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SummaryThe slow progress in the understanding of scuffing (scoring) and runningin of most lubricated surfaces is probably due to an inadequate understanding of the details of asperity deformation and oxide formation. The thickness and properties of oxides influence the stress states imposed on asperities as much as does the liquid lubricant, but the oxides are ignored in theories. Present theories also focus on adhesion as the cause of scuffing and they usually do not take account of the changing surface roughness during sliding. There may indeed be some evidence of adhesion in the later stages of damage but adhesion has not been demonstrated to be the initiating mechanism of scuffing. Plastic fatigue is the more likely explanation, and this can occur without atomic contact between the sliding surfaces.

The formation of transfer film and the consequent effects on the friction and wear behavior of ceramic materisIs during repeat sliding contact were studied. This was done using four structuraI ceramics, namely silicon nitride, silicon carbide, ahunina and zirconia, with a cylinder-on-flat test configuration. The transfer film consists of reattached fme wear debris particles, the film, whenever formed, is strongIy attached, enough to resist being wiped off by the slider. Cahxdations suggest that the Sne particles are attached primariIy by van der Wsals forces and to a lesser extent by electrostatic attractive forces. As a consequence, the formation of transfer ti leads to a decrease in the wear rate because of the 'protecting' role of the SIm. The presence of the ti at the contact interface also results in high friction. The presence of a liquid environment and/or surface active species reduces the particle adhesive forces and hence can inhibit the formation of a transfer f%n.

The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dial~ldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the bounda~ film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

The wear of the piston ring-cylinder wall contact area in fired engines has not been satisfactorily simulated in bench testers so far. This paper reports the development of a successful test device in which the same progression of surface change occurs as in fired engines. These changes were observed by microscopy, by hardness indentations and by the use of the stylus roughness tracer. The test device uses ring and cylinder segments and it oscillates at 350 cycles mm-' over a stroke of 19 mm. It is apparently not necessary to duplicate the temperature, atmosphere, fluid film thickness and other obvious conditions in an engine to achieve simulation. At least, the materials in the engine were not affected by the conditions in the engine that were not duplicated in the laboratory tester. The purpose in developing the tester was to study the role of the honed roughness pattern found on most cylinder walls. Several engine manufacturers attempt to achieve "plateau honing". No manufacturer polishes new cylinder walls. The reason for the honed roughness is to allow a high wear rate, without catastrophic scuffing, in locations of high stress between poorly conforming parts.

The role of the transfer film in reducing wear of polymers is discussed. It is shown that the transfer film forms more readily on roughened surfaces and that it can exist in a solid state and in a low viscosity or fluid state. Each state controls friction and wear of the polymer in a different way.