Investigation on entrance mechanism of calcium and magnesium into micro-arc oxidation coatings developed on Ti-6Al-4V alloys

Micro-arc oxidation (MAO) coatings containing both calcium (Ca) and magnesium (Mg) elements and possessing a proper coating porosity are desirable for surface treatment of medical Ti-6Al-4V alloys. The influences of Na2CaY, Na2MgY, KOH, and final voltage on the Ca content, Mg content, and porosity of MAO coatings developed on Ti-6Al-4V alloys were investigated by an orthogonal experiment under constant voltage control mode. The results indicate that Ca and Mg separately enter into MAO coatings primarily via diffusion, and the competition between these two elements is relatively insignificant. The increasing KOH concentration can increase both Ca and Mg amounts in MAO coatings. High solution pH, which increases with increasing KOH, Na2CaY or Na2MgY concentrations, favors formation of MAO coatings. At the same processing condition, the Ca amount in MAO coatings is higher than the Mg amount, which may be mainly attributed to the smaller solubility product constant of Ca3(PO4)2 than that of Mg3(PO4)2. Final voltage is beneficial to coating formation and has a small impact on Ca or Mg contents but is the most important factor governing coating porosity. With the increasing final voltage, the coating porosity firstly increases from 300 to 350 V, but exhibits a decreasing trend after 350 V.