Effect of porosity on the microhardness testing of brittle ceramics: A case study on the system of NiO-ZrO2

Microhardness tests were conducted on a series of NiO-ZrO2 ceramics with different porosities ranging from 2% to 18%. For each sample, the measured hardness decreases with increasing indentation load, showing a significant indentation size effect. The indentation data were analyzed according to the modified proportional specimen resistance (PSR) model and it was found that the resultant load-independent hardness decreases with increasing porosity. The porosity-dependence of the load-independent hardness was analyzed and a concept of true hardness was proposed to characterize the actual material resistance to indentation-induced plastic deformation. Furthermore, the applicability of the modified PSR model in describing the indentation size effect observed on porous materials is discussed.