Densification behaviour of nanocrystalline hydroxyapatite bioceramics

The sinterability of nanocrystalline hydroxyapatite (HA) particles by microwave sintering was compared with conventional pressureless sintering. The results revealed that microwave heating was effective in producing a useful HA body in a very short sintering cycle without disrupting the HA phase stability. The maximum hardness of 7.21 GPa and 6.38 GPa was obtained for HA sintered at 1050 °C by the conventional method and 1150 °C by microwave sintering, respectively. The maximum fracture toughness measured for the microwave-sintered and conventional-sintered HA was 1.45 MPam1/2 at 1050 °C and 1.22 MPam1/2 at 1000 °C, respectively. Although the relative density of microwave-sintered HA was slightly lower than the conventional-sintered HA throughout the sintering regime employed, taking into account of the heating and soaking periods, the time taken by microwave sintering to achieve a relative density of 96.5% was about 3% of the time consumed for samples sintered by the conventional heating. Microwave heating was found to be an effective technique to produce a useful HA body for clinical applications without causing grain coarsening.