Comparison of the Failures during Cyclic Oxidation of Yttria-Stabilized (7 to 8 Weight Percent) Zirconia Thermal Barrier Coatings Fabricated via Electron Beam Physical Vapor Deposition and Air Plasma Spray
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- Yanar, N.M., Helminiak, M., Meier, G.H. et al. Metall and Mat Trans A (2011) 42: 905. doi:10.1007/s11661-010-0436-7
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The failures during oxidation of electron beam physical vapor deposition (EBPVD) and air plasma spray (APS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) on different bond coats, namely, platinum-modified aluminide and NiCoCrAlY, are described. It is shown that oxidation of the bond coats, along with defects existing near the TBC/bond coat interface, plays a very important role in TBC failures. Procedures to improve TBC performance via modifying the oxidation characteristics of the bond coats and removing the as-processed defects are discussed. The influence of exposure conditions on TBC lives is described and factors such as cycle frequency and thermal gradients are discussed.
The TBC failure modes and lives also strongly depend on the exposure conditions. Bolcavage et al. compared the behavior of several TBCs exposed in a FCT with those exposed in a “Jet Engine Thermal Simulation” (JETS) test. Bolcavage et al. concluded that the FCT test is best suited for those systems in which bond coat oxidation is intimately involved in the failure process (e.g., EBPVD systems), whereas the JETS test is best suited for those systems where fracture in the topcoat is the primary cause of failure (e.g., thick APS TBCs).
compare failures of EBPVD and APS YSZ TBCs and present mechanisms for these failures,
discuss modifications of TBCs that can provide longer lives, and
describe the influence of exposure conditions as well as bond coat type and superalloy substrate on these failures.
All experiments were performed using specimens that were circular discs about 2.5 cm (~1 in.) in diameter and 3-mm (0.12-in.) thick. The platinum aluminide bond coats were prepared by electrolytically depositing 5 to 7 μm of platinum on the superalloys and then aluminizing the platinum-coated coupon using a high-temperature, low-aluminum activity diffusion process. The MCrAlY bond coats were deposited on the superalloy substrate using an argon-shrouded plasma spray process. In order to attempt to minimize the defects in EBPVD TBCs on NiCoCrAlY bond coats, the bond coats were polished to Ra ~ 0.3 μm compared to the Ra ~3 μm for the state of the art bond coat surfaces that were initially grit blasted. Also, some bond coats were plated with ~5 μm of platinum and annealed prior to TBC deposition. The YSZ TBCs were prepared by Praxair Surface Technologies (Indianapolis, IN) using EBPVD or APS. The no bond coat TBCs were deposited at two different times in the EBPVD TBC deposition apparatus, and this is identified as first lot and second lot. In addition, some TBCs were deposited on the superalloy after 5 to 7 μm of platinum had been deposited on the superalloy surface and diffused into the alloy. Some specimens were preoxidized for 50 minutes in air at 1353 K (1080 °C). The APS TBCs were low density (85 pct) and high purity (particularly with regard to alumina and silica impurities). They were deposited at two different thicknesses (375 and 1100 μm). Some of the specimens had dense vertically cracked (DVC) inner layers. The two-layer TBC microstructure was prepared by adjusting the spraying parameters during deposition.
JETS testing was also used in testing of some of the APS TBC coated systems. In this test, disk specimens are loaded onto a large rotating ring and rotated through a series of different positions. In the first position, the specimen is rotated under an oxy-propylene flame for 20 seconds, which heats the topcoat surface to approximately 1673 K (1400 °C), while the backside of the superalloy only reaches temperatures of 1123 K to 1173 K (850 °C to 900 °C). The specimen is then rotated under a nozzle that cools the YSZ surface for another 20 seconds with compressed nitrogen. After this, the specimen is rotated to positions where it undergoes ambient air cooling for two periods of 20 seconds each, and then the heating step is repeated.
The failed TBCs were examined using optical metallography and scanning electron microscopy (SEM). These examinations were performed on the surfaces exposed by the failures as well as by preparing cross sections using conventional metallographic techniques.
3.1 TBC Failures
A substantial number of articles have addressed TBC failures, and failure mechanisms have been proposed. As discussed by Evans et al., it is useful to distinguish between extrinsic and intrinsic TBC failures. The extrinsic category includes damage induced by particle impact and delaminations arising from penetration into the TBC of deposits of calcium-magnesium-alumino-silicate (CMAS), whereas the intrinsic category involves mechanisms that arise because of strain misfit associated with the constituent materials. This article will consider only intrinsic failure mechanisms.
