Relationship between indentation size effect and material properties in the microhardness measurement of some cobalt-based alloys

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.