Chemistry and kinetics of chemical vapor infiltration of pyrocarbon-IV.: Investigation of methane hydrogen mixtures

Chemical vapor infiltration of pyrolytic carbon was studied at a temperature of 1100 degrees C and total pressures of 20, 30 and 50 kPa using methane/hydrogen mixtures with molar ratios of 7:1, 6:1, 5:1, and 4:1. Cylindrically shaped porous alumina ceramics, 20 mm in height and 16 mm in diameter, were used as the substrate. The pore diameters range from 1 to 36 mu m; the total porosity amounts to 23%. The infiltration rates are lowered by hydrogen, but the degrees of pore filling obtained after quasi infinitely long infiltration times are higher than those obtainable without hydrogen. A detailed analysis of the results, based on the Weisz modulus, the pore effectiveness factor, in-pore and surface deposition rates, showed that the overwhelming effect of hydrogen has to be ascribed to strongly lowered surface deposition rates; thus an overgrowing of the pore entrances is suppressed and a complete filling of the pores is possible. (C) 1999 Elsevier Science Ltd. All rights reserved.