The effect of the indenter load on the nanohardness of ductile metals: An experimental study on polycrystalline work-hardened and annealed oxygen-free copper

Abstract

Abstract An experimental investigation of the nanohardness of polycrystalline work-hardened and annealed oxygen-free copper (OFC) for different indenter loads is described. Nanoindentations were made using a Berkovich diamond indenter and the indenter loads used were in the range 1–100 mN. It is shown that by accurately measuring the projected contact areas of nanoindentations with an atomic force microscope the overall nanohardness behaviour of the work-hardened OFC was quite different from that of the annealed OFC. The nanohardness of the former behaved non-monotonically with increasing indenter penetration depth, whereas the nanohardness of the latter decreased monotonically with increasing indenter penetration depth. A three-stage qualitative model has been proposed to explain the nanohardness results. In the first stage, it is argued that, at low penetration depths of less than 150nm (our lowest measured depth), dislocation loops are nucleated at relatively high shear stress values of about G/75, whe...