We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness. We found that both twinning partial slip (TPS) and partial slip parallel with twin boundary (PSPTB) can reduce hardness and therefore should not be ignored when evaluating mechanical properties at nanoscale. There is a critical range of twin thickness λ (~25 Å < λ < ~31 Å), in which hardness of the multilayer films is maximized. At a smaller λ, TPSs appear due to the reaction between partial dislocations and twin boundary accounts for the softening-dominated mechanism. We also found that the combination of the lowered strengthening due to confined layer slips and the softening due to TPSs and PSPTBs results in lower hardness at a larger λ.

The B3-GaN thin film was investigated by performing large-scale molecular dynamics (MD) simulation of nanoindentation. Its plastic behavior and the corresponding mechanism were studied. Based on the analysis on indentation curve, dislocation density, and orientation dependence, it was found that the indentation depths of inceptive plasticity on (001), (110), and (111) planes were consistent with the Schmid law. The microstructure evolutions during the nanoindentation under different conditions were focused, and two formation mechanisms of prismatic loop were proposed. The “lasso”-like mechanism was similar to that in the previous research, where a shear loop can translate into a prismatic loop by cross-slip; and the extended “lasso”-like mechanism was not found to be reported. Our simulation showed that the two screw components of a shear loop will glide on another loop until they encounter each other and eventually produce a prismatic dislocation loop.