Environmental Stability and Oxidation Behavior of HfO2-Si

National Aeronautics and Space Administration

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Environmental Stability and Oxidation Behavior of HfO2-Si and

YbGd(O) Based Environmental Barrier Coating Systems for

SiC/SiC Ceramic Matrix Composites

Dongming Zhu, Serene Farmer, Terry R. McCue, Bryan Harder, Janet B. Hurst

Materials and Structures Division

NASA John H. Glenn Research Center

Cleveland, Ohio 44135

41st International Conference and Expo on Advanced Ceramics and Composites (ICACC’17)

January 22-27, 2017


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NASA EBC and CMC System Development − Emphasize temperature capability, performance and long-term durability

• Highly loaded EBC-CMCs - Prime-reliant coatings

• 2700-3000°F (1482-1650°C) turbine and CMC combustor coatings

• 2700°F (1482°C) EBC bond coat technology for supporting next generation– Recession: <5 mg/cm2 per 1000 h

– Coating and component strength requirements: 15-30 ksi, or 100- 207 MPa

– Resistance to Calcium Magnesium Alumino-Silicate (CMAS), impact and erosion

2400°F (1316°C) Gen I and Gen II SiC/SiC

CMCs

3000°F+ (1650°C+)

Gen I

Temperature

Capability (T/EBC) surface

Gen II – Current commercialGen III

Gen. IV

Increase in T

across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

Gen I

Temperature

Capability (T/EBC) surface

Gen II – Current commercialGen III

Gen. IV

Increase in T

across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

2700°F (1482C)

2000°F (1093°C), PtAl and NiAl bond coats

Step increase in the material’s temperature capability

3000°F SiC/SiC CMC airfoil

and combustor

technologies

2700°F SiC/SiC thin turbine

EBC systems for CMC

airfoils

2800ºF

combustor

TBC

2500ºF

Turbine TBC 2700°F (1482°C) Gen III SiC/SiC CMCs


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Outline

• Advanced 2700°F capable EBC and bond coat developments

- Rare Earth – Silicon, i.e., YbGd-Si (O) and YbGd-Lu-Si (O) and Hafnia-Si

(HfO2-Si) systems

- Early systems cyclic oxidation results and Si composition optimizations

- Focus on oxidation kinetics studies of selected EB-PVD coatings using TGA

- Oxidation mechanisms and degradation mechanisms

• EBC - CMC system thermomechanical - environment testing, particularly

using laser rigs

- A Key step and capability for developments, and help composition optimization

and

• Summary


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NASA Advanced 2700°F Silicide Based Bond Coats – and

EBC Systems Processing for Various Component Applications– Advanced coating systems developed for various processing to improve

Technology Readiness Levels (TRL)– Composition ranges studied mostly from 50 – 80 atomic% silicon

• PVD-CVD processing, for composition downselects - also helping potentially develop a low cost CVD or laser CVD approach

• Compositions initially downselected for selected EB-PVD and APS coating composition processing

YSi YbGdYSi GdYSi

ZrSi+Y YbGdYSi GdYSi

ZrSi+Y YbGdYSi GdYSi

ZrSi+Ta YbGdYSi GdYSi

ZrSi+Ta YbGdSi GdYSi-X

HfSi + Si YbGdSi GdYSi-X

HfSi + YSi YbGdSi

HfSi+Ysi+Si YbGdSi

YbSi YbGdSi

HfSi + YbSi

YbSi

GdYbSi(Hf)

YYbGdSi(Hf) YbYSi

YbHfSi

YbHfSi

YbHfSi

YbHfSi

YbHfSi

YbSi

HfO2-Si 1;

REHfSi

GdYSi

GdYbSi 2

GdYb-LuSi

NdYSi

1,2

Subelment

demos

HfO2-Si

YSi+RESilicate

YSi+Hf-RESilicate

Hf-RESilicate

Used in ERA components as part of bond coat system

Hf-RE-Al-Silicate Used in ERA components as part of bond coat system

PVD-CVD EB-PVD APS*

REHfSi

FurnaceLaser/C

VD/PVD

Process and

composition

transitions

APS*: or plasma spray related

processing methods

US Patent Application S/N: 13/923,450 (LEW 18949-1), US Provisional Patent Serial No.: 62/329,500 (LEW 18949-2), LEW 19435-1.


