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 car bon cloths) were used as substrates for the preparation of low- and hi gh-temperature carbon-carbon (C-C) composites by the liquid-phase impr egnation/carbonization (LPIC) technique. A petroleum pitch was used as the matrix precursor. The carbon cloths were subjected to thermal tre atment (in inert atmosphere) or activation(in CO2) prior to the prepar ation of the composites. In this manner, their porosity and surface ar ea were varied over a very wide range. The oxidation resistance of bot h low-temperature and high-temperature composites and their individual constituents was investigated be nonisothermal thermogravimetric anal ysis. The structure of the starting and partially reacted composites w as 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 rela ted to any intrinsic structural characteristics of the constituents of a composite (e.g., high crystallinity and low reactivity of the carbo n fibers used.) In what appears to be a paradox, improved oxidation re sistance 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 fi bers, a phenomenon akin to ''stress graphitization'' is thought to tak e 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 porou s structure.