Modeling the Effect of Normal Load on Atomic Friction

Simona Skuratovsky, Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel.
The Ilze Katz Institute for Nanoscience and Technology, Ben-Gurion University, Beersheva 84105, Israel.

The widely held tenets of macroscopic friction, who dates back to the works of da Vinci and formulated in Amontons and Coulombs laws, were challenged with the early studies of micro- and nano- scale friction. These studies demonstrated that friction is proportional to the contact area and sliding velocity. The introduction of the Atomic Force Microscope (AFM), paved way to the development of nanotribology, by providing information on the single asperity level with lattice resolution. In these experiments, termed friction force microscopy (FFM), a sharp cantilever tip scans a surface of interest and records lateral friction forces, while being subjected to a normal load. FFM measurements proved themselves as highly useful in the studies of surfaces and surface related phenomena, as they provide high-resolution information on the atomic scale. These measurements employ phenomenological theories that obtain thermodynamic properties from the time dependent measured force traces, through the dependence of the lateral force with the scanning velocity and temperature. Interestingly, although normal load is known to have a dramatic effect on lateral forces, its dependency was not models. Here we use a mathematical ansatz based on experimental FFM measurements of NaCl crystal in ethanol to study the effect of normal loads on atomic scale friction. To this end, we perform FFM simulations using the Langevin equation, and successfully reproduce the measured data, from which we estimate the average corrugation energy of the surface.