Mechanical properties and microarchitecture of nanoporous hydroxyapatite bioceramic nanoparticle coatings on Ti and TiN

Hydroxyapatite (HA) based bioceramic materials are usually prepared at high sintering temperatures to attain suitable mechanical properties. The sintering process usually results in a material which is compositionally and morphologically different from the non stoichiometric nano-crystalline HA phase of hard tissue. At the same time, HA particulates used as precursors in ceramic manufacturing are often very similar to the natural HA nanocrystals. It has been shown that synthetic nanoparticle HA (nanoHA) based materials improve the biological response in vitro and in vivo, but information on the mechanical properties of these materials is scarce. In this work we studied HA nanoparticle (10 - 80 nm mean size) coatings with 30 - 70% porosity prepared by a dip-coating technique on Ti and TiN substrates. It has been found that the mechanical properties of HA nanoparticle coatings are strongly influenced by the initial size, morphology, and surface treatment of nanoparticles. The nanoindentation Young's modulus and hardness of as-deposited nanoHA coatings were in the range of 2.5 - 6.9 GPa and 80 - 230 MPa, respectively. The coatings were stable after annealing up to at least 600 °C, (with Young's modulus reaching 23 GPa and hardness reaching 540 MPa), as well as in simulated body fluids.