The synthesis and properties of two polycarbosilanes that have essentially a “SiH2CH2” composition is described. One of thesepolymers is a highly branched hydridopolycarbosilane (HPCS) derived fromGrignard coupling of CI3SiCH2CI followed by LiAIH4 reduction. This synthesis is amenable to large scaleproduction and we are exploring applications of HPCS as a source of SiCcoatings and its allyl-derivative, AHPCS, as a matrix source for SiC- andC-fiber-reinforced composites. These polymers thermoset on heating at200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiCwith low O content in ca. 80% yield on pyrolysis to 1000 °C. The secondmethod involves ring-opening polymerization of1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight,linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analogof polyethylene. In contrast to high density polyethylene which melts at 135°C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. Onpyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC invirtually quantitative yield. The polymer-to-ceramic conversion was examinedfor PSE by using TGA, mass spec, solid state NMR, and IR methods yieldinginformation regarding the cross-linking and structural evolution processes.The results of these studies of the polymer-to-ceramic conversion processand our efforts to employ the AHPCS polymer as a source of SiC matrices aredescribed.

A process for the fabrication of SiC matrix composites has been developed which employs aliquid, highly branched, polycarbosilane (AHPCS). This polymer thermosets on heating at 200-400 °C (or at 100 °C with catalyst) and yields an amorphous SiC with low excess C and O content in 60-80% yield on pyrolysis to 1000 °C. Preforms consisting of C-coated Nicalon SiC fiber cloth, unidirectional Textron SiC SCS-6 fiber layups, or Mo boats packed with SiC whiskers, were infiltrated with the polymer, cured in an autoclave, and pyrolyzed to 1000 °C. Five to eight infiltration cycles gave net shape composites with final densities at 85-94 % of theoretical. The results of 4-point flexure tests on the as-prepared Nicalon composites indicate flexure strengths (aver. 378 MPa) that are comparable to or better than similarly reinforced CVI-SiC matrix composites. The whisker and SCS-6 composites showed a small weight loss (10-20%) on heating to 1500 °C in Ar and little or no weight change or obvious embrittlement in air at 1000 °C.