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

Abstract Hydroxylapatite (HA) composites with pure zirconia (ZrO 2 ), and 3 and 8% Y 2 O 3 doped ZrO 2 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 ZrO 2 composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO 2 was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO 2 were observed in hydroxylapatite/3% Y 2 O 3 doped ZrO 2 or HA/8% Y 2 O 3 doped ZrO 2 composites. On the other hand, HA and ZrO 2 phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO 2 among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO 2 contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO 2 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.