Oxidation and degradation of amorphous SiAlN coating via forming Si-Si bond

The oxidation and degradation of amorphous SiAlN coatings consisting of Si3N4 and AlN phases, with 750 nm or 300 nm Mo interlayers on Zr alloy in steam environment at 1000 ?C were investigated. After 1 h exposure, no detectable oxide scale formed on SiAlN/Mo (750 nm) coatings whereas SiAlN/Mo (300 nm) formed oxide scale. The downward diffusion of Si followed by relatively faster downward diffusion of N generated excessive Si and lean N, forming Si-Si bond in outermost surface of SiAlN, thereby resulting in oxidation. The degradation mechanism of amorphous nitrides was determined by elemental composition instead of reported amorphous or crystalline status.