(Invited) Developing Environmental Barrier Coatings Resistant to Molten Calcium-Magnesium-Aluminosilicate (CMAS)

Abstract

In order to improve fuel efficiency in commercial aircraft engines, ceramic matrix composites (CMC) are considered a leading material system to replace metal-based turbine engine components, due to their lower density and high-temperature capabilities compared with traditional metallic structural materials. However, silicon-based CMCs are susceptible to oxidation and corrosion in the harsh combustion environment found in air-breathing turbine engines. Environmental barrier coatings (EBCs) are being developed and applied to protect and to enhance durability and service life of CMC components for application in the hot-section of engines. The development of robust EBCs is threatened by sand, volcanic ash and other particulate debris, which are routinely ingested by aircraft engines in varying geographic regions. At temperatures >1200°C, these particulates melt, yielding molten calcium-magnesium-aluminosilicate (CMAS) deposits. These deposits can significantly reduce the durability of present and future engine component materials, particularly EBCs designed to protect silicon-based ceramic matrix composites (CMC). Near target operating temperatures (~1500°C) of future CMC-based aircraft engines, molten CMAS behaves like a viscous melt that can infiltrate and chemically interact with protective coatings, causing premature failure of the EBC system and ultimately the overall CMC engine component. Degradation of candidate EBC materials based on rare-earth silicates by molten CMAS will be presented with a focus on recent work, as well as methods of evaluating the complex high-temperature materials interactions, accomplished at NASA Glenn Research Center.