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The load dependence of the Vickers microhardness of some cobalt-based alloys subjected to heat treatment at different temperatures for various durations, investigated using Hanemann’s method and a PMT-3 hardness tester, has been discussed. The experimental results show that: (1) the indentation size effect is best revealed by the proportional specimen resistance model of Li and Bradt, and (2) the plot of the ratio of indentation load P to indentation diagonal d against d for a sample exhibits two different slopes b with a transition in the slopes occurring at an indentation diagonal d lying between 9 and 20 μm. Analysis of the results revealed that: (1) the origin of indentation size effect is associated with the processes of relaxation of indentation stresses, (2) the load-dependent quantity a and load-independent quantity b for cobalt-based alloys are intimately connected with the grain size and defect structure of the alloys, and the grain-size dependence of their microhardness qualitatively agrees with the modified Hall–Petch relation, (3) in the range of high indentation loads the microhardness of cobalt-based alloys, as determined by using the proportional specimen resistance model, is practically independent of the applied load, and (4) the origin of a critical indentation diagonal dc*≈15 μm, when a change in the slope of the plot of a against b occurs, is associated with the value of indentation diagonal d when frictional resistance begins to contribute to the values of a. [Copyright &y& Elsevier]

The load dependence of the Vickers microhardness of some cobalt-based alloys subjected to heat treatment at different temperatures for various duration has been investigated using Hanemann’s method and a PMT-3 hardness tester. It is found that even for the same sample Hanemann’s method shows a decrease in microhardness with applied load (i.e., normal indentation size effect) while the PMT-3 hardness testing exhibits a reverse effect. The experimental data were analysed using Meyer’s law, Hays–Kendall’s model, the elastic/plastic deformation model of Bull et al., the proportional specimen resistance model of Li and Bradt and the energy balance approach of Bu¨ckle. The analysis of the experimental data revealed that: (1) the Meyer hardness index n decreases linearly with logarithm of the so-called standardised hardness constant A, (2) within the limits of experimental errors, the load-independent microhardness of a given sample predicted by the other models used is constant, (3) the quantities contributing to the load dependence of hardness are inversely related with the constants describing the load-independent hardness, (4) the quantities contributing to the load dependence of hardness are related with the Meyer hardness index n and (5) the constants of the energy balance approach are mutually related but the load-independent microhardness predicted by this approach is erratic with respect to that obtained by the other models. [Copyright &y& Elsevier]