Charge Density Evolution Governing Interfacial Friction

It is well-known that the electron nature of a solid in contact plays a predominant role in determining the many properties of the contact systems, but the general rules of electron coupling that govern interfacial friction remain an open issue for the surface/interface community. Here, density functional theory calculations were used to investigate the physical origins of friction of solid interfaces. It was found that interfacial friction can be inherently traced back to the electronic barrier to the change in the contact configuration of the joints in slip due to the resistance of energy level rearrangement leading to electron transfer, which applies for various interface types ranging from van der Waals, metallic, and ionic to covalent joints. The variation of the electron density accompanying contact conformation changes along the sliding pathways is defined to track the frictional energy dissipation process occurring in slip. The results demonstrate that the frictional energy landscapes evolve synchronously with responding charge density evolution along sliding pathways, yielding an explicitly linear dependence of frictional dissipation on electronic evolution. The correlation coefficient enables us to interpret the fundamental concept of shear strength. The present charge evolution model thereby provides insights into the classic hypothesis that the friction force scales with the real contact area. This may shed light on the intrinsic origin of friction at the electronic level, opening the way to the rational design of nanomechanical devices as well as the understanding of the natural faults.