Surface and Coatings Technology 177–178(2004) 97–102

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2003.06.023

Highly durable thermal barrier coatings made by the solution precursorplasma spray process

Maurice Gell *, Liangde Xie , Xinqing Ma , Eric H. Jordan , Nitin P. Padturea, a b c a

Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, U 136,a

97 North Eagleville Road, Storrs, CT 06269-3136, USAInframat Corporation, Farmington, CT 06032, USAb

Department of Mechanical Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USAc

Abstract

The solution precursor plasma spray(SPPS) process offers the prospect of depositing highly durable thermal barrier coatings(TBCs) of low thermal conductivity. In this study, a Taguchi design of experiments was employed to optimize the SPPS process.The spallation life of SPPS TBCs on a MCrAlY bond coated Ni-base superalloy substrate deposited under the optimizedprocessing conditions was demonstrated to be more than 2.5 times of that of a conventional plasma sprayed TBC with the samesubstrate and bond coat. The superior durability of SPPS TBCs is associated with their novel microstructures, which include:(i)a ceramic matrix containing micrometer and nanometer porosity,(ii) the presence of very fine splats(0.5 to 5-mm diameters),(iii ) through-thickness cracks, and(iv) improved ceramic to bond coat adhesion. The failure of SPPS TBCs occurs within theceramic top coat, near the ceramicybond coat interface. Buckling spallation is the failure mode observed for all tested samples. Itwas also demonstrated that the SPPS process is capable of depositing thick()2 mm) and durable TBCs.� 2003 Elsevier B.V. All rights reserved.

Keywords: Thermal barrier coating; Solution precursor; Plasma spray; Solution precursor plasma spray; Spallation mechanisms

1. Introduction

Thermal barrier coatings(TBCs) are widely used inaircraft engines, marine propulsion and industrial gasturbinesw1–6x. A TBC system usually consists of fourlayers: (1) a metal substrate providing structuralstrength;(2) a bond coat providing oxidation resistance;(3) a ceramic top coat providing the insulation; and(4)a thermally grown oxide(TGO) formed between theceramic top coat and the bond coat due to high temper-ature oxidation of the bond coat. Quality TBCs shouldhave: (a) low thermal conductivity to ensure a highdegree of thermal insulation and(b) good strain toler-ance to relieve the residual thermal stresses generatedduring engine operation.The air plasma spray(APS) process is widely used

for the deposition of TBCsw2,3,5x. In this process,ceramic powder, ZrO -7wt.% Y O(7YSZ), is injected2 2 3

into the high temperature, high velocity plasma jet. The

*Corresponding author. Tel.:q1-860-486-3514; fax:q1-860-486-4745.

E-mail address: mgell@mail.ims.uconn.edu(M. Gell).

powder is melted and propelled toward the substrate.Upon impact, the molten particles solidify and form‘splats’. The accumulation of splats results in the buildupof the ceramic coating. The deposited coating is highlydefective, containing porosity and microcracks, whichcontribute to the low thermal conductivity of APS TBCs.The splat interface contains considerable porosity, haslow toughness, and is the site for crack initiationw2x. Inorder to improve the thermal fatigue resistance of TBCs,Taylor w7x invented a dense vertically cracked(DVC)TBC that has a network of segmentation cracks(crack-density in the range of 20–200yinch), which improvesthe strain tolerance of the coating.Recently, a solution precursor plasma spray(SPPS)

process has been developed to deposit various ceramiccoatingsw8–14x, and the research group at Universityof Connecticut has demonstrated that SPPS offers theprospect of depositing highly durable TBCsw10x. In theSPPS process(Fig. 1), an aqueous chemical precursorfeedstock is injected into the plasma jet. The dropletsundergo a series of physical and chemical reactions priorto deposition on the substrate as 7YSZ coating. The

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Fig. 1. Schematic of the solution precursor plasma spray process.

Table 1Specification of the samples for thermal cycling test

Layer Composition(%) Process Thickness

Top coat ZrO2-7 wt.%Y203 SPPS ;250mmBond coat Co: 20, Cr: 18, Al: 12.5, APS ;100mm

Y:0.6, Si: 0.4, Hf: 0.25,Ni: Balance

Substrate Ni: 57, Cr: 22, Fe:-3, Investment 5 mmW: 14, Mn:0.5, Mo: 2, castingCo:-5

Fig. 2. SEM micrograph of a polished cross-section view of a SPPSTBC showing vertical cracks. Higher-magnification inset shows theunmelted particles, porosity and no large ‘splat’ boundaries.

