FRICTION AND WEAR OF CARBON GRAPHITE MATERIALS AGAINST METAL AND CERAMIC COUNTERFACES

The unlubricated and lubricated sliding friction and wear behaviour of eleven commercially produced carbon-graphite (CG) materials, were evaluated as part of an effort to identify promising tribomaterials for use as piston-wrist couples in energy-efficient, low-emission automotive and truck engines. Graphite has a low density and thus the potential to reduce engine weight and enhance efficiency. The CG materials varied in composition, density and mechanical properties. All eleven grades of CG materials were tested in ball-on-flat reciprocating sliding against M-50 tool steel and NBD 100 silicon nitride spheres under unlubricated conditions. In addition, six of the grades were tested under both hot oil-lubricated conditions and abrasive dust-contaminated hot oil. The friction of all compositions was greatly reduced by lubrication, but wear was significantly increased in almost all cases. Interestingly, the introduction of abrasive mineral dust into the lubricant slightly reduced the friction coefficients and the wear rates of the materials tested. Wear of the counterface materials was generally too small to measure or was obscured by adherent transfer. Two CG compositions experienced mild-to-severe wear transitions in the unlubricated tests, but lubrication with hot oil tended to suppress those transitions.