Measurement of the matrix/fiber interfacial strength of carbon/carbon composites

The fiber/matrix interface properties critically influence the thermal and mechanical properties of composites, and have significant impact on frictional performance of carbon based composites. The effects of heat treatment temperature (HTT) on the fiber/matrix interfacial strength of PAN-based carbon fiber reinforced carbon matrix composites (C/C) were investigated by performing a single fiber push-in test. Commercial C/C brake samples were heat treated at three different temperatures (1800°C, 2100°C and 2400°C). A nanoindenter was used and loads between 20mN and 500mN were applied for push-in tests using a flat end conical indenter. Several thicknesses of samples were tried within the range of 100μm to 1000μm to eliminate the effect of sample geometry during the push-in test. High resolution scanning electron microscopy (HRSEM) was employed to explore any cracks in the vicinity of indents. In addition, High resolution Transmission Electron Microscopy (TEM) was used to observe the microstructure development of the carbon fiber, CVI matrix and the interface as a function of heat treatment temperature. The measured average interfacial debonding stresses ranged from 10MPa to 27MPa for samples heat treated at different temperatures. When compared to as received commercial samples, interfacial strength slightly decreased after heat treatment at 1800°C and subsequently increased with increasing HTT. These changes were ascribed to development of microstructure at the C-fiber/matrix interface.