In discussing intrinsic TBC failures, it is necessary to distinguish between TBCs fabricated via EBPVD and APS as well as the two different bond coats, namely, platinum-modified aluminide and MCrAlY coatings. However, in the case of APS TBCs, platinum aluminide bond coats are not used.
Comparison of EBPVD YSZ TBC Failure Lives on No Bond Coat and Pt-Modified N5 to Optimized Platinum Modified Aluminide and NiCoCrAlY Bond Coats
Failure Time at 1373 K (1100 °C) (Number of 1-Hour Cycles to Failure)
N5 (first lot) grit blast and preoxidation
140, 1280, 700+
N5 (second lot) grit blast
1580, 4100, 1840+, 3660+
N5-Pt overlayer (first lot) grit blast, Pt layer + anneal, grit blast, preoxidation
N5-Pt overlayer (second lot) grit blast, Pt layer + anneal
Optimized platinum modified aluminide bond coat (average of 9 specimens; one did not fail after 3780 cycles)
Optimized NiCoCrAlY bond coat (average of 9 specimens; one did not fail after 1520 cycles)
3.2 Improved TBCs
3.3 Effect of Exposure Conditions
All of the results described previously for both state-of-the-art and improved TBCs were obtained under identical test conditions: exposure in air at 1373 K (1100 °C) in a bottom loading furnace using 1-hour cycles. The lives of TBCs show a strong temperature dependence, and in some cases, the spallation lives and failure mechanisms vary with the type of thermal exposure.
3.3.1 Effect of temperature
Effect of Exposure Temperature on the Cycles to Failure of State-of-the-Art EBPVD TBCs with NiCoCrAlY and Pt-Aluminide Bond Coats (the Number of Specimens Tested is Given in Parentheses)
Temperature, K ( °C)
3.3.2 Effect of cycle frequency
Effect of Cycle Frequency on the Cycles to Failure and Hot Time (Hours) to Failure of State-of-the-Art EBPVD TBCs with NiCoCrAlY and Pt-Aluminide Bond Coats Exposed at 1373 K (1100 °C) (the Number of Specimens Tested is Given in Parentheses)
3.3.3 Effect of thermal gradients
The failures of TBCs can also differ depending on whether the entire specimen (substrate, BC, and topcoat) is heated to a uniform temperature, as in the furnace cyclic test FCT, or exposed under a thermal gradient.
Figure 35 compares the hot time and cycles to failure of the porous APS TBCs on IN 718 substrates (i.e., those in Figure 20) in three different tests: JETS, FCT, and a “pseudo-isothermal test.” The hot time to failure in all three tests is reduced as the topcoat thickness is increased. Also, for a given topcoat thickness, the number of cycles to failure increases but the hot time to failure decreases as the cycle frequency of the test increases.
4.1 TBC Failures
The results obtained with the no bond coat TBC specimens also show that bond coat strength is an important parameter. Both the aluminide and overlay bond coats can be easily deformed at temperatures above 1273 K (1000 °C), and TBC lives on these two bond coats may be improved by making modifications to increase their high-temperature strengths.
Based on the furnace cyclic test data available so far, APS TBC systems with NiCoCrAlY coatings outperformed EBPVD TBC systems with NiCoCrAlY coatings, when both were applied to N5 substrates. The average failure times were around 900 cycles for the APS TBC coated systems, whereas it was around 100 cycles for the EBPVD TBC systems. Moreover, the APS TBCs were thicker than EBPVD TBCs (375 vs 125 μm, respectively). Some variation was present in the cyclic lives of EBPVD TBCs, whereas the cyclic life results were very reproducible in the case of APS TBCs. This might be due to the fact that EBPVD TBC systems tested so far were more prone to failure in the presence of defects than APS TBC systems. The adherence between bond coat/TGO/TBC might be stronger in the case of APS TBC systems, so that the cracks initiated in the vicinity of defects do not propagate to failure. For example, the APS TBC coated systems did not fail despite cracks that were observed as early as 25 pct of their lifetime. More work needs to be done for direct comparison of the adherence in these systems. It would also be useful if comparison is made under JETS testing conditions, since FCT is a more severe test for EBPVD TBCs than APS TBCs.
