Nanoscale friction as a function of activation energies

Abstract

Understanding the scale-dependence of friction is increasingly viewed as a critical quest. With progressively thinner films, mixed and boundary regimes of lubrication have become commonplace. Therefore, at the micro-scale a greater need for mitigating friction is desired in order to improve operational efficiency of many machines and mechanisms. Furthermore, there is a growing tendency to use low friction hard wear-resistant advanced coatings to guard against wear. In parallel, there has been much attention paid to lubricant rheology and formulation. However, only in recent times there has been an emerging view of lubricant–surface combination as a system. In this perspective it is essential to relate the observed and measured friction at component level to the underlying interactions in micro/nano-scales. This is the approach in this paper. Observed phenomenon at micro-scale are related back to the activation energies of lubricant–surface system, providing in particular results for surface modified Ni–SiC coated specimen in combination with formulated lubricants, the combination of which represent the lubricant–surface system of choice in cylinders of high performance race engine. The nano-scale conjunction of an AFM tip with lubricated surface-engineered specimen, subjected to various conjunctional loading and sliding kinematics is investigated. It is shown that the measured frictional characteristics can be adequately described in terms of activation energies in line with the Eyring's thermal activation model for cases of fairly smooth asperity tip contact conjunctions.