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The effect of the presence of a passivation layer on a metal rough surface during contact loading is investigated by means of dislocation dynamics simulations. The metal body is modeled as an FCC single crystal with a self-affine rough surface that is either bare, or covered by a thin coating, impenetrable to dislocations. This analysis permits to isolate the effect of surface roughening driven by dislocation motion: when the surface is bare the dislocations can glide out, leaving crystallographic steps at the surface that modify the local roughness; when the surface is passivated, dislocations are stopped by the interface., This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 681813)., Peer Reviewed, Postprint (published version)

Viscoelasticity and roughness are among the possible causes of the adhesive hysteresis displayed by soft contacts. Viscoelasticity causes an increased effective work of adhesion due to stiffening of the contact, while roughness is responsible for elastic instabilities. Herein, we explore the interplay between viscoelasticity and roughness by simulating in two dimensions the retraction of a rigid cylinder, with wavy surface profile, from a viscoelastic half-space. The wave amplitude and length are varied to induce instabilities in the load–to–area response, while the retraction velocity is increased to promote viscoelasticity. Results show that, in the regime where viscoelasticity is confined to the edges of the wavy contact, the contributions of viscoelasticity and waviness to adhesive hysteresis are nearly independent and additive. At low retraction rates, the instabilities in the load-area curve typical of rough elastic contacts are suppressed by viscoelasticity: the contact stiffens to promote a stable decrease of the contact area with load. This occurs with a minimal change in work of adhesion. However, when the instantaneous limit is met at high retraction rates, mechanical instabilities appear., LN acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 681813)., Peer Reviewed, Postprint (published version)

A seamless 2D dual-scale computational scheme is developed to study contact problems. The model consists of an atomistic domain close to the contact, coupled with an elastic continuum domain away from the contact. The atomistic formulation provides a description of the contact interaction through interatomic potentials and permits to capture atomic wear and defect formation in the contact region. The fields in the continuum domain are calculated by an efficient FFT-based Green’s function method. The novel scheme is validated against full atomistic simulations and applied to study the effect of adhesion on the scratching of a rough copper surface by a rigid smooth spherical tip., Peer Reviewed, Postprint (published version)

A model is proposed herein to investigate the incipient sliding of contacts in the presence of both friction and adhesion, where the interfacial response is modeled based on traction-separation laws. A Maugis-like parameter is defined to characterize the response in the tangential direction. Subsequently, the model is used to investigate the contact between a smooth cylinder and a flat body, where adhesion-friction interactions are strong. A range of behaviors are observed when a tangential displacement is imposed: When the parameter is low, the contact pressure exhibits a relatively constant profile; when it is high, a pressure spike is observed at the edge of the contact. This difference is caused by a significant interface compliance in the former case, which limits the amount of slip. The results for the mid-range values of the Maugis-like parameter can qualitatively replicate various experiments performed using polydimethylsiloxane (PDMS) balls., This work is funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant No. 681813)., Peer Reviewed, Postprint (published version)

According to experimental findings there are types of loading for which it is more difficult to plastically deform small metal structures than the corresponding massive metal. This occurs when at least one of the dimensions of the structure is at the micrometer scale or smaller. It is for instance the case of thin metal coatings: the thinner is the film, the more difficult it is to deform it plastically. This feature is in contrast with what expected on the basis of classical plasticity theories, which predict a size-independent mechanical response. In this thesis the size effect in thin films is studied by means of two-dimensional discrete dislocation plasticity. Plasticity is treated as the collective motion of edge dislocations on prescribed slip systems. The framework contains an intrinsic length scale- the dislocation Burgers vector- which is a necessary condition to capture a size effect. ... Zie: Summary

According to experimental findings there are types of loading for which it is more difficult to plastically deform small metal structures than the corresponding massive metal. This occurs when at least one of the dimensions of the structure is at the micrometer scale or smaller. It is for instance the case of thin metal coatings: the thinner is the film, the more difficult it is to deform it plastically. This feature is in contrast with what expected on the basis of classical plasticity theories, which predict a size-independent mechanical response. In this thesis the size effect in thin films is studied by means of two-dimensional discrete dislocation plasticity. Plasticity is treated as the collective motion of edge dislocations on prescribed slip systems. The framework contains an intrinsic length scale- the dislocation Burgers vector- which is a necessary condition to capture a size effect. ... Zie: Summary