Effect of TiO ₂ doping on densification and mechanical properties of hydroxyapatite by microwave sintering

Hydroxyapatite (HAP) possesses excellent bioactivity/osteointegration properties. Nevertheless, its inferior flexural strength and fracture toughness limit its use in human weight-bearing parts. We investigated a microwave sintering technology which can be effectively used to develop titanium dioxide-hydroxyapatite (TiO ₂ -HAP) ceramics with different amounts of TiO ₂ (0.8,1.6,2.4,3.2,4.0,4.8,5.6 and 6.4 wt%), which contribute to extremely high flexural strength (90–130 MPa) along with a good combination of elastic modulus and fracture toughness. The results of the Rietveld refinement show that multiphase bioceramics (HAP, β-TCP) can be achieved by doping nano-TiO ₂ under microwave sintering. Despite the fact that the main phases of the sintered TiO ₂ -HAP ceramics are HAP and β-TCP, X-ray diffraction confirms the formation of the CaTiO ₃ and CaTi ₂ O ₄ (OH) ₂ phases. Furthermore, the sintering reactions to form these phases are discussed and the results show that an appropriate amount of nano-TiO ₂ can not only effectively inhibit the growth of grain, but also change the fracture mode and increase the relative density. Finally, it is found that doping nano-TiO ₂ by microwave heating is an effective technique for producing HAP/β-TCP composite load-bearing implants in clinical applications without coarsening the size of grain.