Chemical vapor infiltration of pyrocarbon -: III:: the influence of increasing methane partial pressure at increasing total pressure on infiltration rate and degree of pore filling

Chemical vapor infiltration of pyrocarbon was studied at 1100 degrees C and methane pressures ranging from 5 to 100 kPa. A cylindrically shaped porous alumina ceramic, 20 mm in height and 16 mm in diameter, was used as the substrate. The pore diameters of the porous ceramic range from 1 to 36 mu m and the total porosity amounts to 23%. Theoretical considerations based on the Weisz modulus and the pore effectiveness factor suggested an improved infiltration, particularly for the narrow pores, by increasing the methane pressure. This suggestion was confirmed by the experimental results. The degree of pore filling increases with the increase in the methane pressure. The maximum degree of pore filling was achieved at a methane pressure of 20 kPa. This result as well as the result of a preceding study concerning the effect of methane partial pressure suggest that the general idea about optimum conditions for chemical vapor infiltration has to be revised. (C) 1999 Elsevier Science Ltd. All rights reserved.