal-conductivity TEBCs
542 Experimental Procedure
The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining
stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis
MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)
Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)
This mixture was then ball-milled and dried while stirring The dried powder mixture was placed
in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-
02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed
The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil
and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)
in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating
102
rate 1500 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of
the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets
were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box
furnace The top surfaces of the pellets were polished to a 1-μm finish using standard
ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and
the cross-sections were polished to a 1-μm finish
The as-prepared powder was characterized using an X-ray diffractometer (XRD D8
Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was
identified using the PDF2 database The densities of the as-SPSed pellets were measured using the
Archimedes principle with distilled water as the immersion medium
The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl
Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca
Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental
maps were also collected and used to determine homogeneity in the pellets
A transmission electron microscopy (TEM) specimen from a location within the polished
cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI
Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100
F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire
UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns
(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard
procedures
103
543 Solid Solution Confirmation
Although the material was confirmed to be solid-solution by Dong et al [186] they made
samples using a sol-gel process Here the samples were made by mixing oxide constituents and
calcinating the powders Therefore due to the difference in materials processing a confirmation
of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed
Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared
to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and
Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density
of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the
theoretical density obtained by reitveld analysis)
Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet
compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets
Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS
104
maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions
were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at
cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at
cation basis)
Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and
the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si
Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two
higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps
for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS
maps are showing a homogenous material EDS elemental compositions were collected (circled
numbers) and can be found in Table 21
105
Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-
02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are
denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain
boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled
regions are where EDS elemental compositions were obtained and can be found in Table 21
Figure 56B
Figure 56C
106
Table 21 Average EDS elemental composition (at cation basis) from the regions numbered in
the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7
EBC ceramic pellet
Region Yb Y Lu Sc Gd Si
1 11 8 11 8 10 52
2 11 8 11 8 11 51
3 11 8 11 8 10 52
4 12 9 12 9 11 47
TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be
nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of
β-(Y02Yb02Lu02Sc02Gd02)2Si2O7
544 Experimental Thermal Conductivity Results
Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and
can be seen below in Figure 57 Room temperature thermal conductivity of the β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal
conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is
increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-
1middotK-1 at 800 and 1000 degC)
107
Figure 57 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7
Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of
temperature The dashed line represents 1 Wmiddotm-1middotK-1
Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-
component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The
solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000
degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same
temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a
decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher
temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be
attributed to the ldquominimum conductivityrdquo limit [171]
55 Summary
Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7
YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were
108
performed Substitutional-solute point defects are an effective way to scatter phonons and decrease
thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast
solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the
largest decrease in thermal conductivities due to alloying
Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal
conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does
not have as significant a decrease in thermal conductivity as expected by the analytical
calculations
A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and
TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a
single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower
room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate
solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151
Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal
conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]
109
CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED
ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK
BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS
This chapter is unpublished data that may be used in a future publication
61 Introduction
In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with
a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7
and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with
CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was
shown that in some cases solid-solutions can temper the failure mechanisms of the pure
components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC
(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo
experiments to determine the reaction between EBC materials and CMAS glass However the
microstructure of coatings is different to that of polycrystalline pellets Therefore the next step
was to determine how air plasma sprayed (APS) EBCs would interact with CMAS
Unfortunately EBC deposition is still a significant challenge [3940] Conventional air
plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs
typically deposit as an amorphous coating [41] To crystallize the coating during spraying many
researchers have performed APS inside a box furnace where the substrate is heated to temperatures
above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]
has studied the microstructural evolution when a post-deposition heat treatment is performed on
APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant
volume change which can lead to porous coatings Also undesirable phases may form during
110
crystallization However it was determined that a more amorphous coating included less porosity
initially and fewer SiO2 inclusions
In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions
with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings
Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS
processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200
degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor
deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-
silicate EBC which makes the interactions indirect and strongly influenced by the TBC
Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi
ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation
in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little
reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in
the Yb2Si2O7 coatings used by Zhao et al [36]
