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

Carbon-carbon composites have been made using PAN- and pitch-based car bon fibers as reinforcement, and the carbon matrix has been derived fr om 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 graphit ized counterparts. Likewise, for 100% weight loss, the temperature req uired 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 temperatu re of initiation of oxidation decreases in the order R + R > R + P > P + P. The initiation of oxidation has been related to crystallite dime nsions, porosity, and its accessibility to the oxidizing atmosphere an d microstructure of the composites. Composites having pitch as the mat rix leading to open porosity accessible to the oxidizing atmosphere sh ow anisotropy even in the carbonized stage, which results in a lamella r-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 co nverts into a columnar-type microstructure upon graphitization. The co lumnar-type microstructure is more oxidation resistant than the lamell ar-type microstructure. Therefore, composites made with resin as matri x should be more oxidation resistant than composites made with pitch a s the matrix; this was confirmed by TGA results. It is concluded that the microstructure derived from the matrix, the porosity, and its acce ssibility to the oxidizing atmosphere are the factors which control th e reactivity of the carbon-carbon com sites.