GRAIN-ORIENTATIONAL DEPENDENCE OF CRISTOBALITE FORMATION IN POLYCRYSTALLINE BETA-SIC FILMS ON THERMAL-OXIDATION

We studied the dependence of cristobalite formation on grain orientati on of polycrystalline beta-SiC films upon thermal oxidation. Sample fi lms with various surface morphologies and microstructures were synthes ized on graphite by chemical vapor deposition (CVD), and were oxidized in methane combustion exhaust fas or in air at a pressure of 100 kPa and a temperature of 1773 or 1873 K, SiO2 overlayers formed on the SiC films consisted of amorphous silica and cristobalite; The amount of c ristobalite formed was found to increase with increasing. content of [ 111]-oriented grains on the film surfaces both in methane combustion e xhaust as and in air. We discuss the possibility of epitaxial cristoba lite crystallite formation on the {111} surfaces of B-sic grains and t he reasons why [111]-oriented SiC grains are more likely to form crist obalite despite the pebbled surface of the SiC films, which is commonl y seen in CVD-SiC films.