Modification of the fibre-matrix bonding in a brittle carbon-carbon composite by controlled oxidation

Unidirectional, pitch-based-matrix composites fabricated from PAN-based Courtaulds fibres were oxidised in air at temperatures of 420 and 730 degrees C to low extents of burn-off (less than 3.5%). Material loss occurred primarily at fibre-matrix interfaces at the edge regions of the composites and, in the case of the lower temperature heat-treated materials, in the areas close to internal pores and cracks. Three point flexure tests showed a change in the mode of failure for the lower temperature samples from a totally brittle mechanism to a more progressive, ''graceful'' failure mode. The materials heat treated at 730 degrees C continued to fail catastrophically after carbon burn-off. Significant increases in the mechanical properties of the samples was observed, flexural strength and ultimate failure strain increased by up to 85 and 120%, respectively for a weight loss of 1.4%. The higher temperature treated composites suffered larger weight losses and mechanical properties were adversely affected as a result.