Overlap Model for Chemical Vapor Infiltration of Fibrous Yarns

Abstract

Attention is focused on fabrication of fiber‐reinforced ceramic composites by chemical vapor infiltration. A model is proposed for infiltration of a cylindrical fiber bundle, or yarn strand, comprised in turn of small cylindrical fibers. Along any cross section perpendicular to the yarn axis, the centers of the fibers are assumed to be randomly distributed throughout the cross‐sectional area without fiber overlap. As infiltration/densification proceeds, growth is assumed to occur via sequential deposition of uniform layers such that the fiber‐matrix composite consists of growing cylinders whose edges eventually overlap. Based on the random overlap model, expressions for key time‐dependent properties are developed for both the reaction‐limited case and for cases with significant diffusional limitations. An analytical solution to the resulting equations is obtained for the reactionlimited case whereas numerical solutions are required for the diffusion‐limited case. The effects of geometric, kinetic, and diffusional parameters on the infiltration dynamics are explored.