Densification of Porous Materials by Chemical Vapor Infiltration

A mathemat ica l mode l that describes the combined diffusion and reaction of trichloromethylsilane in a porous pre-fo rm under typical chemical vapor infiltration (CVI) conditions is presented. The mode l utilizes a single pore to demon-strate the importance of the pore geometry, totalgaseous pressure, temperature, and imposed temperature gradient on the degree of achieved densification. By focusing attention on a single pore, it is possible to account for changes in the pore shape that occur during deposition of silicon carbide on the pore boundaries. Process conditions for operation in the reac-tion-controlled reg ime in order to attain complete densification are quantitatively identified even for the longer pores and the relatively higher temperatures used. Compos i te materials are becoming increasingly impor-tant in the aerospace, automobile, nuclear, solar energy, and manufactur ing industries. Carbon fibers imbedded in a carbon matr ix exhibit significant advantages, including low density to strength ratio, and excellent behavior to-wards friction and ablation at even high temperatures. Moreover, the use of multidirectional reinforcement lead