Rapid densification of nanocrystalline hydroxyapatite for biomedical applications

The effect of rapid sintering by microwave heating on the properties of nanocrystalline hydroxyapatite (HA) bioceramic powder was studied. The starting high-purity nano HA crystals was synthesized using a wet chemical precipitation technique. The HA compacts were microwave-sintered over the temperature range 1000–1300 °C, using a rapid sintering schedule of 30 min for each temperature. Shrinkage of all compacts was uniform and the sintered material did not exhibit any cracks. XRD phase analysis indicated that the HA phase stability was not disrupted throughout the sintering regime employed. A relative density of 90–91% was attained for samples sintered at 1000 °C, whereas a higher relative density of above 96% and ∼98% was measured for HA sintered at 1100 °C and 1300 °C, respectively. The present work revealed that microwave heating was beneficial in preventing grain growth, particularly when sintered at temperatures <1200 °C, and coupled with improved mechanical properties, i.e., high fracture toughness of 1.45 MPa m1/2 and hardness of 6.38 GPa were measured for HA compacts sintered at 1050 °C and 1150 °C, respectively.