CVD mullite coatings in high-temperature, high-pressure air-H2O

Crystalline mullite was deposited by chemical vapor deposition (CVD) onto S iC/SiC composites overlaid with CVD SiC, Specimens were exposed to isotherm al oxidation tests in high-pressure air + H2O at 1200 degrees C, Unprotecte d CVD SiC formed silica scales with a dense amorphous inner layer and a thi ck, porous, outer layer of cristobalite, Thin coatings (similar to 2 mu m) of dense CVD mullite effectively suppressed the rapid oxidation of CVD SiC. No microstructural evidence of mullite, volatility was observed under thes e temperature, pressure, and low-flow-rate conditions. Results of this prel iminary study indicate that dense, crystalline, high-purity CVD mullite is stable and protective in low-velocity, high-pressure, moisture-containing e nvironments.