Pressureless sintering of nanocrystalline hydroxyapatite at different temperatures

In this work, the sintering behaviour of hydroxyapatite (HA) at different temperatures was studied. Nano HA powders synthesized by sol-gel technique were uniaxially pressed at 30 MPa into pellets and cold isostatically pressed at 200 MPa. The pellets were sintered in air at temperatures ranging from 900 A degrees C to 1400 A degrees C with a holding time of 2 h. It was observed that, at a sintering temperature of 1200 A degrees C when the material was composed of pure HA phase, the samples exhibited densities of > 98.5 % of the theoretical value and possessed a hardness value of 5.89 GPa. Decomposition of HA into the secondary phases of TCP and CaO was found to occur at 1300 A degrees C and 1400 A degrees C, respectively. Changes in the microstructure, relative density and hardness of the sintered HA ceramics with the sintering temperature were also analyzed. The variation in the hardness was found to be dependent on the relative density up to a threshold grain size limit of 2 mu m. However, beyond this threshold, no correlation existed between the two properties. Porosity and grain size were found to play an important role in determining the properties of the sintered HA compacts.