Abnormal behaviour during graphitisation of carbon/carbon composites made with pitch based carbon fibers

Abstract

Carbon/carbon composites have been made with coal tar pitch as matrix precursor and mesophase pitch based carbon fibers with different microstructures as reinforcements. Composites are made with and without intermediate graphitisation steps. It has been found that composites made with carbon fibers having parallel sheet-like microstructure exhibit expansion in fiber direction and increase in flexural strength during first graphitisation, whereas those made with radial and mixed microstructure exhibit no change in fiber direction but decrease in flexural strength after first graphitisation. Studies on structural parameters as evaluated from x-ray diffraction patterns of the fibers and composites have revealed that in former composites the fiber/matrix interactions lead to stretching of the fibers during graphitisation of the composites resulting in increase in length, flexural strength, and structural parameters. This phenomenon is inhibited in latter composites due to mixed microstructure of the fibers.