Molecular dynamics study of size effects and deformation of thin films due to nanoindentation

The indentation response of Ni thin films of thicknesses in the nano scale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. Simulations were performed in single crystal films in the [111] orientation with thicknesses of 7nm and 33nm. In the elastic regime, the loading curves observed start deviating from the Hertzian predictions for indentation depths greater than 2.5% of the film thickness. The observed loading curves are therefore dependent on the film thickness. The simulation results also show that the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The deformation mechanism consists of the emission of dislocation loops in the area underneath the indenter. The loops are emitted in multiple directions with more dislocations emitted as the films thickness increases. This effect is interpreted as due to the back stress created by the first dislocations emitted when they reach the film/substrate interface and cannot cross into the substrate.