The influence of the microstructure on the hardness of sintered hydroxyapatite

The Vickers hardness, HV, was measured for 42 sintered monophase hydroxyapatite (HAP) specimens having volume fraction porosities, P, that ranged from 0.02 to 0.31 and average grain sizes between 1.7 μm and 7.4 μm. The HV versus porosity behavior of the sintered HAP specimens were successfully described by the exponential function of porosity, HV=H0 exp(–bP), where the least-squares fit values of the zero-porosity hardness H0 and the constant b were 6.00±0.7 GPa and b=6.03, respectively. The function HV=H0 exp(−bP) is consistent with a minimum solid area model proposed by Rice. In contrast to the strong dependence of hardness on porosity, there was no clear trend of hardness as a function of grain size, for the grain size range included in this study.