Size-dependent hardness of nanoscale metallic contacts from molecular dynamics simulations

Abstract

We characterize how size and shape affects the hardness of nanoscale metallic contacts using large-scale molecular dynamics (MD) simulations. High-aspect-ratio contacts continue the experimentally observed trend of hardening with decreasing contact size down to the sub-10-nm regime. However, we find that this effect is shape dependent and the rate of hardening with decreasing contact size diminishes as the aspect ratio of the asperities becomes smaller. Interestingly, low-aspect-ratio asperities that can support simple dislocation glide exhibit softening with decreasing size. A detailed analysis of the MD trajectories reveals the dislocation mechanisms that govern these complex size effects.

Multiscale contact mechanics model for RF–MEMS switches with quantified uncertainties

“…Hardness calculations. Asperity/asperity and flat/asperity contacts with various asperity heights are simulated via MD using an approach used previously for metal-metal contacts and described in prior publications [11]; we provide only a brief description here. Contact simulations are performed via constant-energy MD simulations (NVE ensemble) with an increasing external force applied to both slabs with equal magnitude and opposite direction.…”

“…In order to compute the asperity hardness from the MD simulations we need to compute the local stress in the asperities and identify the plastic deformation during loading. In order to obtain the local stress on the contacts we compute the effective contact area (A cont ) from each atomistic structure; see [11]. We define contact stress as σ cont = F ext /A cont in terms of instantaneous values of force and effective contact area.…”

“…This increase corresponds to the elastic load of the asperities and a change in slope denotes plastic relaxation. The local stress corresponding to this change in slope is defined as the asperity hardness [11,14] and are marked by circles in figure 7. Perhaps not surprisingly, the results for amorphous contacts presented here show a less drastic plastic relaxation than crystalline metals [11,14] contact size (nm) Results: size-dependent hardness.…”

Key role of excess atomic volume in structural rearrangements at the front of moving partial dislocations in copper nanocrystals

“…It is known that the atomic density (atomic volume) is a thermodynamic variable which determines the phase state of condensed matter 12 . The phase diagrams plotted in terms of temperature, concentration, and atomic density not only extend the knowledge of phase states in a solid under changes of its atomic density but can also explain the instability and mechanical melting of its lattice when the solid expands to a critical specific volume 13 .…”

“…In the papers cited above 12–19 , the increase in volume was taken uniform. In this context, the questions arise of what effects can be produced by a local volume change and whether it will precede the formation of lattice defects, and if so, what types of these defects are.…”