Place: Materialwissenschaftliches Kolloquium, TU Darmstadt
Large-scale molecular dynamics simulations of nanoindentation have been performed to study the influence of different length scales on the hardness of the material. Typical numerical samples contained between 2 and 5 million atoms interacting with an embedded atom method (EAM) potential. The indenter was modeled essentially as a hard sphere with a purely repulsive potential. The indentation was performed in a displacement controlled fashion and at a constant finite temperature. The simulations show that the calculated hardness decreases with indentation depth, which is consistent with the indentation size effect found in experiments. To calculate the “true” material property hardness from nanoindentation experiments or simulations, a physical model of the deformation processes and their dependence on the microstructure is necessary. A careful analysis of the dislocation microstructure resulting from the numerical simulations reveals that the average dislocation density in the plastic zone increases with indentation depth. Hence, neither strain gradient theory nor Taylor hardening are able to explain this size effect in hardness. The origin of the size effect seems to be rather that the indentation is performed into a perfect, dislocation-free single-crystal, where the first dislocations nucleate homogeneously at the theoretical shear strength. To verify this assumption, indentation simulations close to grain boundaries are performed and compared with experiment. The results show that the grain boundary in fact simplifies deformation in the early stages, but also provides hardening due to the confinement of the plastic zone.