Microstructural Studies of In-Situ Mesophase Transformation in the Fabrication of Carbon-Carbon Composites

Injection of a low viscosity mesophase pitch into CVD-rigidized preforms can be an effective approach for fabrication of carbon-carbon composites. Here flow-induced microstructures are stabilized by oxidation such that upon carbonization the fibrous carbon needles running through the flow channels are retained. While the injection method is effective in making highly densified materials with controlled microstructure it is not without limitations. The method is best applied to well-defined geometries such as aircraft brakes and the use of rigidized preforms is preferred to resist compaction under injection pressure. The Air Force Research Laboratory has developed a rapid, low-cost impregnation process that can be used to rigidize preforms or for further densification of C/C composites. The Air Force impregnation method is based on the mesophase pitch synthesis process advanced by Mochida and co-workers. The principle behind the impregnation strategy is that pure wetting monomer mixed with a catalyst can easily penetrate a fiber bundle or pore space in a preform by capillary forces. Once the preform is completely filled, in-situ polymerization of the monomer leads to formation of mesophase and hardening to coke. In this paper, the microstructures of mesophase formation in bulk and within preforms were examined at various stages in the temperature range critical to mesophase formation and hardening. Results of multiple cycle in-situ transformation also are compared with impregnation using fully transformed mesophase pitch and will be presented at the conference. (1 table, 3 figures, 10 refs.) Prepared in cooperation with the Department of MAE, University of California, San Diego, CA and ERC Inc., Edwards AFB, CA. The original document contains color images.