Characterization of Chemical Vapor-Deposited (CVD) mullite+CVD alumina+plasma-sprayed tantalum oxide coatings on silicon nitride vanes after an industrial gas turbine engine field test

Silicon nitride ceramic vanes coated with chemical vapor-deposited (CVD) mullite, CVD alumina, and plasma-sprayed tantalum oxide were exposed to field tests in an industrial gas turbine engine. Results varied due to expected non-uniformities in the CVD coating microstructures, but dense CVD mullite/alumina showed excellent stability and protective capacity after 1148 h of engine testing. Surfaces without CVD coatings experienced massive intragranular subsurface oxidation and/or rapid recession of the ceramic substrate due to volatilization of silica species formed by oxidation. These results suggest that thin (<5 μm), dense, high-purity CVD mullite and CVD alumina are viable components for an environmental barrier coating system to protect structural ceramics in combustion environments.