An experimental investigation was made of the nano-hardness of polycrystalline work-hardened and annealed O-free material under various indenter loads. Nano-indentations were made by using a Berkovich diamond indenter, using loads which ranged from 1 to 100mN. It was shown, by accurately measuring the projected contact areas of nano-indentations using an atomic force microscope, that the overall nano-hardness behaviour of the work-hardened material was quite different to that of the annealed material. The nano-hardness of the former behaved non-monotonically with increasing indenter penetration depth, while the nano-hardness of the latter decreased monotonically with increasing indenter penetration depth. A 3-stage qualitative model was proposed in order to explain the nano-hardness results. In the first stage, at penetration depths of less than 150nm (the lowest measured depth), dislocation loops nucleated at the relatively high shear stress values of about G/75; where G was the shear modulus of the material. For larger indenter penetration depths, the dislocation loops glided and expanded without there being any significant effect of prior plastic strain upon the specimens. The nano-hardness decreased, with increasing indenter penetration, because of the lower shear stress which was required in order to expand loops of larger diameter. At still greater penetration depths, a second stage applied and the effect of prior strain became significant. Because the nano-hardness reflected the resistance to dislocation glide, which would be higher for work-hardened material, the nano-hardness of the work-hardened material remained higher - at all indenter penetration depths - than did that of annealed material. In the final stage, it was suggested that when the dislocation density around an indentation became sufficiently high the nano-hardness behaviour obeyed a pattern which was similar to that of a spherical indentation with a constant a/R ratio of 0.6, where a and R were the radii of the indentation and indenter, respectively.

The Effect of the Indenter Load on the Nanohardness of Ductile Metals - an Experimental Study of Polycrystalline Work-Hardened and Annealed Oxygen-Free Copper. Y.Y.Lim, M.M.Chaudhri: Philosophical Magazine A, 1999, 79[12], 2979-3000