INFLUENCE OF MATRIX PRECURSOR ON THE OXIDATION BEHAVIOR OF CARBON-CARBON COMPOSITES

Carbon-carbon composites have been made using PAN- and pitch-based carbon fibers as reinforcement, and the carbon matrix has been derived from phenolic resin (R) and coal tar pitch (P) and a mixture of the two. The temperature of initiation of oxidation in the case of carbonized composites is about 150-200-degrees-C lower than that of their graphitized counterparts. Likewise, for 100% weight loss, the temperature required is 150-200-degrees-C higher for graphitized samples compared to carbonized samples. Comparing the different matrix systems (R + R, R + P, and P + P) and the same fiber, whether PAN or pitch, the temperature of initiation of oxidation decreases in the order R + R > R + P > P + P. The initiation of oxidation has been related to crystallite dimensions, porosity, and its accessibility to the oxidizing atmosphere and microstructure of the composites. Composites having pitch as the matrix leading to open porosity accessible to the oxidizing atmosphere show anisotropy even in the carbonized stage, which results in a lamellar-type microstructure on graphitization; whereas composites made with resin as the matrix leading to amorphous carbon having closed porosity show an isotropic microstructure at the carbonization stage, which converts into a columnar-type microstructure upon graphitization. The columnar-type microstructure is more oxidation resistant than the lamellar-type microstructure. Therefore, composites made with resin as matrix should be more oxidation resistant than composites made with pitch as the matrix; this was confirmed by TGA results. It is concluded that the microstructure derived from the matrix, the porosity, and its accessibility to the oxidizing atmosphere are the factors which control the reactivity of the carbon-carbon com sites.