Microstructure and bioactivity of Ca, P and Sr doped TiO₂ coating formed on porous titanium by micro-arc oxidation

In the present study, bioactive coatings enriched with micro-pores and Ca-P-Sr elements were formed on pore walls of porous titanium by micro-arc oxidation (MAO). It is found that pore size plays a significant role on the MAO treatment of porous titanium. For samples with pore size smaller than 90μm, whatever applied voltage and treatment time were employed, MAO coatings were formed only in the near surface region. As to the sample with average pore size of 150μm, MAO coatings were formed on both outer and inner pore walls throughout the depth. Compared with the untreated one, the specific surface area dramatically increased about 460 times. Further studies found that pore size, thickness and amounts of O, Ca, P and Sr elements of the coating on the outer pore walls were obviously higher than those on the inner pore walls. Additionally, the coating on the outer pore walls was composed of anatase and rutile TiO₂ and other complex Ca-P-Sr phases, in comparison with anatase TiO₂ formed on the inner pore walls. In spite of the distinct features of coatings on the different locations of pore walls, MAO-treated porous titanium overall showed a good apatite-inducing ability.