Microstructure of as-coated thermal barrier coatings with varying lifetimes

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1 Surface and Coatings Technology (2004) Microstructure of as-coated thermal barrier coatings with varying lifetimes a a b b, A.J. Burns, R. Subramanian, B.W. Kempshall, Y.H. Sohn * a Siemens-Westinghouse Power Corporation, A Siemens Company, Orlando, FL , USA b Advanced Materials Processing and Analysis Center, University of Central Florida, Aerospace Engineering, Orlando, FL , USA Abstract The microstructures of the thermally grown oxide (TGO) in as-coated electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) were examined by scanning transmission electron microscopy (STEM) as a function of demonstrated TBC lifetime in a thermally cycled environment. As-coated TBC specimens were selected for analysis based on individual statistical significance in overall lifetime variation (i.e. early failures to long-lasting). Weibull probability distribution developed through cyclic furnace tests (CFT) served as the medium to determine statistical significance. For selected TBC specimens that represent the variation in lifetime, site-specific specimen preparation for STEM was carried out by focused ion beam (FIB) equipped with in-situ lift-out (INLO) technique. Differences in microstructure of TGO particularly in the mixedoxide and continuous-oxide zone were examined and correlated to the lifetime characteristics of the TBCs. The TGO for durable TBC consisted of fine grained, uniformly thick, and dense mixed-oxide zone in addition to a uniform and smooth microstructural transition into the continuous oxide zone Elsevier B.V. All rights reserved. Keywords: Microstructure; Thermally grown oxide (TGO); Thermal barrier coatings (TBC); Scanning transmission electron microscopy; Physical vapor deposition 1. Introduction Thermal barrier coatings (TBCs) are widely used for thermal protection of hot section components in turbines for power generation w1 5x. Reliable and durable TBCs contribute to improved performance and efficiency of advanced gas turbines. TBC systems, in general, consist of a thermally insulating outer ceramic (ZrO - 7 to 8 2 wt.%y O ; YSZ) coating, a thermally grown oxide 2 3 (TGO) scale, an oxidation resistant metallic bond coat, and a superalloy substrate. Commonly, YSZ coatings are deposited either by air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD). Bond coats are often deposited by low-pressureyvacuum plasma spray in the case of MCrAlY (MsNi andyor Co). Several factors and phenomena can influence the failure of TBCs; in general, failure is defined as spallation of the YSZ coating. Oxidation of the bond coat, *Corresponding author. Tel.: q ; fax: q address: ysohn@mail.ucf.edu (Y.H. Sohn). sintering of the TBC and phase instability within the TBC are the most recognized factors contributing to TBC spallation w1 10x. In particular, for EB-PVD YSZ TBC systems, oxidation of bondcoat and growth of TGO has been identified as one of the main modes of system degradation. When considering bond coat oxidation and TGO growth, additional considerations must be given to statistical variation of TBC lifetimes dependant upon processing parameters. Thus, factors influencing oxidation-related TBC failure and statistical variation can include, among many, surface finish of the bond coat, deposition conditions within the PVD chamber, resultant as-received microstructure, transient and steady state oxide growth kinetics, microstructure and composition of the bond coats, duration of thermal cycling, development of residual stresses, loss of aluminum reservoir increasing the probability of breakaway oxidation and damage to the passive alumina scale (TGO) leading to rapid oxygen ingress. This investigation focuses on the characterization of as-coated microstructures of EB-PVD TBCs with an emphasis on TGO morphology. Microstructure analysis /04/$- see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.surfcoat

90 A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96 Fig. 1. A schematic illustration of specimen geometry for TBCs examined in this study.

2 90 A.J. Burns et al. / Surface and Coatings Technology (2004) Fig. 1. A schematic illustration of specimen geometry for TBCs examined in this study. is then compared to TBC lifetime in an oxidizing thermal cyclic environment. The thermal cyclic test was designed to induce oxidation-related failure in a TBC system based on degradation of the bond coatyysz interface through growth of the TGO. All specimens were treated similarly during testing to minimize any externally induced variability. Detailed microstructure analysis was carried out using scanning transmission electron microscopy (STEM) via specimen preparation by the focused ion beam (FIB) in-situ lift-out (INLO) technique w11x. 2. Experimental procedure 2.1. Sample procurement, preparation and testing IN-738 (Ni 16Cr 8.5Co 3.5Al 3.5Ti 2.5 W 1.75Mo 1.6Ta 7Nb 11C) superalloy pins with diameter of 8 mm were bond coated with CoNiCrAlY (Co Fig. 2. A schematic illustration showing temperature-time plot of cycling furnace test (CFT) employed in this study to evaluate TBC lifetime. 32Ni 21Cr 8Al 7Y) using a low-pressure plasma spray (LPPS) method. The bond coat was then surface finished with a proprietary production procedure to impart similar surface finish on the bond-coated pins as achieved on production components. The bond-coated pins were then EB-PVD coated with ZrO wt.%y O (YSZ) in an effort to mimick several normal 2 3 operating conditions within an EB-PVD coater. Normal Fig. 3. Secondary electron focused ion beam (FIB) images showing sequence of the FIB in-situ lift-out (INLO) TEM specimen preparation process (a d) for a representative TBC specimen.

A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96 91 Fig. 4. A schematic illustration of TBC failure distribution using Weibull statistics.

After procurement from the coating vendor; the specimens were cut into four furnace segments, one asreceived (AR) segment and a bullet end (BE) according to the schematic shown in Fig. 1.

3 A.J. Burns et al. / Surface and Coatings Technology (2004) Fig. 4. A schematic illustration of TBC failure distribution using Weibull statistics. operating conditions indicate the state of the coating process during different times in coating campaigns. After procurement from the coating vendor; the specimens were cut into four furnace segments, one asreceived (AR) segment and a bullet end (BE) according to the schematic shown in Fig. 1. The furnace segment specimens were subjected to high temperature 24-h cyclic oxidation under atmospheric conditions until the TBC failed. TBC failure was defined as a 10%- spallation in area or propagation of a visible crack Fig. 5. Typical microstructure of TBCs in the as-received state: (a) Bright field (BF) image and (b) High angle annular dark field (HAADF) image in the vicinity of the TGO. Fig. 6. Typical microstructure of TBC specimen I in the as-received state in the vicinity of the TGO: (a) Bright field (BF) image with convergent beam electron diffraction (CBED inset) demonstrating the presence of a-al O in the TGO and (b) HAADF image. 2 3

92 A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96 within the TBC for a length of greater than 10% the length of the specimen.

2.2. TEM and STEM preparation and analysis procedure The cross-sectional samples were embedded in transparent cold-setting epoxy and ground using 600-grit SiC paper followed by polishing with diamond

4 92 A.J. Burns et al. / Surface and Coatings Technology (2004) within the TBC for a length of greater than 10% the length of the specimen. A representation of the thermal cycle employed in this investigation is schematically presented in Fig TEM and STEM preparation and analysis procedure The cross-sectional samples were embedded in transparent cold-setting epoxy and ground using 600-grit SiC paper followed by polishing with diamond paste down to 1y4 mm. The polished surface of the specimens was then sputter coated with Au Pd to create adequate conduction during FIB imaging and milling operations. A transmission electron microscopy (TEM) specimen was prepared from the cross-section of each TBC system utilizing the focused ion beam (FIB) in-situ lift-out (INLO) technique w11x. An FEI FIB200 FIB workstation with an Omniprobe in-situ tungsten (W) lift-out probe was employed to prepare the TEM lift-out specimens. With the FIB technique, a wedge shaped sample is cut free from the bulk material by means of FIB milling. The sample is then attached to the in-situ W probe using ion beam assisted Pt chemical vapor deposition (CVD) as shown in Fig. 3a. Next the ion beam assisted Pt chemical vapor deposition is used to attach the sample to a 2=1 mm Cu TEM grid that had been previously cut to accommodate access to the grid with the in-situ W probe as shown in Fig. 3b. Once it is secured to the TEM grid, the sample is milled free from the in-situ W probe and then subsequently FIB milled to electron transparency (approx. 100 nm) as shown in Fig. 3c,d. Fig. 8. Typical microstructure of TBC specimen II in the as-received state in the vicinity of the TGO: (a) BF image and (b) HAADF image. The microstructure of the TBC coating systems was analyzed using TEM and scanning TEM (STEM) techniques. A PhilipsyFEI Tecnai F30 300KeV TEM, equipped with high angle annular dark field (HAADF) and EDAX X-ray energy dispersive spectroscopy (XEDS), was employed in this investigation to examine the microstructure of the bond coat, TGO, and YSZ in as-coated TBCs. 3. Results and discussion 3.1. Lifetime characteristics of thermal barrier coatings Fig. 7. High magnification HAADF image from TBC specimen I showing the large grain structure, large scale porosity in a relatively thin mixed-oxide zone and vertically oriented pores beginning at the edge of the continuous zone (a-al2o 3) of the TGO. Note that pores appear to be centered above the grain boundaries of the continuous zone. Results from the CFT-induced TBC failures allowed a mathematical representation of TBC failure probability to be developed utilizing Weibull statistical analysis w12x. A schematic representation of the CFT-induced failure distribution is presented in Fig. 4. Utilization of the Weibull probability distribution allows development of a methodology to separate modes of coating failure by a variety of means including microstructure analysis, statistical mathematical analysis, or quantitative mechanistic analysis.

A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96

5 A.J. Burns et al. / Surface and Coatings Technology (2004) Fig. 9. (a) Higher magnification HAADF images from TBC specimen II with (b) elemental line scan analysis (Black lines in the photomicrographs indicate the location on respective distance axis below in line scan) Microstructure analysis Four EB-PVD TBC coated specimens were examined in this investigation, they were individually picked to represent specimens from different failure times; early failures (specimens I and II), mid-term failures (specimen III) and long-lasting specimens (IV). Macroscopically, all four EB-PVD TBC specimens exhibited good ceramic adhesion in the as-received state. Microstructure of the TBC specimens in the asreceived condition was examined utilizing both bright field (BF) and HAADF images in addition to XEDS. A typical TBC microstructure analyzed in this fashion is shown in Fig. 5. The contrast in the HAADF micrographs is analogous to the Z contrast observed in backscatter electron (BSE) images in scanning electron microscopy (SEM) where a higher average Z yields higher intensity in the photomicrograph. Specimen I represents a TBC that exhibits high probability of early failure; representative microstructures in BF and HAADF imaging is shown in Fig. 6. TBC specimen I has a TGO consisting of continuous a-al2o3 approximately 200 to 400 nm in thickness. Between the columnar YSZ and this continuous a- Al2O3 layer is a zone of mixed oxide particles that contain Zirconium and Yttrium dispersed in an Al 2 O 3 matrix. Specimen I exhibited vertically aligned porosity which appeared to initiate at the continuous zone Al2O3 grain boundaries and extend upward through the mixed-oxide zone to the nucleation point of the columnar YSZ. The particle size within the mixed zone is relatively large on the order of 200 nm as presented in Fig. 7. Large-scale porosity was found around the particles in the mixed zone, distinct from the vertically oriented porosity. There is a very abrupt and porous transition at the interface between the columnar (vertically oriented) YSZ and the equiaxed mixed oxide zone in the TGO. TBC specimen II was another specimen showing high probability of early failure; representative microstructures in BF and HAADF are shown in Fig. 8. Higher resolution images of specimen II are shown in Fig. 9. The mixed oxide zone is non-uniform in thickness varying from negligible (approx. 5 nm) to over 400 nm in thickness; it is also filled with large pores distributed throughout but concentrated predominantly at the interface between the mixed oxide and continuous zones within the TGO. XEDS line scans across the TGO

94 A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96 mixed-oxide region; presence of this porosity was not segregated to any specific area.

Specimen IV represents the TBC system with the longest lifetime during CFT; its representative microstructure is presented in Fig. 12 and the higher magnification images are presented in Fig. 13.

6 94 A.J. Burns et al. / Surface and Coatings Technology (2004) mixed-oxide region; presence of this porosity was not segregated to any specific area. Transition regions between the mixed-oxide zone to the columnar YSZ coating is predominantly uniform with only certain areas showing an abrupt (non-uniform) transition. Specimen IV represents the TBC system with the longest lifetime during CFT; its representative microstructure is presented in Fig. 12 and the higher magnification images are presented in Fig. 13. The mixed-oxide zone exhibits particle sizes on the order of 10 to 100 nm with no measurable porosity (near 100% density). Transition of the mixed zone to the columnar YSZ microstructure is smooth and uniform resulting in a consistent transition interface from the equiaxed mixed structure to the vertically oriented grains in the TBC. Fig. 10. Typical microstructure of TBC specimen III in the as-received state in the vicinity of the TGO: (a) BF image and (b) HAADF image. thickness in multiple locations shows the proliferation of large-scale porosity and non-(zr,y,al) based oxide particles containing Ni and Co. Particle sizes within the mixed oxide zone range from approximately 75 to 300 nm. Most transition areas of the mixed-oxide zone to the columnar YSZ are abrupt (non-uniform), however, certain locations show a more uniform transition. TBC specimen III exhibited a higher probability of lasting longer than specimens I and II; representative and high magnification microstructures are presented in Figs. 10 and 11, respectively. The mixed-oxide zone has relatively fine scale structure with particles sizes in the range of 75 to 200 nm, and ranges from 200 to 400 nm in thickness. Small-scale porosity was observed in the Fig. 11. High Magnification HAADF image from TBC specimen III with elemental line scan analysis (Black lines in the photomicrographs indicate the location on respective distance axis below in line scan).

A.J. Burns et al. / Surface and Coatings Technology 177 178 (2004) 89 96 95 Fig. 12.

7 A.J. Burns et al. / Surface and Coatings Technology (2004) Fig. 12. Typical microstructure of TBC specimen IV in the as-received state in the vicinity of the TGO: (a) BF image and (b) HAADF image. Fig. 13. High magnification HAADF images from TBC specimen IV showing dense and fine grained mixed-oxide zone: (a) Mixed-oxide zone thickness ranges from 75 to 200 nm, (b) Microstructure and elemental map of mixed-oxide zone at higher magnification.

