Fiber–matrix unbonding and plastic deformation in C/C composites under tribological loading

Carbon/carbon composites exhibit specific tribological behaviour during friction, i.e. the friction coefficient and wear rate decrease as temperature in the contact increases. In order to better understand the wear mechanisms involved at low and high temperatures, friction experiments were performed under air on a tribometer in disc-on-disc configuration and the worn surfaces were characterized at different length scales by OM, SEM and AFM techniques. It was seen that the high wear rate occurring at low temperature was due to severe mechanical abrasion. Worn and scratched surfaces were observed, with the presence of fiber–matrix unbonding and broken and torn-out fibers and the formation of high amounts of third body, demonstrating the fragile behaviour of the C/C composite at low temperature. On the contrary, the low wear rate obtained at high temperature was due to a soft mechanical process, since the main phenomena observed were pyrocarbon matrix exfoliation and expansion and plastic deformation of Z-fiber surfaces, modifying the stress field close to the rubbed surfaces. This involved the detachment of small particles, leading to smooth surfaces.