THE SURFACE-MORPHOLOGY AND STRUCTURE OF CARBON-CARBON COMPOSITES IN HIGH-ENERGY SLIDING CONTACT

The surface morphology and microstructure of a carbon-carbon composite material in sliding contact have been investigated. The carbon-carbon composite sample is made from an organic binder-impregnation process. Chopped carbon fiber felt is impregnated with phenolic resin and pitch. A ring-on-ring specimen configuration with fiber randomly oriented in the plane of sliding is used to simulate aircraft brakes. The relative sliding speed between two composite rings decelerates from an initial speed of 23 m s-1 to a complete stop under a load of 3 100 N to simulate a high energy aircraft braking process. Two types of surface morphology can be distinguished on the sample surface: a dull-looking grey surface area with a machine-finished appearance, and a lustrous black area with a mirror-like polished appearance under room light. The sliding surface on the grey area is rough. Patches of wear debris and wear tracks on top of both the fiber and the matrix are clearly visible. Large blisters formed from the compaction of wear debris are sometimes observed on this surface. The sliding surface on the lustrous area is covered with a layer of thin debris film of the order of 1 mum thick. This film is composed of aggregates of equiaxial particles and thus exhibits no preferred crystallite orientation on the surface. The existence of two types of surface morphology is due to a difference in the local contact pressure. In the grey surface area the contact pressure is higher, which leads to a rougher surface without continuous debris film coverage. In the lustrous surface area the contact pressure is lower, which allows the maintenance of a debris film. The difference in the contact pressure is due to the non-uniform frictional heat generation which causes unequal thermal expansion of the contact surface as often observed in tribological tests involving high energy dissipation rate (J.R. Barber, Wear, 10 (1967) 155-159; J.R. Barber, Proc. R. Soc. London, Ser. A, 312 (1969) 381-394).