There is little known about the interaction between CMAS and solid-solution ie
Yb1Y1Si2O7 APS coatings
Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and
Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =
076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-
mechanical mechanisms of these interactions and to use this understanding to guide the design
and development of future CMAS-resistant low thermal-conductivity TEBCs
111
62 Experimental Procedures
621 Air Plasma Sprayed Coatings
The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073
Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon
Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder
The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay
Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma
sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC
console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-
plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary
gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of
712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed
was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders
622 Heat Treatments
Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which
will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets
for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)
623 CMAS Interactions
The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52
MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition
of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was
112
prepared using a procedure described elsewhere [7086] CMAS interaction studies were
performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-
treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then
placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace
at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings
were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm
finish
624 Characterization
The as-sprayed and heat-treated APS coatings were characterized using an X-ray
diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity
The phases present were identified using the PDF2 database In-situ high-temperature XRD of the
as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were
conducted to determine the temperature needed for the coatings to crystallize A ramping rate of
10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was
performed
The densities of the as-sprayed and heat-treated coatings were measured using the
Archimedes principle with distilled water as the immersion medium
Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were
observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany
or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy
(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS
113
elemental maps particularly Ca and Si were also collected and used to determine CMAS
penetration into the pellets
63 Results
631 As-sprayed and Heat-Treated Coatings
As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM
micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and
some porosity is observed There are lighter and darker gray regions in this microstructure
indicating a change in silica concentration Lighter regions have lower amounts of silica which
was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS
coating XRD was collected on both the top and bottom of the coating Slight differences can be
seen between the top to bottom of the coating but both confirm that the coating is mostly
amorphous with small amounts of un-melted particles
Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low
and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
114
Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7
APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and
darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick
Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns
were collected on both the top and bottom of the coating The bottom of the coating is almost
purely amorphous The top of the coating shows more peaks indicating it contains more un-melted
Yb1Y1Si2O7 particles Both show a mostly amorphous coating
Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)
low and (B) high magnification The lighter gray regions in these images contain less silica (C)
Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides
showing a mostly amorphous coating
To determine the heat treatment needed to crystallize the coatings in-situ high-temperature
XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25
and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The
coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the
coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD
remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat
treatment temperature for the APS coatings
115
Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from
room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100
1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and
the square markers and dashed line index the Yb1Y1SiO5 phase
Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and
61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of
all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment
is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99
However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present
which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density
compared to pure Yb2Si2O7
116
Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD
patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides
showing both Yb2Si2O7 and Yb2SiO5 are present
Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-
treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings
Coatings Density
(Mgm-3)
Theoretical
Density (Mgm-3)
Relative
Density
Open
Porosity
Yb2Si2O7 As-sprayed 639 615 104 4
Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5
Yb1Y1Si2O7 As-sprayed 492 5045 98 4
Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3
Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline
Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is
also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the
Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also
confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7
APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-
of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed
due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-
117
mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of
the pure Yb1Y1Si2O7
Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS
coating at (A) low and (B) high magnification The lighter gray regions in these images are
Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed
XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom
sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present
632 NAVAIR CMAS Interactions
All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)
APS coatings
Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the
region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
118
Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7
APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box
indicates the region where (B) was collected (B) A higher magnification image and its
corresponding Si Ca and Yb elemental EDS maps
Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a
Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E
are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS
elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a
thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)
can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is
also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further
into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found
119
Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 23
Table 23 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Ca Si Phase
1 45 12 43 Yb-Ca-Si Apatite (ss)
2 47 10 43 Yb-Ca-Si Apatite (ss)
3 62 - 38 Yb2SiO5
4 44 - 56 Yb2Si2O7
5 61 - 39 Yb2SiO5
6 45 - 55 Yb2Si2O7
7 61 - 39 Yb2SiO5
Ideal Compositions
500 125 375 Yb8Ca2(SiO4)6O2 Apatite
500 - 500 Yb2Si2O7
667 - 333 Yb2SiO5
120
Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has
interacted with CMAS at 1500 degC for 24 h Figure 65B is a higher magnification image of the
region indicated in Figure 65A and its corresponding Si Ca and Yb elemental EDS maps No
CMAS glass is observed on the top of the coating The dashed line indicates the approximate
CMAS penetration
Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)
Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The
