Chemical vapor infiltration of pyrolytic carbon was studied at a temperatue of 1100 deg C and total pressures of 20,30 and 50 kPa using methane/hydrogen mixtures with molar rations 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 porousity 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 anaysis of the results, based on the Weisz modulus,the pore effetiveness factor, in-pore and surface deposition rates, showed that the overwhelming effect of hydrogen has to be ascribed to strongly lowered surface deposition rater; thus an overgrowing of the pore entrances is suppressed and a complete filling of the pores is possible.
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