Structure and high-temperature stability of compositionally graded CVD mullite coatings

Abstract

Dense, uniform and crack-free mullite (3Al 2O 3·2SiO 2) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al 2O 3 nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and α-alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.