Fabrication and Properties of Fiber- and Particulate-Reinforced SiC Matrix Composites Obtained with (A)HPCS as the Matrix Source

A polymer precursor-based route to fiber and particulate-reinforced SiC-matrix composites is described. This route employs a proprietary allyl-substituted hydridopolycarbosilane [(A)HPCS] as a liquid phase source of near-stoichiometric SiC. The polymer can be handled in air and has physical and chemical properties appropriate to SiC-matrix formation by liquid phase filtration of the reinforcement phase, followed by thermal crosslinking and pyrolysis, or by resin transfer molding using particulate/polymer mixtures. Subsequent reinfiltration/crosslinking/pyrolysis cycles can be used to build up the density (and reduce the open porosity) of the composite producing, after 5-10 cycles, a high density ( > 90% of theoretical) structure which has little or no open porosity and relatively high flexure strengths, depending on the nature of the reinforcement employed. This process has been used to prepare continuous and chopped SiC fiber (Nicalon, Hi-Nicalon, Tyranno LOX-E) and SiC-powder/whisker reinforced SiC/SiC composites having a wide range of (controllable) porosities and final shapes/sizes. The fully-dense composites show improved uniformity, overall density and flexure strengths compared to comparable reinfroced CVI-SiC matrix composites. Complex parts can be made by near-net shape molding followed by machining after 2-3 cycles with low-cost tooling. Partial densification has been used to produce components of intermediate density and open porosity that are of potential interest as high temperature gas and/or liquid metal filtration media. Preliminary results of SiC/SiC composite joining experiments which employ this same polymer as a binder/SiC-source are also described.