Effect of Thermal Property Variation on Surface Grooving

Carbon-carbon composites are used as friction materials because of their exceptional mechanical and thermal properties. In particular, carbon fibers offer high specific stiffness and high thermal diffusivity along the fiber axis. However, it was observed that after being subjected to the braking process, especially after rapid deceleration, the friction surfaces across the rubbing tracks become very wavy. These grooves alter the contact characteristics at the sliding interface and may reduce the friction performance. During the rubbing process, a sliding surface is damaged. In this paper, the effect of thermal property variations on the friction surface is presented. Multiple carbon samples were fabricated with different crystallinity distributions and orientations. All experiments were conducted using a Friction Assessment Screening Tester (FAST) under controlled atmospheric conditions. An experiment using a laser beam to provide the temperature variations across the surface was also performed. The results show that the formation of macro-grooves on the friction surface is a function of thermal or/and mechanical property variations, crystallinity distribution and crystallite orientation with a respect to the rubbing direction. However, if the distribution of crystallites is non-uniform, micro-grooves are always observed.