The microstructural origin of the elastic-plastic response of a Cu substrate during nano-indentation was studied by using molecular dynamics simulations. The elastic response was found to deviate from the Hertzian behavior which was observed experimentally. The discrepancy was traced to the small tip radius used in the simulation. Further penetration led to the development of an inhomogeneous microstructure. At a given strain rate, various parts of the contact surface could deform via different mechanisms. Some deformed elastically, some via dislocation bow-out and some via the nucleation and growth of Shockley partials that sometimes interacted to form stair-rod locks. The resultant effect produced the observed quasi-elastic behavior of the load-displacement curve; characterized by interspersed minor yields. The computer simulations reproduced the corresponding dislocation structure development in some detail. Stair-rod lock formation was found to provide a more satisfactory explanation for the experimentally observed time-delayed occurrence of pop-in below the spontaneous pop-in load.

Dislocation Nucleation in the Initial Stage during Nanoindentation. H.Y.Liang, C.H.Woo, H.Huang, A.H.W.Ngan, T.X.Yu: Philosophical Magazine, 2003, 83[31], 3609-22