Molecular dynamics simulation of the Stribeck curve: Boundary lubrication, mixed lubrication, and hydrodynamic lubrication on the atomistic level

Lubricated contact processes are studied using classical molecular dynamics simulations for determining the entire range of the Stribeck curve. Therefore, the lateral movement of two solid bodies at different gap height are studied. In each simulation, a rigid asperity is moved at constant height above a flat iron surface in a lubricating fluid. Both methane and decane are considered as lubricants. The three main lubrication regimes of the Stribeck curve and their transition regions are covered by the study: Boundary lubrication (significant elastic and plastic deformation of the substrate), mixed lubrication (adsorbed fluid layer dominates the process), and hydrodynamic lubrication (shear flow is set up between the surface and the asperity). We find the formation of a tribofilm in which lubricant molecules are immersed into the metal surface-not only in the case of scratching, but also for boundary lubrication and mixed lubrication. The formation of a tribofilm is found to have important consequences for the contact process. Moreover, the two fluids are found to show distinctly different behavior in the three lubrication regimes: For hydrodynamic lubrication (large gap height), decane yields a better tribological performance; for boundary lubrication (small gap height), decane shows a larger friction coefficient than methane, which is due to the different mechanisms observed for the formation of the tribofilm; the mixed lubrication regime can be considered as a transition regime between the two other regimes. Moreover, it is found that the nature of the tribofilm depends on the lubricant: While methane particles substitute substrate atoms sustaining mostly the crystalline structure, the decane molecules distort the substrate surface and an amorphous tribofilm is formed.