SPPS TBC has a unique microstructure with verticalcracks in a porous matrix and the absence of coarsesplats.The work of this paper involves the optimization of

the SPPS process. The microstructure, bond strength anddurability of the coating deposited under the optimizedprocessing conditions are presented.

2. Experimental procedures

2.1. Coating fabrication

Coatings were deposited using a robot-operated Metco9 MB plasma torch(Sulzer Metco, Westbury, NY) froma 7YSZ solution precursorw10,13,15x. A Ni-base super-alloy, bond coated with CoCrAlY(Table 1) was usedas the substrate for the coatings deposited for micro-structural evaluation and thermal cycling test. Mild steelcylinders(bored and threaded on one end for attachmentto pull test equipment) were used as substrate for bondstrength measurement samples. The plasma power usedwas in the range of 35–45 kW, and Ar and H were2

used as primary and secondary plasma gases, respec-tively. The solution precursor atomizing gas was N .2

Taguchi design of experiments(DOE) w16x wasemployed to optimize the process. In this study, sevenprocessing variables were selected based on previousresults w17x, and three levels of each variable wereinvestigated. The selected Taguchi DOE orthogonalarray is L (3 ).13

27

2.2. Characterization

The microstructure of the SPPS TBCs was character-ized using an optical microscope(Macrophot�, Nikon,Japan) andyor a scanning electron microscope(JSM6335F, JEOL, Japan) equipped with a field emis-sion source. Bond strength of the deposited coating wasmeasured according to ASTM standard ASTM C633.

2.3. Thermal cycling test

SPPS TBCs together with conventional APS TBCs(approx. 250-mm thick, 7YSZ) on the same bond-coatedsuperalloy substrates(Table 1), which was obtainedfrom a commercial source were thermally cycled in airusing a CM� rapid temperature furnace(CM Inc.Bloomfield, NJ). The thermal cycle consisted of a 10min heat-up from room temperature to 11218C (20508F), a 40 min hold at 11218C (2050 8F) and a 10 minforced air quench. A sample was considered to havefailed when the TBC failure area reached 50% of thetotal area.

3. Results

3.1. Microstructure

A typical polished cross-section of the TBCs producedusing SPPS is shown in Fig. 2. It can be seen that thereare no coarse splat boundaries, which are always presentin the conventional APS coatings. Most of the verticalcracks are through the thickness of the coating and theaverage distance between adjacent vertical cracks wasfound to be in the range of 50–300mm, depending on

99M. Gell et al. / Surface and Coatings Technology 177 –178 (2004) 97–102

Fig. 3. (a) Top surface,(b) cross-section of the splats formed in theSPPS process showing the diameters of the splats are in the range0.5–5mm.

Fig. 4. Distribution of nanometer porosity in the SPPS TBC:(a) Pol-ished cross-section,(b) fractured cross-section.

the processing conditions. Also present in the coating isporosity and unmelted particles(inset of Fig. 2), whichare located preferentially along:(a) the inter-pass bound-aries, i.e. the boundary formed between two consecutivepassage of plasma torch across the same location and(b) the vertical crack surface.The fractured cross-section of the SPPS coating(Fig.

3b) reveals the presence of the columnar grain structure,which is an indication of the occurrence of melting andrapid solidification. The diameters of the splats, asshown in Fig. 3a(the top surface of the depositscollected from the core of the plasma torch) w15x, rangefrom 0.5 to 5 mm, whereas the splat diameters in atypical APS coating are in the range 50–100mm w18–20x. The finer splat size improves the contact betweensplats.Fig. 4a shows the distribution of both micrometer and

nanometer porosity in the coating. The micrometerporosity is located preferentially along the inter-passboundaries(inset of Fig. 2). The nanometer porosity isuniformly distributed in the matrix of the coating andin the unmelted particles(Fig. 4b).

3.2. Effect of processing variables on coating structure

Coating microstructures produced in the Taguchi DOEvary in a wide range with regard to coating density,average distance between adjacent through-thicknesscracks, and degree of delineation of inter-pass bounda-ries. The effects of the processing parameters on thesemicrostructural features were determined and the opti-mum condition for the deposition of TBCs was definedw21x. This optimum condition was used to depositcoatings for thermal cycling test and to fabricate thickTBCs.

