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 ZrO ₂ 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.