Fabrication of Sic Matrix Composites by Liquid Phase Infiltration with a Polymeric Precursor

ABSTRACTA 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.

Carbonization and graphitization of polyimide Upilex

Two-step carbonization was found on a polyimide film, Upilex, similar to that observed on another polyimide, Kapton: large weight loss up to about 22 wt. % and linear shrinkage along the film surface to 30% in a narrow temperature range from 500 to 650 °C, and an additional small weight loss and shrinkage in a wide range between 700 and 1000 °C. The yield of carbonization agreed roughly with that calculated by assuming the release of non-carbon atoms as simple species of CO, O2, and N2. A 25 μm Upilex film yielded a relatively high degree of graphitization after heat treatment at 3000 °C: a layered structure under SEM, high magnetoresistance, and a sharp 002 x-ray diffraction peak at a d-spacing of 0.336 nm. The degree of graphitization was lower than for a similar Kapton film, which is attributed to the lack of flatness in the Upilex molecule. A very much lower degree of graphitization was observed for 50 μm Upilex films. This difference is tentatively attributed to constraints that must have influenced the orientation of the Upilex molecules near the surface of the films as they were formed.