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ASME Turbo Expo 2007 Power for Land, Sea and Air May 14-17, 2007, Montreal, Canada GT 2007-27685 ENVIRONMENTAL BARRIER COATINGS FOR MONOLITHIC SILICON NITRIDE – BOND COAT DEVELOPMENT Tania Bhatia, G.V. Srinivasan, Sonia V. Tulyani, Robert A. Barth, Venkat R. Vedula and William K. Tredway United Technologies research Center, East Hartford, CT, USA ABSTRACT Environmental barrier coatings (EBCs) are being developed for silicon carbide (SiC) based composites and monolithic silicon nitride (Si3N4) to protect against the accelerated oxidation and subsequent silica volatilization in high temperature high- pressure steam environments encountered in gas turbine engines. It has been found that the application of EBCs developed for SiC-based composites (EBCSiC) to monolithic silicon nitride results in a loss of room temperature mechanical strength of the monolithic substrate. In this paper, we discuss the development of a bond coat system tailored for monolithic silicon nitride that helps retain the strength of the substrate. Some of the unique requirements and challenges associated with the processing of non-line-of-sight EBCs for Si3N4 will also be discussed. Preliminary results from coating of airfoils will be presented. INTRODUCTION Silicon-based ceramics (monolithic silicon carbide and silicon nitride) and ceramic-matrix composites (CMCs) are attractive materials for use in gas turbine engine hot sections due to their high temperature mechanical and physical properties as well as lower density than metals. Potential gas turbine ceramic components include combustor liners, vanes, rotors and shrouds. One of the key material requirements for these components is that they withstand high temperatures and pressures for extremely long durations under steam-rich environments. Steam stability has been shown to be a problem in SiO2 forming ceramics (SiC and Si3N4) [1-5] due to the accelerated oxidation and recession of silicon carbide and silicon nitride owing to the formation of volatile Si(OH)4 species in high temperature exposure to steam. Environmental Barrier Coatings (EBCs) have been developed for SiC/SiC CMCs and are key to realizing the potential of Si-based materials for high temperature gas turbine applications [6]. EBCs developed for SiC/SiC CMCs have been successfully deployed for protection of CMC combustor liners in Solar Centaur 50S engines and have accumulated over 75,000 hours of engine tests at temperatures in the 1150 - 1200 °C (2100-2200F) range [6-10]. Engine tests provide valuable insights and lessons that guide the development of a newer generation of coatings [6-10]. Early attempts at adapting the EBC developed for SiC(EBCSiC) for silicon nitride have shown that coating provides adequate environmental protection to the substrate [11-13]. However, the coating results in an undesirable reduction in the strength of the ceramic substrate caused by the CTE mismatch between the coating and substrate [13]. It was also shown that the mechanical property knockdown contributed to the failure of EBCSiC coated SN 282 vanes during proof testing by Solar Turbines Inc. [13, 14]. It was clear that a new coating system needed to be developed for Si3N4 components. 144 Copyright © 2007 by ASMEPDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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