Composites of hydroxyapatite with cubic zirconia with MgO or MgF2 were pressureless sintered at temperatures from 1000 to 1300 degrees C. The reactions and transformations of phases were monitored with X-ray diffraction. For the hydroxyapatite and zirconia composites with MgO, calcium diffused from hydroxyapatite into the zirconia, and hydroxyapatite decomposed to tri-calcium phosphate at sintering temperatures higher than 1000 degrees C. Above about 1200 degrees C, CaZrO3 was formed. Composites containing the MgF2 decomposed slower than the composites with MgO which was verified by the changes in the lattice volume of the hydroxyapatite left in the composites. Fluorine ions in MgF2 diffused into hydroxyapatite which resulted in thermal stability at high sintering temperatures. Composites with MgF2 had higher hardness than those with MgO. The lowest porosity was found in a composite initially containing 10 wt% cubic zirconia and 5 wt% MgF2. (c) 2008 Elsevier B.V. All rights reserved.