THERMALLY ACTIVATED PROCESSES DURING NANOINDENTATION IN ATOMISTIC SIMULATIONS

The nanoindentation test at high temperatures is becoming increasingly popular nowadays due to the recent affordability to perform the measurements during heating of the specimen. At elevated temperature thermally activated processes become dominant in contrast to dislocation glide at low temperature. Therefore, it is imperative to understand the underlying physics behind the plastic deformation during high temperature indentation tests and in particular on the nanoscale, where every single dislocation interaction is momentous [1]. Since experiments do not capture the ongoing dislocation mechanisms, we perform nanoindentation simulations of thin films by employing atomistic simulation [2].

The simulation is performed by using an open source code LAMMPS. An initial dislocation structure is created beneath the indent by indenting an atomically relaxed film. The system is then heated in the canonical ensemble to the desired temperature, while keeping the spherical indenter in place, and kept at constant temperature thenafter until the dislocation density stabilizes. In order to analyze and characterize the dislocation structures during the simulations, order parameters of the local atomic structure were calculated after introducing numerical techniques to minimize the thermal noise.

Having an accelerated cross-slip process at high temperatures and, hence, the dislocation annihilation, the film is left with the fewer dislocations than the expected number of geometrically necessary dislocations, mostly of edge character in different slip planes. The dislocation annihilation was found to be faster at higher temperatures and the residual dislocation densities seems to exhibit a linear dependency with temperature. This observation suggests that annihilation of dislocation structures should be introduced in indentation models, such as in the one proposed by Nix and Gao [3].

References:

[1] http://www.sciencedirect.com/science/article/pii/S1359645415300276

[2] http://www.sciencedirect.com/science/article/pii/S1359645415001676

[3] http://www.sciencedirect.com/science/article/pii/S0022509697000860