Formation mechanism of HA-based coatings by micro-arc oxidation

Hydroxyapatite (HA)-based coatings were prepared by micro-arc oxidation (MAO) in an aqueous electrolyte containing calcium acetate and β-glycerophosphate disodium at high applied voltage. The structure and evolution of phase components in the coatings as a function of duration time (1–20 min) were investigated. It was observed that some granules started to appear in the electrolyte adjacent to anode after MAO duration for about 5 min and become more in amount with prolonging duration time. The obtained results show that the coatings consist of HA- and TiO2-based two layers, in which the HA-based outerlayer contains small amounts of α-TCP (α-Ca3(PO4)2) and CaCO3 while the TiO2-based innerlayer contains CaTiO3. The granules are negatively charged amorphous HA, TCP and CaCO3. It is suggested that formation of the granules in the electrolyte adjacent to anode, electro-migration of the granules onto the pre-formed TiO2-based layer and sintering-caused phase transformation of the adhered granules are possible mechanism for HA-based layer formation.