Nano-Scale Effect in Adhesive Friction of Sliding Rough Surfaces

Study of contact and friction at multiple length scales is necessary for the effective design and analysis of surfaces in sliding microand nano-electromechanical systems (MEMS/NEMS). As loading forces decrease in such applications, the size of the asperity contacts tends to decrease into the nano scale regime. Also with the increase in surface area to volume ratio in such systems, the surface force or adhesion becomes more prominent in contributing to surface interaction effects. Since the friction force depends on the real area of contact, which is strongly influenced by the presence of surface forces and surface roughness, it is important to analyze the effect of adhesion and roughness on the frictional behavior of small scale sliding systems. In the present study, the Hurtado and Kim model for the behavior of the friction stress is incorporated into the multi-asperity adhesive contact model of Roy Chowdhury and Ghosh which includes the asperity adhesion forces using the Johnson-Kendall-Roberts adhesion model. The well-established elastic adhesion index along with the plasticity index is used to consider the different conditions that arise as a result of varying load, material parameters and contact size. Results are obtained as the variation of coefficient of friction versus normal load for different combinations of the controlling parameters. It is found that the nano-scale effect in multi-asperity contacts is dominant for low values of adhesion index, small normal load and elastic contact conditions.