A Mechanism for Lubrication between Surfaces with Atomic Level Roughness

Besides the micron scale roughness characterized by the fact that solid surfaces in contact are only in contact at micron scale aperities, there must also be atomic level roughness at the interface between two asperities in contact. One likely form of such atomic level roughness is that the top atomic layer of the surfaces of the each of the two asperities at the region of contact could be incomplete in a random fashion, resulting in randomly distributed steps. Such roughness can put the surfaces in the strong pinning regime, in which the two surfaces are locked together (i.e., there is static friction). Based on the results of simulations done on films of molecules trapped between surfaces with similar atomic level roughness, it is proposed that lubricant molecules adsorbed on an interface with such roughness can increase the number of atomic scale points of contact at the interface between two contacting asperities, which can switch the interface from the strong to weak pinning regime, resulting in a large reduction in the static friction. This is proposed as a possible mechanism for the reduction of friction by boundary lubrication for at least some nonmetallic surfaces in contact.