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Oxidation Kinetics and Furnace Cyclic Behavior of RESi EBC

Bond Coats -

Oxidation kinetics

0

1

2

3

4

5

0 100 200 300 400 500 600

Specific weight gain, mg2/cm4Spec

ific

wei

ght

gai

n,

mg

2/c

m4

Time, hours

YGdSi bond coat on SiC/SiC, 1500°C

SiC

RESi(O)

RE2Si2O7-x

Transition region

between Si-rich

and silicate regions

40 mm

An example of cross-section

TGA tested specimen

-

– Some early multi-component PVD processed systems showed excellent oxidation

resistance and furnace cyclic test (FCT) durability at 1500°C

– FCT and steam tests also performed for more advanced RESiO-Hf systems

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

60 65 70 75 80 85

RESi(O)RESi(O)RESi(O)

RESi(O)RESi(O)

RESi(O)

RESi(O)

RESi(O)RESi(O)RESi(O)

RESi(O)

RESi(O)

HfO2-SiHfO2-Si

Kp,

mg

2/c

m4-s

ec

Silicon composition, at%

500 hr test,

1500°C in O2


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Sil

ico

n c

on

cen

trat

ion

, at

%

6

Oxidation Kinetics and Furnace Cyclic Behavior of RESi EBC

Bond Coats - Continued

FCT life, Testing in Air at 1500°C, 1 hr cycles

SiC

RESi(O)

RE2Si2O7-x

Transition region

between Si-rich

and silicate regions

40 mm

An example of cross-section

TGA tested specimen

-

– Some early multi-component PVD processed systems showed excellent oxidation

resistance and furnace cyclic test (FCT) durability at 1500°C

– FCT and steam tests also performed for more advanced RESiO-Hf systems

– FCT durability found to be closely related to temperature capability and oxidation

resistance of the coating systems

Monosilicides/monosilicates

Di-silicides/disilicates


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Oxidation Resistance of Plasma sprayed Based HfO2-Si─ TGA weight change measurements in flowing O2

─ Parabolic oxidation kinetics generally observed

─ Solid-state reaction is also involved with the systems, and more complex behavior at 1400

and 1500°C

─ Improved oxidation resistance through APS plasma spray powder processing optimization

(AE10219 II; Sulzer/Oerlikon Metco)

0.0

1.0

2.0

3.0

4.0

5.0

0.00055 0.0006 0.00065 0.0007 0.00075 0.0008

lnK

p, m

g2/c

m4

1/T, K-1

Activation energy 101 kJ/mol

1200°C1300°C1400°C

R2=0.98258

1500°C

10218 Clad

10219 Clad II

10219 Clad Ia

- AE 10219: first Generation HfO2-30wt%Si composite APS powders

- AE 10218 is HfO2-30wt%Si composite APS powders used in NASA

ERA liner component demonstrations

- AE 10219 Clad II is second Generation HfO2-30wt%Si composite

APS powders

0

10

20

30

40

50

60

0 2 4 6 8 10 12

1200C

1300C

1400C

1500C

Spe

cifi

c w

eigh

t ga

in, m

g/cm

2

Time, hour1/2

AE 10219 Clad II

HfSiO4

HfO2

Si

Polished specimen microstructure after

1400°C test (Hot pressed sample)

Plasma spray processed


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Microstructures of Furnace Cyclic Tested GdYbSi(O) EBC Systems— Cyclic tested cross-sections of early PVD processed YbGdSi(O) bond coat

— Self-grown rare earth silicate EBCs and with some RE-containing SiO2 rich phase separations

— Relatively good coating adhesion and cyclic durability

1500°C, in air, 500, 1 hr cycles

AB

B

A

Composition (mol%) spectrum Area #1

SiO2 67.98

Gd2O3 11.95

Yb2O3 20.07

Composition (mol%) spectrum Area #2

SiO2 66.03

Gd2O3 10.07

Yb2O3 23.9

- Complex coating architectures

after the testing

- Designed with EBC like

compositions – Self-grown

EBCs


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Microstructures of Cyclic Tested GdYbSi(O) EBC Systems-

