The friction behavior of atomically stepped metal surfaces under contact loading is studied using molecular dynamics simulations. While real rough metal surfaces involve roughness at multiple length scales, the focus of this paper is on understanding friction of the smallest scale of roughness: atomic steps. To this end, periodic stepped Al surfaces with different step geometry are brought into contact and sheared at room temperature. Contact stress that continuously tries to build up during loading, is released with fluctuating stress drops during sliding, according to the typical stick-slip behavior. Stress release occurs not only through local slip, but also by means of step motion. The steps move along the contact, concurrently resulting in normal migration of the contact. The direction of migration depends on the sign of the step, i.e., its orientation with respect to the shearing direction. If the steps are of equal sign, there is a net migration of the entire contact accompanied by significant vacancy generation at room temperature. The stick-slip behavior of the stepped contacts is found to have all the characteristic of a self-organized critical state, with statistics dictated by step density. For the studied step geometries, frictional sliding is found to involve significant atomic rearrangement through which the contact roughness is drastically changed. This leads for certain step configurations to a marked transition from jerky sliding motion to smooth sliding, making the final friction stress approximately similar to that of a flat contact.

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The impingement of edge dislocations on nano-scale interfaces formed when bringing in contact aluminum crystals is investigated using molecular dynamics simulations. Dislocations, inserted in the bottom crystal, glide towards the contact when the two crystals are pressed together. There, dislocations are absorbed and upon further loading new dislocations are nucleated from the impinging site. Absorption and nucleation are events that affect the length of dislocation pile-ups and therefore the plastic behavior of crystals under contact loading. While it is possible to track absorption and nucleation at the nano-scale with molecular dynamics simulations, larger scale models, which are suitable to study plasticity, do not have the right resolution and neglect these events. The goal of this work is to gain a better understanding of dislocation impingement and to assess to which extent absorption and re-nucleation would play a role at the larger scale. The contacts are here characterized by their initial atomic scale roughness for which a simple, novel definition is introduced. Results show for the first time that roughness controls dislocation nucleation from the contact. This is true for both dislocation-free crystals and for crystals containing one or more dislocations before application of contact loading. In dislocation-free crystals nucleation occurs at decreasing load for increasing roughness. When a dislocation impinges on the contact, it affects its local roughness, by that decreasing the load necessary for dislocation nucleation. Only when the initial roughness of the contact is above a given threshold, dislocation impingement does not affect the load required for nucleation. If instead of a single dislocation, a train of dislocations impinges on the same site, dislocation nucleation is even more facilitated. However, even in this case the contact pressure required to nucleate dislocations is in the order of one GPa, rather high compared with the pressure required to sustain plastic deformation when macro-scale bodies are in contact.

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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.@en