dashed box indicates the region where (B) was collected (B) A higher magnification image and
its corresponding Si Ca Y and Yb elemental EDS maps
Figures 66A 66B and 66D are higher magnification cross-sectional SEM images of a
Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and
66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS
elemental compositions of regions 1 to 8 are reported in Table 24 The top of the coating has a
layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite
(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite
phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5
present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map
121
in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and
Yb2SiO5 (regions 6 and 8) can be found
Figure 66 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted
(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher
magnification images were obtained (B D) The higher magnification SEM micrographs and (C
E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)
correspond to locations where elemental compositions were obtained using EDS and they are
reported in Table 24
122
Table 24 Average EDS elemental composition (at cation basis) from the regions indicated in
the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with
CMAS at 1500 degC for 24 h
Region Yb Y Ca Si Phase
1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)
2 24 18 - 58 Yb1Y1Si2O7
3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss)
4 24 18 - 58 Yb1Y1Si2O7
5 22 20 - 58 Yb1Y1Si2O7
6 33 25 - 42 Yb1Y1SiO5
7 22 20 - 58 Yb1Y1Si2O7
8 30 27 - 43 Yb1Y1SiO5
Ideal Compositions
250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite
250 250 - 500 Yb1Y1Si2O7
333 333 - 334 Yb1Y1SiO5
64 Discussion
Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter
3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio
= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7
pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials
than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite
formation is found in the Yb1Y1Si2O7 APS coating
Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in
the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al
[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio
= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]
Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered
polycrystalline Yb2SiO5 pellets
123
In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite
(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the
surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and
Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb
YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the
elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150
μm) This could be due to the placement of the cross-section (slightly off center of the CMAS
interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb
YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the
RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The
example reaction for the pure Yb system is shown
4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)
Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from
the melt
4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)
In fact excess SiO2 from the Yb2Si2O7 is added into the melt
In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain
boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5
(RE = Yb YbY) is present and another reaction with the CMAS can occur
Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)
This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si
elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which
have approximately the same Si concentration this is the CMAS interaction zone Below that in
124
the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions
the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =
Yb YbY) in these APS coatings
The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could
be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
65 Future Work
There is ongoing work for the APS coatings and CMAS interaction studies Currently a
post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She
is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM
The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be
determined through high-resolution XRD and rietveld analysis
CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in
Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete
a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA
= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)
formation
66 Summary
Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A
heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline
125
coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD
and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-
RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings
The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs
Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied
CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to
the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS
melt Therefore no remaining CMAS glass was observed in either coatings
The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be
due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the
RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the
formation of a dilatation gradient
126
CHAPTER 7 CONCLUSIONS AND FUTURE WORK
71 Summary and Conclusions
Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and
fibers are showing great promise in the enginersquos hot-section due to their inherently high
temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-
gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession
[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings
(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements
[131617]
Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-
aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris
or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines
Since the surface temperatures of EBCs are expected to be well above the melting point of most
CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS
attack of EBCs is expected to be severe due to the high operating temperatures and the fact that
all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]
Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS
to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for
choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS
and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth
RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant
EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough
screening criterion based purely on chemical considerations The actual reactivity will depend on
127
many other factors including the nature of the cations in the EBC ceramics the CMAS
composition and the relative stability of the reaction products
In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense
polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio
= 076) glass have been explored as part of a model study Despite the fact that the optical basicities
of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS
occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions
of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12
(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7
and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y
in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly
different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)
in Chapter 3
Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7
and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of
reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases
of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC
ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to
the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the
CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1
vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy
phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the
dilatation-gradient
128
Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of
tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC
ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of
environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-
x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the
CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic
Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0
Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2
Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks
Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease
