Effect of deposition parameters on the strength of CVD. beta. -SiC coatings

Abstract

Silicon carbide coatings have been deposited on graphite specimens by thermal decomposition of dichlorodimethylsilane. The effects of flow variables, substrate temperature and gas composition, on the tensile strength of chemically vapor deposited silicon carbide coatings are discussed. The ratio H{sub 2}:(CH{sub 3}){sub 2}SiCl{sub 2} was varied from 4:1 to 6:1 at substrate temperatures of 1,100{degree}, 1,150{degree}, 1,200{degree}, and 1,250{degree}C. The strength of VCD SiC coatings was strongly dependent on the substrate temperature and the gas composition. Highest strength values were obtained at low temperatures (1,100{degree}) and low concentration ratios (4:1) of carrier/precursor gas. At 1,150{degree} and 1,200{degree}C, the curves tend to exhibit minimum strength at a specific ratio of carrier to precursor gas. The effects of substrate temperature and gas composition on the stoichiometry and structure of the SiC and its relation with the strength of the coatings are also discussed.