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

Molecular dynamics (MD) simulation is conducted in order to study effects of atomic scale roughness on sliding surfaces on the ultra-thin film lubrication. The model lubricants are hydrocarbons including n-hexane, cyclohexane and n-hexadecane, which are confined and sheared by iron surfaces having transverse roughness. Velocity distribution and shear force computed in MD simulations are compared with those obtained through the conventional hydrodynamic (HD) lubrication theory. Although some similarity is seen in velocity distributions in MD and HD results, the MD simulation reveals roughness effects that are quite different from those derived in the continuum HD approach. Atomic surface roughness affects formation and breakdown of layered structures, movement of bridging molecules between the layers, and slip between the molecules and that between the molecules and the solid surface. When surfaces have roughness, particularly when both stationary and moving surfaces have roughness, disturbances caused in molecular movement are far greater, resulting in higher traction coefficient than in the case when surfaces are atomically flat. All these effects depend on surface asperity shape and molecular structure of lubricants.