4.2 Improved TBCs
The failure of the EBPVD TBCs on both bond coats is very susceptible to defects, and TBC lives increase as defects are minimized. Bond coat oxidation is a very important factor in determining TBC lives. More transient oxidation takes place for NiCoCrAlY bond coats compared to platinum aluminide bond coats, but such transient oxidation of the NiCoCrAlY bond coats can be minimized by polishing the bond coat or by enriching the bond coat surface with platinum. It is believed that the increased life of the EBPVD TBC is due to improved adherence and the development of pure and slow growing TGO on the platinum-coated NiCoCrAlY. In the case of hand-polished specimens, the improved lives were attributed mainly to the lack of defects along the TGO/TBC interface (mainly TBC defects and transient oxides).
Since rough surfaces are conducive to bond coat rumpling because defects exist in the TBC that permit the bond coat to recede, a light grit blast was used on Pt aluminide coatings to attempt to produce a TBC system with fewer of these defects. Significant improvements in lives were observed, which are attributed to the inhibition of the ratcheting type of failure.
Based on testing of APS TBCs with different thicknesses, the trend was that thick TBCs failed after significantly shorter exposure times than thin TBCs. However, similar and relatively longer cyclic lives of thin and thick APS TBCs with DVC layers suggested that the presence of inner DVC layers provide some strain relief preventing early failure of thick TBCs in the lower portion of the TBC, where failure usually takes place. Improvement in lives despite the presence of some cracking observed along the interface between lower and upper TBC layers indicates the possibility of further improvements by better controlling the spraying parameters.
4.3 Effect of Exposure Conditions
4.3.1 Effect of temperature
The Arrhenius relationship between the failure time of EBPVD TBCs and temperature, which gives an activation energy that is close to that of the parabolic rate constant for the growth of α-alumina, suggests a significant role of TGO growth rate on the failure of these TBC systems. Therefore, the stored energy in the TGO is believed to be one of the major driving forces in the failure of these systems.
4.3.2 Effect of cycle frequency
Increasing the cycle frequency decreased the lives of EBPVD TBCs on Pt aluminide coatings, whereas it increased the lives of EBPVD TBCs with NiCoCrAlY coatings. The decreased lives of Pt aluminide coatings are due to development of voids under rapid cycling conditions. The mechanism for profuse void development for rapid cycled Pt aluminide coatings has not been completely characterized, but it is believed that the large number of cycles produces severe deformation of the bond coat due to the thermal expansion mismatch between bond coat and substrate and the martensitic transformation in the coating, both of which contribute to rumpling for slower cycling. The severe deformation presumably forms the voids by a cavitation process. In the case of NiCoCrAlY coatings, such cavitation was not observed, and it is believed that some kinds of stress relief processes are responsible for the improved lives under rapid cycling conditions.
4.3.3 Effect of thermal gradients
Exposing under a thermal gradient was observed to have an effect on the failure time as well as the failure mechanism of APS TBC systems. More testing and characterization is required for a better understanding of thermal gradient effects.
5 Concluding Remarks
The failures of EBPVD and APS TBCs were compared. Such a comparison must be done with care since the thicknesses of APS TBCs are usually much greater than those of EBPVD TBCs (>300 and ~200 μm). The failures of both TBCs very often occur at or near the TBC/TGO or TGO/bond coat interfaces due to the strong influence of oxidation of the bond coats on TBC failure. It has been shown that the oxidation of bond coats with EBPVD TBCs can be influenced by controlling the smoothness of the bond coat or adding platinum to the surface of the bond coat. Polishing of APS TBCs is not practical for APS TBCs, since a rough TBC/bond coat interface is necessary to mechanically bond the TBC to the bond coat.
The APS TBCs are more susceptible to failure out in the TBC away from the TBC/TGO interface due to the presence of splat boundaries in the TBCs.
It is also shown that exposure conditions significantly affect TBC failures, as may be expected, but the failures depend on the bond coats, which respond in different ways.
Financial support of this research by ONR (MURI Contract No. N00014-02-1-0801) and National Energy Technology Laboratory under RDS Contract No. DE-AC26-04NT41817 and TBC specimen preparation by Howmet and GE Aircraft Systems are gratefully acknowledged.