in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite
formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size
Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This
suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions
Chapter 5 introduces a new concept based on the formation of solid-solutions thermal
environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC
and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated
The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and
compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was
also studied Between room temperature and 600 degC a large decrease in thermal conductivity
compared to the other materials studied in this chapter was observed However at higher
temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal
129
conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-
(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit
Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC
ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma
spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS
coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been
explored as part of a model study Before CMAS testing could occur the APS coatings needed to
be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as
well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found
the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed
Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but
also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the
APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets
was not observed in the APS coatings which could be due to the depletion of CMAS or the
porosity in the coatings
72 Future Work
Although we have gained insight into potential coatings used as EBCs on hot-section
components in gas-turbine engines there is more that needs to be researched In the context of
dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be
studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of
CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind
this is not known and should be investigated further
130
Another area of focus will be water vapor corrosion studies on the dense polycrystalline
solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and
Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of
Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data
are still in the process of being analyzed to determine the recession of the pellet and the reaction
products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM
micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si
elemental EDS maps can be seen in Figures 67Indash67L respectively
Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7
Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement
zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The
corresponding Si elemental EDS maps to (E-H) respectively
The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest
moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a
homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure
131
RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been
studied
Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using
combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted
As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-
solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should
also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction
studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will
be done to complete this systematic study
These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy
facility shutdown
132
REFERENCES
[1] NP Padture M Gell EH Jordan Thermal Barrier Coatings for Gas-Turbine Engine
Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609
[2] R Darolia Thermal barrier coatings technology critical review progress update remaining
challenges and prospects International Materials Reviews 58 (2013) 315ndash348
httpsdoiorg1011791743280413Y0000000019
[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-
turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232
[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)
804ndash809 httpsdoiorg101038nmat4687
[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides
MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234
[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069
httpsdoiorg101126science1179327
[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology
John Wiley amp Sons Hoboken NJ USA 2014
[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine
efficiency American Ceramic Society Bulletin 95 (nd) 7
[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature
materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for
Energy Conversion and Storage Elsevier 2020
[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York
2005
[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by
SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-
Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834
httpsdoiorg101111j1151-29161999tb02005x
[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered
Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA
2016
133
[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J
American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-
29161993tb03684x
[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the
American Ceramic Society 80 (1997) 197ndash205 httpsdoiorg101111j1151-
29161997tb02810x
[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface
and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-
8972(00)00889-6
[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for
SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25
(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013
[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper
Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal
of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-
29162003tb03466x
[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride
International Journal of Applied Ceramic Technology 1 (2004) 362ndash373
httpsdoiorg101111j1744-74022004tb00187x
[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions
of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash
2932 httpsdoiorg101111jace16972
[21] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-
Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588
httpsdoiorg101146annurev-matsci-071312-121636
[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐
velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726
httpsdoiorg101111jace16915
[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush
SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic
coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102
(2019) 2948ndash2964 httpsdoiorg101111jace16113
[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS
glass with yttrium disilicate environmental barrier coating material Ceramics International
44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058
134
[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion
behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33
(2013) 3419ndash3428 httpsdoiorg101016jjeurceramsoc201305030
[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical
interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating
materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067
httpsdoiorg101016jjeurceramsoc201906051
[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of
composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta
Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043
[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior
of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of
Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004
[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture
mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal
exposure Acta Materialia 103 (2016) 448ndash460
httpsdoiorg101016jactamat201510019
[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative
study on high-temperature performance and thermal shock behavior of plasma-sprayed
Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646
httpsdoiorg101016jsurfcoat201806056
[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal
Properties of Yb 2 SiO 5 A Promising Material for TEBCs Applications J Am Ceram Soc
99 (2016) 1404ndash1411 httpsdoiorg101111jace14085
[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase
composition on microstructure and thermal properties of ytterbium silicate coatings deposited
by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash
3985 httpsdoiorg101016jjeurceramsoc201804047
[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of
calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer
environmental barrier coatings Acta Materialia 105 (2016) 189ndash198
httpsdoiorg101016jactamat201512016
[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of
calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials
Surface and Coatings Technology 284 (2015) 44ndash50
httpsdoiorg101016jsurfcoat201508069
135
[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability
and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am
Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262
[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma
sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162
httpsdoiorg101016jsurfcoat201512053
[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin
Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3
[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7
in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889
httpsdoiorg101016jjssc201105034
[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental
Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary
Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024
httpsdoiorg101007s11666-017-0574-1
[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-
Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings
Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055
[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed
Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486
httpsdoiorg101016jjeurceramsoc201811018
[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley
Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in
water vapor Acta Materialia 106 (2016) 1ndash14
httpsdoiorg101016jactamat201512053
[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier
coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083
httpsdoiorg101016jjeurceramsoc201404027
[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer
environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I
Processing Surface and Coatings Technology 205 (2010) 258ndash265
httpsdoiorg101016jsurfcoat201006029
[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings
under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826
httpsdoiorg101016jjallcom201712297
[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage
evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under
136
thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326
httpsdoiorg101016jsurfcoat201507042
[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation
of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high
temperature Materials Letters 193 (2017) 176ndash178
httpsdoiorg101016jmatlet201701085
[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor
deposited mullite environmental barrier coatings for Si-based ceramics Journal of the
European Ceramic Society 28 (2008) 437ndash445
httpsdoiorg101016jjeurceramsoc200703007
[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings
Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015
[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of
nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)
192ndash195 httpsdoiorg101016jmatchemphys200311028
[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation
rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521
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to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965
httpsdoiorg101111jace12974
[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the
Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in
combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267
httpsdoiorg101016jjeurceramsoc201507019
[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and
thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC
composites Ceramics International 43 (2017) 13075ndash13082
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[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of
thermodynamic properties for phase transitions in minerals European Journal of Mineralogy
(1998) 621ndash691 httpsdoiorg101127ejm1040621
[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A
REVIEW (2013) 18
[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability
of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic
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137
[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie
Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and
Combustor Liners of Advanced Industrial Gas Turbines 2011
httpsdoiorg1021721004879
[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of
thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941
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[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials
in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)
641ndash651 httpsdoiorg10111512906754
[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war
JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663
[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface
temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings
Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5
[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High
Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223
[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG
Evans Mechanisms of cracking and delamination within thick thermal barrier systems in
aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials
Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006
[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal
Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the
American Ceramic Society 89 (2006) 3167ndash3175 httpsdoiorg101111j1551-
2916200601209x
[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying
the minimum level to initiate damage (2010)
httpdxdoiorg101016jijrmhm200907005
[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially
Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay
Journal of the American Ceramic Society 94 (2011) 925ndash931
httpsdoiorg101111j1551-2916201004166x
[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition
on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J
Turbomach 135 (2013) httpsdoiorg10111514006571
138
[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and
degradation mechanisms of advanced environmental barrier coatings Surface and Coatings
Technology 237 (2013) 79ndash87 httpsdoiorg101016jsurfcoat201308036
[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP
Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature
attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844
httpsdoiorg101016jactamat201009013
[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP
Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)
2419ndash2424 httpsdoiorg101002adma201004783
[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion
behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental
barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332
httpsdoiorg102109jcersj216211
[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic
Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a
Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357
[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation
of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials
Science and Engineering A 528 (2011) 7214ndash7221
httpsdoiorg101016jmsea201106041
[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine
Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power
129 (2007) 135ndash143 httpsdoiorg10111512181181
[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle
Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and
Power 130 (2008) 051503 httpsdoiorg10111512903901
[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20
httpsdoiorg101038509020a
[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture
2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part
I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949
httpsdoiorg101111jace13210
[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954
[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)
139
[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash
Geophysical Research Letters 7 (1980) 949ndash952
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[82] WH Chesner User guidelines for waste and by-product materials in pavement construction
US Dept of Transportation Federal Highway Administration Research and Development
Turner-Fairbank Highway Research Center McLean VA 1998
[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH
Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)
1ndash14
[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and
Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)
