Computer experiments on nano-indentation: A molecular dynamics approach tothe elasto-plastic contact of metal copper

Molecular dynamics simulations are used to investigate the micro-mechanisms of nano-indentation for tip to substrate contact. The method combines a ma ny-body interatomic potential derived from the nearest-neighbor EAM and bro wnian dynamics (BD) approach to simulate a rigid tip indenting Cu (001) sur face. Elastic contact and plastic instability of the crystal are investigat ed through the loading-unloading cycle, the variations of the system potent ial energy versus the tip approach, the atomic stress distributions and the portraits of atomic trajectories and configurations. For elastic indentati on, we find that atomistic stress distributions resembling roughly to those of the continuum Hertzian fields, except for a jump-to-contact phenomenon in the initial contact stage. When the tip approach is beyond some critical value, plastic instability of the substrate occurs, and both the contact l oad and potential energy decrease dramatically. Detailed calculations revea l that material yield at the atomic level is still governed by the von Mise s shear strain-energy criterion, while atomistic trajectories show that the displacements in (010) plane of atoms near the contact region is similar t o that in Johnson's cavity model, accompanied by atomic cross-layer movemen ts in [010] direction to release the strain energy. The crystal defects aft er plastic indentation include subsurface cavities, surface atomic steps an d plastic indent. (C) 2000 Kluwer Academic Publishers.