Novel Processing Techniques for Low Porosity Carbon / Carbon Composites and Their Characterization

It is well acknowledged that carbon-carbon (C/C) composites contain higher percentage of process related defects as compared to conventional polymeric composites such as graphite/epoxy, due to the numerous processing steps involved. The geometry created by the tows, variations in stacking sequence of lamina and the flow characteristics of the precursor and the formation of closed porosity are factors that lead to process-related flaws. These defects influence the end properties (mechanical and functional) adversely. The resulting composite exhibits brittle failure, reduced thermal stability and very low fracture toughness and impact resistance. In this work, a novel approach is taken to minimize the process-related evolution of defects and to improve toughness of carbon/carbon by incorporating a lamina-by-lamina curing scheme initially using a new polyimide resin, which exhibits much better thermal stability than conventional phenolic/epoxy resins. Use of polyimide results in developing an effective network of open cracks for densification of the composite, leading to significant improvement of toughness of the material. The resulting composite was characterized after each processing stage for its thermal, mechanical, nondestructive and microstructural properties. These properties have been compared with C/C composites processed through the conventional route. The modified composite showed remarkable improvement in toughness and higher thermal stability in air.