4483ndash4489 httpsdoiorg101111j1551-2916201104747x
[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-
sprayed coatings tested simultaneously under CMAS and thermal gradient cycling
conditions Surface and Coatings Technology 205 (2010) 2287ndash2295
httpsdoiorg101016jsurfcoat201009008
[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are
resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash
6745 httpsdoiorg101016jactamat200708028
[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria
stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society
31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006
[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier
Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674
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[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium
aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash
2914 httpsdoiorg101016jjeurceramsoc201503032
[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with
multiple coatings (1999)
[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and
coating method therefor (2003)
[92] GE Witz Multilayer thermal barrier coating (2012)
[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as
protective overlay for thermal barrier coatings against CMAS degradation Surface and
Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055
140
[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier
Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly
Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957
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[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity
concept Journal of Non-Crystalline Solids 21 (1976) 373ndash410
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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide
glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50
httpsdoiorg102109jcersj11344
[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the
American Ceramic Society 72 (1989) 2012ndash2013 httpsdoiorg101111j1151-
29161989tb06022x
[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-
Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9
[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium
gallium and indium (2005)
httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003
(accessed February 25 2020)
[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides
I Journal of Applied Physics 79 (1996) 1736ndash1740 httpsdoiorg1010631360962
[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF
OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION
POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal
of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250
[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of
the American Ceramic Society 80 (2005) 1416ndash1420 httpsdoiorg101111j1151-
29161997tb02999x
[104] JA Duffy Relationship between Cationic Charge Coordination Number and
Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141
httpsdoiorg101021jp040330w
[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical
basicity assessment (2005)
141
httpswwwingentaconnectcomcontentsgtpcg20050000004600000001art00001
(accessed February 25 2020)
[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of
lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136
httpsdoiorg101016jphysb200611015
[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[108] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between
oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)
2323ndash2328 httpsdoiorg101039DT9870002323
[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to
viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49
httpsdoiorg102298JMMB1001041G
[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity
of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of
hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-
5861(00)00380-1
[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and
Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8
[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including
a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated
Articles (2011)
[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate
Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction
with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511
httpsdoiorg101111j1551-2916201003916x
[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation
by Molten Glassy Deposit in the Presence of Water Vapor (2011)
[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-
Containing Materials US Patent No 7862901 2011
[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38
(2018) 3905ndash3913 httpsdoiorg101016jjeurceramsoc201803021
142
[117] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier
coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate
(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic
Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010
[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier
Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-
Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)
[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings
(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154
(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032
[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3
and NdGaO3 Applied Physics Letters 86 (2005) 251911
httpsdoiorg10106311944901
[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase
Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr
Metallkunde v92 1083-1097 (2001) 92 (2001)
[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties
of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and
KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98
(2005) 103514 httpsdoiorg10106312128696
[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction
and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-
Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900
httpsdoiorg101111jace12618
[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical
Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541
httpsdoiorg101111j1551-2916200701803x
[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI
52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967
httpsdoiorg101016jwear200812018
[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245
httpsdoiorg102478s13536-012-0091-3
[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater
Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173
[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating
ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta
Materialia 60 (2012) 5437ndash5447 httpsdoiorg101016jactamat201206053
143
[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-
magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal
barrier coatings Scripta Materialia 112 (2016) 118ndash122
httpsdoiorg101016jscriptamat201509027
[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top
coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at
1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187
httpsdoiorg101111jace14854
[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate
Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91
(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x
[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma
sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten
CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916
httpsdoiorg101016jsurfcoat201203051
[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier
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γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier
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[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7
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[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in
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Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609
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(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity
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Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced
Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512
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A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)
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(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of
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and mechanical properties of sintered mullite ceramic as an environmental barrier coating
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