Comparative investigation of hydroxyapatite coatings formed on titanium via phosphate chemical conversion

Phosphate chemical conversion coatings have attracted much attention on surface modification of metallic implant, due to their in-situ growth pattern and good mechanical performances. Nevertheless, it is difficult to use this method for preparing a pure hydroxyapatite (HA) coating. In this study, a pure HA coating was prepared on titanium (Ti) in an acidic phosphate aqueous via a simple single-step chemical conversion without alkali post-treatment. The microstructure, chemical composition, electrochemical and mechanical performances of the HA coating were characterized in comparison with those of a precursor dicalcium phosphate (CaP) coating which was alkali post-treated (CaP-OH). Results revealed that the one-step chemically converted HA coating was homogeneously nano-structured with higher phase purity and crystallinity. Moreover, the HA coating was thicker (~6.5 μm) than the CaP-OH one (~40 μm) but with better mechanical properties and corrosion resistance. The results encouraged the direct preparation of crystalline HA coating with superior properties on surface of Ti via a single-step phosphate chemical conversion. •A one-step preparation of HA coating on titanium is proposed using chemical conversion technique.•The HA coating is nanostructured with higher phase purity and crystallinity compared to the two-step prepared one.•The HA coating performs better in the mechanical and electrochemical characteristics.•Both the nucleation and morphology of the HA crystals are influenced by the ultrasonic irradiation.