Effect of heat treatment of carbon fibers on the mechanical properties of C/C composites

Untreated and 2 500 °C heat treated carbon fiber felts were densified by chemical vapor infiltration. The mechanical properties of the as-prepared carbon/carbon (C/C) composites were examined by three point bending test. Furthermore, texture of the pyrolytic carbon, surface morphology of carbon fibers and fracture surface morphology of the broken samples were observed by polarized microscopy and scanning electron microscopy. The results showed that after the high temperature heat treatment, the surface of the fibers became smoother and numerous grooves can be observed. The matrix of the both deposited C/C composites had an extinction angle of about 21°, indicating it was a high textured pyrolytic carbon. Compared with the untreated carbon fiber felt reinforced C/C composite, the high temperature treated carbon fiber felt reinforced C/C composite had a sharp decrease in flexural strength and modulus, and its failure exhibited a typical pseudo plastic mode. It cannot be found pyrolytic carbon wrapping the pulled-out fibers which illustrated that the heat treatment significantly weakened the fiber-matrix interface resulting in the low mechanical properties and pseudo plastic failure of the C/C composites.