Processing and mechanical properties of hydroxyapatite reinforced with hydroxyapatite whiskers.

Hydrothermally synthesized HAp fine crystals/HAp whiskers mixtures have been used for the preparation of HAp/0-30% (whiskers) composites. The composites have been fabricated by pressureless sintering and hot-pressing. The best mechanical properties and the highest densities have been achieved for composites hot pressed at 1000 degrees C (2 h, 30 MPa in flowing Ar). Their density was in the range of 90-97% of the theoretical density. Fracture toughness (Klc) of the composites reflected their microstructure and had the value of 1.4 MPa m1/2 (as compared with Klc = 1.0 MPa m1/2 for the non-reinforced HAp matrix). Compressive prestressing of the HAp matrix and crack deflection (both derived from the residual stress field) contributed to the increase of fracture toughness. Other toughening mechanisms have not been observed. HAp/HAp (whiskers) composites exhibited improved toughness without degradation of biocompatibility, because the HAp whiskers acted both as a reinforcement and as a biocompatible phase. Problems related to biocompatibility and mechanical properties of available HAp-based composites were also discussed.