Phase Composition and Microstructural Responses of Graded Mullite/YSZ Coatings Under Water Vapor Environments
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E. Garcia, J. Mesquita-Guimara˜es, P. Miranzo, M.I. Osendi, C.V. Cojocaru, Y. Wang, C. Moreau, and R.S. Lima (Submitted April 29, 2010; in revised form October 5, 2010) Mullite-based systems have been considered as environmental barrier coatings (EBCs) for high temperature protection of Si-based ceramic (Si3N4, SiC) substrates against water vapor corrosion, for application in forthcoming turbine engines. Graded mullite/Y-ZrO2 composites plasma sprayed over Hexoloy SiC substrates were analyzed as EBCs. All feedstock materials were purposely prepared and singular spraying conditions were used to assure superior crystallization. The different coated specimens were subjected to temperatures of 1300 °C for 100-500 h under water vapor environment. The effect of water corrosion on the exposed coatings was investigated by focusing on their phase and microstructure changes.
Keywords
environmental barrier coating, graded mullite/ ZrO2, layered coatings, plasma spray, thermal aging, water vapor corrosion
1. Introduction Various single layer and multilayered environmental barrier coatings (EBCs) have been intended to protect Sibased ceramic (Si3N4, SiC) components of gas turbines against severe conditions, in particular, water vapor and high temperatures (Ref 1, 2). For this purpose, mullite coatings have been tried either as a single or intermediate layer in the EBC systems (Ref 1-5) because of its low density, low thermal conductivity, stability in oxidizing environments and its thermal expansion coefficient that is close to those of Si3N4 and SiC. Plasma sprayed mullite coatings commonly develop high amount of amorphous phase, which after thermal exposure crystallizes producing extensive cracking (Ref 3, 4). Efforts have been put into reducing the presence of amorphous phases in the coatings This article is an invited paper selected from presentations at the 2010 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Thermal Spray: Global Solutions for Future Applications, Proceedings of the 2010 International Thermal Spray Conference, Singapore, May 3-5, 2010, Basil R. Marple, Arvind Agarwal, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2011. E. Garcia, J. Mesquita-Guimara˜es, P. Miranzo, and M.I. Osendi, Institute of Ceramics and Glass (ICV), CSIC, Madrid, Spain; and C.V. Cojocaru, Y. Wang, C. Moreau, and R.S. Lima, National Research Council of Canada (NRC), Boucherville, QC, Canada. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
by raising the substrate temperature during thermal spraying (Ref 4, 5), thus generally enhancing their mechanical integrity. Nevertheless, even fully crystalline mullite coatings exhibited some silica volatilization and alumina scale formation under high pressure steam atmospheres (Ref 5). Consequently, different solutions based on mullite have been tried to improve the coating lifetime in this harsh environme
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