Single asperity nanocontacts: Comparison between molecular dynamics simulations and continuum mechanics models

Using classical molecular dynamics, atomic scale simulations of normal contact between a nominally flat substrate and different atomistic and non-atomistic spherical particles were performed to investigate the applicability of classical contact theories at the nanoscale, and further elucidate the phenomena that govern the perceived breakdown of continuum in nanocontacts. Using a rigorous definition of the real contact area, the corresponding surface energies were calculated via molecular dynamics simulations and validated with values reported in the literature. The contact force and radius, as well as the contact stress distribution and dislocation propagation were then investigated in detail. The Hertz elastic contact model was shown to sufficiently capture the results of molecular dynamics simulations prior to the onset of plastic deformation for non-adhesive contacts (non-atomistic particles). Moreover, the results show that the Lommer-Cottrell locks observed during contact acted as obstacles to dislocation motion, and cross-slipping of atoms at these points was the deformation mechanism responsible for force drops during loading. In the case of the adhesive atomistic particles, the results were compared to the limiting cases of the Johnson-Kendal-Roberts (JKR) and Derjaguin-Muller-Toporov (DMT) continuum models. The atomistic systems showed deviations from the classical models, which could be related to energy loss and changes in the effective work of adhesion, as well as the anisotropic properties of the atomistic systems. The findings of this work support the published literature on the subject, and further contribute to our understanding of discrepancies between atomistic and continuum descriptions of contact. (C) 2014 Elsevier B.V. All rights reserved.