Friction from nano to macroforce scales analyzed by single and multiple-asperity contact approaches

Understanding friction and wear at different force and length scales is an interesting aspect in tribology. The variety of processes that make friction such a complex phenomenon includes mechanical, chemical and atomic interactions, each operating at their own time, length, and force scale.Extending local tribological information obtained from a single-asperity contact study (e.g. by lateral force microscopy LFM experiments) to contacts with multiple-asperity interactions is a challenging task. In this work, we analyze the localized friction recorded during a single-asperity LFM experiment, and compare it with that of multiple-asperity sliding contacts. Such a detailed study is relevant for miniature contacts which prevail in microelectromechanical systems of devices. In this work, the friction loops (i.e., tangential force versus displacement curves) obtained from nano to macroforce and different length scales were used to understand the scale effects of friction. Experiments were performed in reciprocating sliding conditions with a ball-on-flat configuration on diamond-like carbon and titanium nitride coatings.With decreasing contact size and stiffness of the measuring equipment, the effect of roughness is strikingly evident. The effect of surface roughness is clearly visible on the recorded friction loops as fluctuations, which were later attributed to geometrical and adhesive components. Based on our observations, a simple contact model was proposed to describe the nature of localized friction in single and multiple-asperity configurations during a sliding pass. The existing friction model for smooth and chemically interacting surfaces was extended to rough surfaces and it was found that most of the existing experimental claims could be explained as special cases of the proposed model.