Atomic-scale simulations of nanoindentation-induced plasticity in copper crystals with nanometer-sized nickel coatings

The mechanical behaviour of very small volumes differs from what is typically observed on the macrolevel. In particular, thin metal films with nanometer-sized coatings possess interesting mechanical properties. Here, we use molecular dynamics simulations to elucidate details of plastic deformation and the underlying deformation mechanisms during nanoindentation of thin copper films with epitaxial nickel coatings. Several deformation mechanisms, such as dislocation pile-up on the interface, dislocation cross-slip and movement of misfit dislocations, are observed. Most interestingly, copper films are significantly strengthened by thin nickel coatings. This effect is especially pronounced at a very low temperature. For nickel coatings as thin as 1.6 nm (at low temperature) or 2.5 nm (at room temperature), the coated copper adopts the hardness of bulk nickel, even though the operating deformation mechanism is unique in each case considered and the hardness of bulk copper is significantly lower. The critical thickness at which a coated copper film achieves hardness of bulk nickel depends not only on the temperature, but also on the radius of the indenter tip. Sharper indenters yield higher hardness. The obtained results suggest that thin surface coatings or oxide layers at nanoscale contacts may have a pronounced effect on the perceived hardness of the surface.