3.3. Bond strength

ASTM 633 standard test reveals that the bond strengthbetween the SPPS TBCs is 24.2 MPa, while that forconventional APS TBCs is 19.9 MPa. Both data are theaverage of six measurements. The substrates were pre-pared at the same time and picked up randomly forSPPS or APS deposition. APS TBCs were depositedfrom Metco 7YSZ powder and using the manufacturer’ssuggested spraying parameters.

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Fig. 5. Thermal cyclic life of SPPS TBCs and conventional APS TBCs, data of DVC TBCs and EB-PVD TBCs are also included for reference.

Fig. 6. Micrographs of the failed SPPS TBCs.(a) optical micrographs of the top surface and(b) polished cross-section showing the overview ofa buckle,(c) polished cross-section showing that the failure occurs within the ceramic coat, near the ceramic coat and bond coat interface.

3.4. Thermal cycling

A total of eight SPPS TBCs together with fourcommercially obtained conventional plasma-sprayedTBCs, used as baseline were cycled to failure. The SPPSTBC consists of;250-mm thick SPPS 7YSZ top coat,;100-mm thick APS MCrAlY bond coat and a 5-mmthick Ni-base superalloy substrate(Table 1). The life-time of the SPPS TBC ranged from 935 to 1090 cycles,with an average life of 1018 cycles. Fig. 5 comparesthe thermal cyclic life of SPPS-deposited TBCs and

commercially obtained conventional plasma-sprayedTBCs, which only differ in the ceramic top coat micro-structures. The data of EB-PVD TBCs(EB-PVD 7YSZon a single-crystal Ni-base superalloy substrate with agrit blasted(Ni, Pt)Al bond coat) w22x and DVC TBCsw23x previously tested under the same conditions werealso included for comparison. Note that the thermalcyclic lives of SPPS TBC is more than 2.5 times ofthose of conventional APS TBCs and DVC TBCs, andeven 1.7 times of the thermal cyclic lives of EB-PVDTBCs.

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As observed from the morphology of failed samples,top surface(Fig. 6a) and cross-section(Fig. 6b,c), allof the samples were failed due to buckling of the topceramic coat. The buckle showing in Fig. 6 has adimension of 20-mm length, 10-mm width and 1-mmamplitude. Fig. 6c reveals that the failure of SPPS TBCsalso occurs within the top coat and close to the YSZybond coat interface, which is similar to the failurescenario of conventional APS TBCsw24,25x.

3.5. Thick TBCs

Without any modification of the process or processingparameters, TBCs as thick as 2.2 mm has been depositedsuccessfully. The ceramic is well bonded to the metalsubstrate and the structure of the coating is similar toSPPS TBCs of regular thickness,;250mm. In the 1-h1121 8C thermal cycling test, the durability of 625-mmand 1-mm thick TBCs were found to be 900 cycles and555 cycles, respectively. Thus, their durability is greaterthan that of the conventional APS TBCs of regularthickness(approx. 250-mm thick) (Fig. 5).

4. Discussion

4.1. Superior durability of SPPS TBCs

The superior durability of SPPS TBCs over that ofAPS TBCs, DVC TBCs or even that of EB-PVD TBCsis shown in Fig. 5. This superiority may be attributedto the improved strain tolerance and ceramic toughnessof the SPPS TBCs.The failure of both APS and SPPS TBCs is primarily

within the top ceramic coat, near the YSZyTGO inter-face (Fig. 6c) w24,25x. Prior to the final failure of thetop ceramic coat, the following events occur in the TBCsystemw24,26,27x. (i) The growth of thermally grownoxide, which will induce the increase of the amplitudeof the tensile stress normal to the YSZyTGO interfacewithin the ceramic top coat;(ii) the propagation oflateral cracks within the ceramic top coat with thermalcycling due to the tensile stress normal to the YSZyTGO interface;(iii ) when the length of the lateral cracksreaches a certain critical value, the coating fails bydelamination or buckling when the strain energy releaserate exceeds the toughness of YSZ coating. Therefore,the durability of TBC can be improved by reducing thenormal stress within the ceramic coat andyor increasingthe toughness of the YSZ coat.The effect of segmentation cracks on the improvement

