Development and environmental durability of mullite and mullite/YSZ dual layer coatings for SiC and Si3N4 ceramics

Development of refractory oxide environmental coatings for SiC and Si3N4 ceramics, interaction with simulated combustion environments, and long-term durability under thermal cycling in air are discussed. Mullite coating performance data indicate its ability to limit the molten salt corrosion. Two-layer mullite/YSZ coating showed promise as a barrier coating limiting enhanced silica volatilization by water vapor in a simulated lean-burn combustion environment. The mullite/YSZ dual layer coatings showed excellent resistance to cracking and debonding for 1000 h using 2-h thermal cycles in air at 1200-1300 degrees C. A CVD SiC bond coat developed for Si3N4 dramatically enhanced the adherence of mullite coatings. Contamination of silica scale by alkali and alkali earth metal oxides resulted in enhanced oxidation and pore formation which is an issue for the long-term environmental durability of these coatings.