Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO₂ phase diagram

Composites of hydroxylapatite (HA) with pure zirconia, and 3 and 8% Y₂O₃ in zirconia, were pressure-less sintered at temperatures from 900 to 1300 ◦C, and hot-pressed at 1200 ◦C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000 ◦C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200 ◦C, CaZrO₃ was formed. These reactions and transformations were interpreted in terms of the ZrO₂-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ sintered at 1300 ◦C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon airsintering.