of strain tolerance of the brittle ceramic coating havebeen well recognizedw28–31x. The SPPS TBCs depos-ited under the optimized condition have a thickness of;250mm, and a network of through-thickness verticalcracks with an average distance between adjacent cracksof ;100 mm. Results from modeling show that the

stress reduction due to periodic cracks in coatingsincreases as the spacing between cracks(L) increases,and also increases as the stiffness of the substratedecreasesw29,31x. The exact stress reduction for crackspacing and elastic modulus of the present coatingcannot be calculated becausehyL (h: the coating thick-ness) ranges approximately 1–3, which is outside thevalidity range of the formulae used inw29x. However,the results inw29x can be used forhyLs0.35. In thiscase, the stress in the center of the crack segment isreduced to 50% of its original value and is less at allother locations for our coatings. The stress reduction forour coating withhyL between 1 and 3 will be muchgreater, which will drastically diminish the driving forcefor the initiation and propagation of lateral cracks, andalso the strain energy stored in the ceramic coat.In APS TBCs, the contact area between splats is

approximately 40% of the total area of the splatw18–20x, which greatly reduces the toughness of the splatinterface. In SPPS TBCs, the contact between splats isgreatly improved due to the finer splat size(Fig. 3a,b).The improved contact between splats explains the higherbonding strength between the ceramic top coat andsubstrate, relative to that of APS coatings. As a result,coating toughness is enhanced.The improved durability of the SPPS compared to

EB-PVD TBCs in the furnace cycle tests may beattributed to a number of factors:(a) since the spacingof the through-thickness cracks is less than one-half ofthe coating thickness, the strain tolerance should bemaximizedw29x, and at least equal to that of EB-PVDcolumns;(b) the substrate and bond coat are differentfor SPPS and EB-PVD TBCs and it is known thatsubstrate and bond coat composition and bond coatprocessing can significantly influence TBC durabilityw32,33x.

4.2. Thick and durable TBCs deposited using SPPS

The thickness of the ceramic coating deposited usingthe conventional APS process is limited because thethermal stress developed during spraying will causespallation of the deposited coating during depositionw34x. The SPPS process is capable of depositing thickTBCs ()2 mm). Steffens et al.w34x reported that thethermal cyclic life of TBC decreases with the increaseof coating thickness and he attributed that to the dete-rioration of the bonding strength of the coating to thesubstrate. The decrease of thermal cyclic life with theincrease of coating thickness was also observed in SPPSTBCs, comparing 1018 cycles for 250mm, 900 and 555cycles for 625-mm and 1-mm thick coating, respectively.Note that even for the 1-mm thick coating, the thermalcyclic life is still significantly longer than that of 250-mm thick conventional APS TBCs(Fig. 5). The abilityto deposit thick TBCs using the SPPS process and the

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high durability of the deposited thick coating may beattributed to the improved strain tolerance of the coatingand the better bonding between the top coat and thesubstrate, as discussed in Section 4.1. These thickceramic coatings can provide additional insulation, espe-cially for static engine components like turbine vanesand combustors, and can be used for abradable bladeouter air seals.

5. Summary and conclusions

The solution precursor plasma spray process has beenoptimized using the Taguchi design of experiment. TBCsdeposited using the optimized conditions exhibited supe-rior durability relative to TBCs produced using the APS,DVC and EB-PVD processes. Thick and durable TBCswere also deposited using the SPPS process. The follow-ing conclusions can be made on the basis of thepresented results:

1. SPPS offers the prospect to deposit coatings with awide range of microstructures and thicknesses.

2. SPPS TBCs demonstrated superior durability to APSTBCs, DVC TBC and even EB-PVD TBCS in the 1h @ 11218C thermal cycling test. The high durabilityis attributed to the improved strain tolerance andtoughness of the coating.

3. SPPS TBCs fail by large scale buckling of the topceramic coat, and the failure occurs within the 7YSZcoating, near the 7YSZybond coat interface.

Acknowledgments

The authors thank Dr T.D. Xiao, Prof. B.M. Cetegenand Mr A. Ozturk for fruitful discussions. This worksupported by US Office of Naval Research(Grant No.N000014-02-1-0171) managed by Drs Lawrence Kaba-coff and Steven Fishman.

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