Surface Effects on Boundary Friction with Additive-Free Lubricating Films: Coupled Influence of Roughness and Material Properties

This work deals with the friction mechanisms of non-conforming contacts lubricated with additive-free poly-alpha olefins in the boundary regime at mean pressures between 0.3 and 0.9 GPa. To analyse the coupled influence of surface roughness and material, the surfaces involved in these experiments were made of steel or DLC-coated steel with a roughness from the nanometer to the micrometer. On the one hand, DLC–DLC contacts presented the lowest friction level with hardly any influence of the surface roughness on the boundary friction coefficient: we showed that the high hardness of these surfaces prevented from significant wear and that boundary friction arose from the shearing of a nanometric fluid layer, physisorbed on the surfaces. On the other hand, steel–steel and steel–DLC contacts presented roughness-dependent higher friction coefficients. Two trends arose according to the contact composite roughness. For rough ones, friction was controlled by the plastic deformation of micrometric asperities. For smooth ones, the dulled asperities did not allow significant local pressure rises, but they occupied a large fraction of the contact area. The boundary friction level was then controlled by the area left available for the lubricant between these conforming asperities and valleys.