TRIBOLOGICAL PROPERTIES OF METAL MATRIX GRAPHITE PARTICLE COMPOSITES

The tribological behaviour of metal matrix composites containing graphite particles is reviewed. After outlining the processes for the synthesis of these composites, present theoretical understanding pertaining to the friction and wear of composites in the presence of a thin lubricating film is described. The experimental results show that friction and wear rate in metal matrix-graphite particle composites are significantly reduced compared with those in matrix alloys, as a result of the incorporation of graphite particles. When the graphite content of metal matrix composites exceeds about 20 vol.-%, the friction coefficient approaches that of pure graphite and becomes independent of the matrix alloy. This is an indication of effective intervention of a thin film of graphite between the matrix and the counterface. Initially during sliding, the film of graphite is not present but it forms as a result of surface and subsurface deformation resulting in transfer of graphite to the tribosurface. A dynamical steady state of the film characterised by its friction and wear rate sets in. The influences of variables such as normal pressure, sliding velocity, and composition on the steady state friction and wear rate are discussed. The presence of graphite particles in the matrix of aluminium alloys increases their seizure resistance and enables them to run under boundary lubrication without galling. Copper-graphite and silver-graphite composites have found applications in electrical brushes and contact strips. Aluminium-graphite composites have been successfully tested as materials for bearings, pistons, and liners in engines and electromechanical machinery; their use resulted in improvements in performance and sometimes reduction in cost. These metal matrix composites are emerging as an important class of tribological materials and deserve a complete understanding of their friction, wear, and seizure behaviour. This article concludes by identifying various gaps in understanding where future investigations may be directed.