On the oxidation resistance of carbon-carbon composites: Importance of fiber structure for composite reactivity

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.