Effects of surface roughness on behaviors of lubricant molecules in ultra-thin film lubrication

A molecular dynamics simulation was carried out to study the effects of atomic scale roughness on sliding surfaces on ultrathin film lubrication. Model lubricants were hydrocarbons, e.g., n-hexane, cyclohexane, and n-hexadecane, which were confined and sheared by iron surfaces having transverse roughness. Lubricant molecules remaining in the valley region of roughness and the velocity of molecules located in the position far from the wall surface had an influence in the case when asperity was present on the stationary surface. In the case when roughness was present on the solid wall surface, the confluence and the segregation of molecular layers occurred inside a lubricating film. The movement of molecules following after a change of the molecular structure occurred in a wider range in the case when roughness was present on both the stationary surface and the moving surface than in the case when roughness was on the stationary surface only. The sliding between the wall surface having roughness and lubricant molecules occurred less intensively. Traction between two surfaces increased by the above effect induced by the presence of asperity on the solid wall surface.