Three commercial carbons (an activated cloth and two rayon-derived carbon cloths) were used as substrates for the preparation of low- and high-temperature carbon-carbon (CC) composites by the liquid-phase impregnation/carbonization (LPIC) technique. A petroleum pitch was used as the matrix precursor. The carbon cloths were subjected to thermal treatment (in inert atmosphere) or activation (in CO2) prior to the preparation of the composites. In this manner, their porosity and surface area were varied over a very wide range. The oxidation resistance of both low-temperature and high-temperature composites and their individual constituents was investigated be nonisothermal thermogravimetric analysis. The structure of the starting and partially reacted composites was investigated by X-ray diffraction and scanning electron microscopy. The synergistic oxidation resistance effects reported previously were observed again for the high-temperature composites. They are not related to any intrinsic structural characteristics of the constituents of a composite (e.g., high crystallinity and low reactivity of the carbon fibers used.) In what appears to be a paradox, improved oxidation resistance is invariably obtained if porosity is developed in the fibers prior to composite preparation; when subsequent carbonization of the matrix takes place in the constrained space within the pores of the fibers, a phenomenon akin to "stress graphitization" is thought to take place. The resulting carbon (at the fiber-matrix interface) is thus more oxidation-resistant than that obtained when matrix carbonization occurs in the presence of fibers that do not possess a developed porous structure.
All Science Journal Classification (ASJC) codes
- Materials Science(all)