Development and optimisation of hydroxyapatite-ß-TCP functionally gradated biomaterial.

Use of hydroxyapatite (HAP) for biomaterials is widely established, often in a combination with titanium alloy substrates in orthopaedic and other implants. Porous HAP-based coatings undergo sintering and heat treatment processes to achieve proper level of density yet avoiding undesirable reactions and phase changes. Thermal expansion mismatch between constituents may also lead to cracking of the coating due to excessive thermal stresses. Thus control of mechanical properties and stress state in the coating during the whole processing cycle represents some challenges when trying to combine conflicting objectives. In this work a method of optimisation of mechanical behaviour of an FGM biomaterial coating or scaffold composed of HAP and beta-tricalcium phosphate (ß-TCP) is considered. For coating on titanium-based substrates which have anatase (TiO2) top layer, thermodynamic equilibrium was analysed and the "safe" compositional path was found to ensure preservation of original ceramic phases during processing. Experimental studies of HAP-TCP sintering kinetics of different compositions were carried out with dilatometry to obtain true shrinkage curves and thermal expansion functions. These data were integrated with thermo-mechanical calculations of stresses and strains in HAP-ß-TCP FGM during processing, which were optimised with respect to minimal stresses, stresses derivatives and curvatures. The beneficial effect of compositional gradation on behaviour of these materials is shown.