Characteristic of microarc oxidized coatings on titanium alloy formed in electrolytes containing chelate complex and nano-HA

Microarc oxidized (MAO) TiO₂-based coatings containing Ca and P on titanium alloy were formed in electrolytes containing nano-hydroxyapatite (nano-HA), calcium and phosphate salts. The effects of HA concentration on the thickness, micropore size and number of the MAO coatings were not pronounced. However, the surfaces of the MAO coatings become rough and the crystallinity of anatase increases with increasing HA concentration. In addition, the Ca and P concentrations on the surfaces of the MAO coatings decrease, since the chelate complex of CaY²⁻ (Y = ₂(OOC)NCH₂CH₂N(COO)₂]⁴⁻) and phosphate ions are hindered to be incorporated into the MAO coatings by HA. In vitro experiments indicate that the apatite-forming abilities of the MAO coatings decrease with increasing HA concentration. Furthermore, with increasing HA concentration, the solubility of Ca and P of the MAO coatings decreases, which could lower the supersaturation of the SBF with respect to apatite near the surfaces of the MAO coatings, further leading to the decreased apatite-forming ability. The results indicate that the HA addition in the electrolytes has an important effect on the structure and in vitro bioactivity of the MAO coatings.