Competition between dislocations and cracks in molecular dynamics simulations of metal nanoimprinting

Direct metal nanoimprinting of a gold thin layer is studied by means of quasi-static molecular dynamics simulations. The aim of this study is to understand if it is possible to obtain a reproducible nanopattern with features of a few nanometers, that closely resembles the shape of the template. The majority of our simulations show an unexpected competition between crack formation and dislocation plasticity upon retraction of the template, which leads in some cases to an imprint and in other cases to a flat surface. These results are at odds with previous simulations of metal nanoimprinting, which always predicted formation of an imprint. The reason for this discrepancy lies in the much lower (and thus more realistic) imprinting velocity used in this work. The most interesting finding of this paper is that the competition between crack and dislocations for certain loading conditions and geometry of the crystals is driven by thermal fluctuations of the atomic velocities. Local events, namely atomic fluctuations and dislocation nucleation, determine the global mechanical response of the system, i.e. whether an imprint is obtained or not. The relevance of thermal fluctuations is confirmed by the fact that any of the simulations presented here, if repeated at 10 K, leads to a brittle material behavior.