The effect of transfer layers on the surface contact and wear of carbon-graphite materials

Solid carbon-graphite materials are widely used in a variety of arduous tribological applications in which their ability to run either unlubricated or in the presence of fluids with unpropitious tribological properties can be exploited. The usual arrangement in such devices as seals and bearings is to run a carbon component against a much harder metallic or ceramic counterface of appropriately low surface roughness. In practice, it is often observed in these circumstances that, although the initial wear rate of the relatively soft carbon is high, a satisfactory long-term performance can be achieved if, and only if, a stable layer of carbonaceous material is generated both on the trace of the carbon and as a transfer layer on the harder counterface: in the absence of this film the high initial wear rate continues unabated. The range of carbon materials available commercially is large, but can be classified by the extent to which they have been graphitized—those with a higher graphitic content have a lower elastic modulus and generally higher wear rate than less graphitic specimens. In this paper, we consider the development of the contact conditions with component life and how these can be associated with the alleviation of the initial severity through the development of a cushioning layer of consolidated wear debris and thus offer an explanation of the experimental observations in terms of a contact mechanics model.