Study of the Critical Factors Controlling the Synthesis of Ceramic MatrixComposites from Preceramic Polymers

摘要

In this three year program, vinylmethylsilane co-polymers which were processableat ambient pressure were studied as precursors for silicon carbide matrix composites. Using measurements of curing characteristics, rheological properties, and pyrolysis kinetics, fabrication cycles for processing carbon fabric reinforced composites were identified. Composites exhibiting a range of useful mechanical properties were demonstrated. The primary shortcomings of a commercially available vinylmethylsilane polymer used in composite fabrication were identified as extreme sensitivity to oxygen contamination during processing and excess carbon in the pyrolyzed products. Analytical methods showed that 900 C pyrolysis products were amorphous ceramics consisting primarily of SiO-C complexes. The amorphous ceramic was unstable above 1200 C where CO and CO2 release initiated. Ceramic products converted at 1440 C showed increases in Beta-SiC and and SiO2 content at the expense of Si-O-C content. Polymer modifications were pursued to improve the converted ceramic thermal stability and stoichiometry. Novel methylvinylsilanes with reactive endgroups exhibited significantly reduced oxygen sensitivity and high ceramic convertibility.