Effect of microcracking on the energy-balance relationship for hardness testing of ceramics

Energy-balance model has been widely employed to analyze the indentation size effect in the measured hardness. This model predicts that there is a linear relationship between P/d and d, where P and d are the indentation load and the resultant indentation size, respectively. In this paper, the effect of indentation-induced microcracking on the energy-balance model was examined. It was shown that, for the indentation events accompanied by the occurrence of microcracking, the observed linear relationship between P/d and d is only an experimental artefact resulting from the narrowness of the test load range used. Consequently, caution should be taken when the slope of the P/d– d straight line is measured in a narrow range of test loads as the measure of the so-called load-independent hardness, unless it is certain that no microcracking due to indentation occurs during the test.