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

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 150 nm (our lowest measured depth), dislocation loops are nucleated at relatively high shear stress values of about G/75, where G is the shear modulus of the OFC. For larger indenter penetration depths, the dislocation loops glide and expand without there being a significant influence of the prior plastic strain in the specimens. The nanohardness decreases with increasing indenter penetration because of the lower shear stress required to expand loops of larger diameters. For still larger penetration depths, the second stage is applicable and the influence of the prior strain becomes significant. As the nanohardness values reflect the resistance to dislocation glide, which would be higher for the work-hardened OFC, the nanohardness of the work-hardened OFC remains higher at all indenter penetration depths than that of the annealed OFC. In the final stage, it is suggested that, when the dislocation density around an indentation becomes sufficiently high, the nanohardness behaviour follows a pattern similar to that of a spherical indentation of a constant a/R value of 0.6, where a and R are the radii of the indentation and indenter respectively. With this approach, the behaviours of both the work-hardened and the annealed OFC have been correlated reasonably satisfactorily using their p(m) versus a/R response corresponding to spherical indenters of different radii, where p(m) is the mean indentation pressure.