Continued

1500°C, in air, 500, 1 hr cycles

Outlined area SpotComposition (mol%)

SiO2 66.72

Gd2O3 8.62

Yb2O3 24.66

Composition (mol%)

SiO2 96.15

Gd2O3 1.2

Yb2O3 2.64

— Cyclic tested cross-sections of early PVD processed YbGdSi(O) bond coat

— Self-grown rare earth silicate EBCs and with some RE-containing SiO2 rich phase separations

— Relatively good coating adhesion and cyclic durability


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Experimental: NASA Yb,Gd,Y Rare Earth Silicate EBCs

— Yb,Gd(Nd),Y (or RE-Silicate) Multi-Component Rare Earth Silicate EBCs

— Sometime using fine alternating HfO2 and the silicates for top coats

— EB-PVD bond coat systems mostly focused on YbGdSi, YbGd-LuSi, and YbNdSi,

and HfO2-Si

— Initial compositions optimized for the EBC bond coats: RE:Si 1:2; and Hf:Si 1:2 –

1:1

— Coating processed on SiC/SiC ceramic matrix composites for studies

— Processed using Directed Vapor EB-PVD at Directed Vapor Technologies

EBC

Bond Coat

SiC/SiC CMC


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Experimental: Oxidation and Durability Tests

— Test specimens with dimensions 25 mm diameter disc specimens for

oxidation, laser heat flux and furnace cyclic test (FCT) – briefly reviewed

— Thermogravimetric analysis (TGA), using 0.5”x1” CVI SiC/SiC specimens

— Laser long-term thermomechanical fatigue + steam/CMAS water vapor cyclic

test using 0.5”x6” dogbone specimens

Cooling shower

head jets

Test specimen

High

temperature

extensometer

Laser beam

delivery optic

system

High heat flux tensile TMF and rupture testing High heat flux tensile TMF and rupture, with high

velocity steam testing


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Oxidation Kinetics of EB-PVD Processed YbGdSi(O) Based

Coating

– Oxidation kinetics obtained at various temperatures in flowing O2 for

YbGdSi(O) (not necessarily processing optimized)

– Parabolic oxidation kinetics generally observed after initial transient stages

– Activation energy determined 110 kJ/mol

12

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600

1100°C

1200°C

1300°C

1400°C

1500°C

Spec

ific

wei

ght

gai

n, m

g2/c

m4

Time, hours

YbGdSi bond coat/YbGdY silicate EBC on SiC/SiC in O2

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

0.0005 0.00055 0.0006 0.00065 0.0007 0.00075 0.0008

Ln K

p

1/T, K-1

Activation energy, 110.6 kJ/mol


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Oxidation Kinetics Comparisons of Several Advanced EB-PVD

Processed EBC Systems Compared

– The EB-PVD Systems showed comparable oxidation rates and good oxidation

resistance, tested up to 500 h

– Kinetics compared with LuGdSi (O) and HfO2-Si (O) systems

– Further process improvements help improved oxidation resistance and

durability

13

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

0.0005 0.00055 0.0006 0.00065 0.0007 0.00075 0.0008

YbGdSiLuGdSiLuGdSi/with PVD YbGdSi layerHfO2-Si

Ln

Kp

, m

g2/c

m2-h

1/T, K-1

Activation energy 110.6 kJ/mol

Activation energy 136.5 kJ/mol


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Microstructures of the Advanced EBCs after the Oxidation

Tests

– RE-Si system: forming RE silicate “scales”, fully compatible with EBCs

– Reaction and oxidation mechanisms are being further studied, particularly RE

containing SiO2 phase stability

– Further process improvements can help improve the oxidation resistance and

durability

14

EBC layer 1

EBC layer 2

EBC bond coat

SiC/SiC CMC

Cross-section micrograph of

YbGdSi(O) tested at 1500°C, 500hr

EBC bond coat“scales”

Yb,Gd,Y silicate EBC Layer 2

Yb,Gd silicate scale


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Microstructures of the Advanced EBCs after the 500 hr

