A MODEL OF THE DYNAMICS OF BOUNDARY FILM FORMATION

The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperatu re. Tests are performed on a cylinder-on-disk machine using mineral oi l with various concentrations of zinc dialkyldithiophosphate (ZDP). Th e thickness and refractive index of the boundary films during step loa d test were monitored in situ with an ellipsometer, and the compositio n of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film form ation rate, while the film removal rate increases owing to (a) the dec rease of durability of the boundary film material and (b) the reductio n of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanis ms, and this balance is reflected in the boundary 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 par ticular, the OIC is effective in reducing wear of the opposing surface s by covering the OMM.