Preparation and mechanical properties of dense polycrystalline hydroxyapatite through freeze-drying.

High purity hydroxyapatite (HAp) powders were synthesized through the wet-chemical method and subsequent freeze-drying or heat-drying, respectively. Dense polycrystalline HAp ceramics were obtained by sintering the powder compacts in air in the temperature range 1100-1350 degrees C. Results show that the freeze-drying method can produce highly homogeneous, fine-grained HAp powders. The relative density, Vickers hardness and flexural strength of the sintered freeze-dried HAp ceramics increased with the sintering temperature, reaching a maximum at around 1350 degrees C, and the highest values obtained were 99.0%, 820 and 110 MPa, respectively. All these mechanical properties of the freeze-dried HAp ceramics are much higher than those of the conventionally heat-dried HAp ceramics. The drying method can significantly influence the properties of the HAp powders as well as the HAp ceramics.