Oxidation Tests in O2- Continued

– HfO2-Si bond coat: forming HfSiOx based scales bond coat, compatible with

EBCs

– Reaction and stability being studied

– Further process improvements can help improve the oxidation resistance and

durability

15

EBC bond coat

Cross-section micrograph of HfO2-Si

tested at 1500°C, 500hr

HfSiOx


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Tests - Continued

– Surface Morphologies of YbGdSi Bond Coat only on CMC after Oxidation at

1400C, 300 hr

16

A

Composition (mol%)

Gd2O3 3.49

Yb2O3 13.84

SiO2 82.67

Area – all image region

Composition (mol%)

Gd2O3 7.73

Yb2O3 30.54

SiO2 61.73

Area A Composition


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Tests - Continued

17

– Surface Morphologies of YbGdSi Bond Coat only on CMC after Oxidation at

1400°C, 300hr

– Observed SiO2 rich phase separation with fine rare earth silicate phases

– Solubility of HfO2 and rare earth oxides/silicates also being studied using TEM


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CMAS Resistance for the Rare Earth-Silicon Coatings– CMAS resistance of Yb-GdSi (O) at 1500°C, 100 hr

– Higher stability and CMAS resistance observed due to its High Melting Point Coating

Compositions

– Observed the Apatite phase formation

CMAS layer

Coating layer

Coating layer


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High Heat Flux Thermomechanical fatigue Tests of Advanced

NASA EBC-Bond Coats Systems on CMCs• Laser High Heat Flux themomechanical fatigue testing of a HfO2-Si and NASA advanced

EBC baseline with steam at 3 Hz, 2600-2700°F, and 69 MPa maximum stress with stress

ratio 0.05, completed 500 h testing

• Tsurface = 1500-1600°C

• Tinterface= 1320-1350°C

• Heat Flux = 170 W/cm2

• Specimen had some degradations 3hz fatigue testing at 10 ksi loading

Completed 500 hr testing

Testing proving vital failure

mechanisms in a simulated test

environments

EBCs

EBCs

Higher Si content HfO2-Si


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NASA EBC-Bond Coats Systems on CMCs - Continued

- NdYb silicate EBC-RESi bond coat EBC coatings on 3D-architecture CVI-PIP

SiC-SiC CMC (EB-PVD processing), tested in combined CMAS and steam

thermomechanical fatigue, completed ~300 h testing

Steam and CMAS attacked coating

surface at 2700°F


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NASA EBC-Bond Coats Systems on CMCs - Continued

- NdYb silicate EBC-RESi bond coat EBC coatings on 3D architecture CVI-PIP

SiC-SiC CMC (EB-PVD processing), tested in combined CMAS and steam

thermomechanical fatigue, completed ~300 h testing

Steam and CMAS attacked coating surface at

2700°F

SiO2 rich phases separation in CMAS

Nd and Yb dissolutions

Oxide

Component

Mole

Conc.

Yb2O3 3.90

Nd2O3 6.36

Y2O3 1.00

CaO 2.30

SiO2 84.09

MgO 2.36

100.00


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Summary• RE - Silicon and HfO2-Si bond coats with multicomponent rare earth silicate

EBCs processed using EB-PVD, and the oxidation kinetics investigated

• The coatings generally showed very good oxidation and cyclic resistance for

CMCs with targeted designed bond coat compositions, at 1500°C and up to 500

h tests

• The EBC bond coats grow rare earth silicates or HfSiOx “scales”, compatible with

the EBC systems

• Stability of RE, Hf containing SiO2 rich phases from the phase separation being

further evaluated

• Long-term environment durability testing conducted to evaluate the coatings in

more complex load, CMAS and/or steam environments, simulating turbine airfoil

conditions

• The results helping further design and processing improved environmental

barrier coating systems, for achieving more robust, prime-reliant EBC systems


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Acknowledgements

The work was supported by NASA Transformational Tools and Technologies

Project.

The authors are grateful to

- LME and LMC colleagues, Kang N. Lee, Gustavo Costa, Narottam Bansal,

Valerie Wiesner and others for helpful discussions

- Ron Phillips for assisting thermomechanical tests

- John Setlock and Don Humphrey for assisting Thermogravimetric analysis

(TGA) and oxidation tests

- Sue Puleo and Rick Rogers for X-ray analysis

Documents

Environmental Stability and Oxidation Behavior of HfO2-Si