Enhanced phase stability in hydroxylapatite/zirconia composites with hot isostatic pressing

Hydroxylapatite (HA) composites with pure zirconia (ZrO2), and 3 and 8% Y2O3 doped ZrO2 were pressure-less sintered in air and hot isostatically pressed (under 120 MPa gas pressure) at 1100 °C for 2 h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO2 composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO2 was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO2 were observed in hydroxylapatite/3% Y2O3 doped ZrO2 or HA/8% Y2O3 doped ZrO2 composites. On the other hand, HA and ZrO2 phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO2 among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO2 contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO2 and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites.