Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5 mol% Y2O3) containing 2, 5, 7.5 and 10 wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100 nm) the TZP particles were only of spherical nature (∼50 nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO2. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5 wt% TZP, while it was 92% for 7.5 wt% and 90% for 10 wt% TZP compositions. X-ray analysis of sintered samples shows that with 2 wt% TZP, the retained phases were only HA and t-ZrO2. However, for 5, 7.5 and 10 wt% TZP addition both TCP and CaZrO3 were also observed along with HA and t-ZrO2. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72 MPa, it was 35.5 MPa for diametral compression. The strength shows a decreasing trend at higher ZrO2 content. SEM pictures show near uniform distribution of ZrO2 in HA matrix. The reduction in sintered density at higher ZrO2 content could be related to difference in the sintering behaviour of HA and ZrO2.