Toughening mechanisms in iron-containing hydroxyapatite/titanium composites.

Pure hydroxyapatite (HA) is brittle and it cannot be directly used for the load-bearing biomedical applications. The purpose of this investigation was to develop a new iron-containing HA/titanium composite via pressureless sintering at a relatively low temperature with particular emphasis on identifying the underlying toughening mechanisms. The addition of iron to HA/titanium composites led to a unique and favorable core/shell microstructure of Ti-Fe particles that consisted of outer titanium and inner iron, and good interfacial bonding with HA matrix. While the relative density, hardness and Young's modulus reduced, the flexural strength, fracture toughness, fatigue resistance, and the related fracture surface roughness increased significantly with increasing amount of Ti-Fe particles. Different toughening mechanisms including crack bridging, branching and deflection were observed in the composites, thus effectively increasing the crack propagation resistance and resulting in a substantial improvement in the mechanical properties of the composites.