8 96 A.J. Burns et al. / Surface and Coatings Technology (2004) Table 1 Specimen description and the microstructure characteristics of thermally grown oxide in specimens of demonstrated variable TBC lifetime Specimen ID I II III IV Lifetime characteristics Earliest Early Intermediate Long Phase constituent of bond coat g bqg bqg bqg in contact with TGO Average TGO thickness (nm) Continuous TGO zone thickness (nm) Mixed TGO zone thickness (nm) 50 to Mixed TGO zone microstructure Large grains and particles Small (fine) grains and particles Notable microstructure in TGO Small tear shaped Large voids Small clustered Small interspersed voids, some larger voids voids voids Approximate bondcoat grain size (mm) XEDS elemental line scans and area maps confirm the uniform transition. The roughness of the metalyceramic interface is a local artifact associated with specimen and no correlation was observed between the bond coat surface roughness and TBC lifetime in this study. The emphasis of the microstructural analysis carried out in this study includes thickness, grain size and porosity of the overall TGO distinguished by the continuous zone and the mixed-oxide zone. The microstructural characteristics of continuity and transition for these regions into each other, and into the columnar YSZ were also a consideration. A summary of the microstructure characteristics of the TGO observed from the specimens analyzed is presented in Table 1. The relative life of the TBC specimens and the corresponding microstructure indicate that a fine-grained mixed-oxide zone is preferred. The representative specimen with the longest TBC lifetime (specimen IV) had a fine particle microstructure (10 to 100 nm) that uniformly transitioned from the equiaxed grains to the vertically oriented grains of YSZ. In general, the probability of longer survival for EB-PVD TBCs was observed to increase with: 1. A decrease in the grain size of mixed-oxide zone. 2. Uniformity in thickness of the mixed-oxide zone. 3. An increase in the density of the mixed-oxide zone. 4. Uniform and smooth microstructure transition without porosity from mixed-oxide zone to the continuous zone. 5. Uniform and smooth microstructure transition from equiaxed mixed-oxide zone to the beginning of the vertically oriented zirconia grains of the EB-PVD YSZ structure. 4. Summary Microstructure of thermally grown oxide (TGO) in as-coated electron beam physical vapor deposited (EB- PVD) thermal barrier coatings (TBCs) with CoNiCrAlY bondcoat was examined as a function of cyclic furnacetest demonstrated TBC lifetime. As-coated TBC specimens were selected for analysis based on individual statistical significance in overall lifetime variation. Weibull probability distribution developed through cyclic furnace tests served as the tool to determine statistical significance and failure probability. For selected TBC specimens that represent the variation in lifetime, sitespecific specimen preparation was carried out by focused ion beam equipped with in-situ lift-out technique for scanning transmission electron microscopy. Microstructure of durable TBC exhibited fine grained, uniformly thick, and dense mixed-oxide zone in the TGO in addition to a uniform and smooth microstructural transition to continuous oxide zone. References w1x R.A. Miller, Surf. Coat. Technol. 30 (1987) 1. w2x R. Subramanian, S.M. Sabol, J. Goedjen, M. Arana, Thermal barrier coatings for the ATS engine, Proceedings of ATS Annual Review Meeting, Pittsburgh, PA, w3x A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2000) 505. w4x N. Padture, E.H. Jordan, M. Gell, Science 296 (2002) 280. w5x P.K. Wright, Curr. Opin. Solid State Mater. Sci. 4 (1999) 255. w6x M. Gell, K. Vaidyanathan, B. Barber, J. Cheng, E. Jordan, Metall. Mater. Trans. A 30A (1999) 427. w7x V.K. Tolpygo, D.R. Clarke, in: N.B. Dahotre, J.M. Hampikian, J.E. Morral (Eds.), Elevated Temperature Coatings: Science and Technology IV, TMS, Warrendale, PA, 2001, p. 93. w8x A.G. Evan, M.Y. He, J.W. Hutchinson, Acta Metall. 45 (1997) w9x K.S. Murphy, K.L. More, M.J. Lance, Surf. Coat. Technol (2001) 152. w10x Y.H. Sohn, J.H. Kim, E.H. Jordan, M. Gell, Surf. Coat. Technol (2001) 70. w11x L.R. Herlinger, S. Chevacharoenkul, D.C. Erwin, Proceedings of 22nd International Symposium for Testing and Failure Analysis 96, ASM International, Materials Park, OH, 1996, pp w12x M. Barsoum, Fundamentals of Ceramics, 1st ed, McGraw-Hill, New York, 1997, pp

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