CORRELATION BETWEEN THE OXIDATION BEHAVIOR AND THE MICROSTRUCTURE OF SIC COATINGS DEPOSITED ON GRAPHITE SUBSTRATES VIA CHEMICAL-VAPOR-DEPOSITION

The dependence of high temperature oxidation behavior of beta-SIC coat ings on their microstructures were investigated. Sample coatings with various surface morphologies and microstructures were synthesized on g raphite by chemical vapor deposition, and were exposed to air or metha ne combustion exhaust gas at a pressure of 100 kPa and temperatures ra nging from 1500 to 1700 degrees C. It was found that the formation of cristobalite decreased with decreasing content of [111]-oriented grain s at the specimen surfaces both in air and methane combustion exhaust gas. Because the formation of cristobalite is known to reduce the prot ectability against oxidation of SiC coatings because of the cracking a nd peeling of the protective oxide overlayer by thermal expansion and the fast transformation of cristobalite during cooling to room tempera ture, the present result indicates that oxidation protectability incre ases with decreasing [111]-oriented grains in the SiC coatings. (C) 19 98 Elsevier Science S.A.