Friction and plasticity of micro- and nano-scale metal contacts

Abstract

Sustainability has become an integral part of todays society. A thorough understanding of friction as a major cause of energy dissipation is therefore highly relevant. Friction of rough surfaces in contact is a physical phenomenon that involves multiple length- and time-scales, complicating a full understanding of friction. The fundamental study presented in this thesis aims at extending the existing knowledge of friction. In this work we follow a bottom-up approach and investigate friction and plasticity of metal contacts at the nano- and micro-scale using computational methods. At the micro-scale, the plastic shear response of single asperities is studied using discrete dislocation dynamics. This is a method that averages over atoms, but still accounts for the intrinsic length scale of plastic flow (i.e. the Burgers vector), making it capable of capturing size-effects. One of the main findings is that the contact area, more than the volume of the asperity, controls the plastic response. Studying contact and friction at the nano-scale requires atomistic simulations. Dislocation impingement on metal interfaces can significantly affect the plastic response of systems during contact. Therefore, the impingement behavior of edge dislocations on metal contacts is studied using molecular dynamics. A novel contact characterization is introduced: the atomic scale contact roughness. The roughness is found to be controlling the dislocation impingement behavior, i.e. absorption and re-nucleation. Impingement of dislocations on interfaces results in stepped contacts. The friction behavior of such atomically stepped nano-scale contacts is studied using molecular dynamics simulations. Multiple relaxation mechanisms, such as local contact slip and step motion, occur simultaneously. Step motion leads to local contact migration perpendicular to the contact plane, resulting in vacancy generation in the re-crystallized part of the crystal, which could affect dislocation behavior at larger scales. It is found that friction of atomically stepped contacts has a self-organized critical state. Interestingly, sliding friction of contacts with certain step configurations leads to significant atomic rearrangement at the contact (self-organization of the steps), leading to a marked transition from jerky sliding to smooth sliding. This thesis provides insight into different energy dissipation mechanisms during friction of micro- and nano-scale metal contacts. The fundamental insights from this work can be used in the development of multi-scale models of friction.