Preparation and formation mechanism of copper incorporated micro-arc oxidation coatings developed on Ti-6Al-4V alloys

Micro-arc oxidation (MAO) was used to fabricate light-red coatings with a maximum Cu amount of 3.06 wt% on Ti-6AI.4V alloys in electrolytes composed of Na2CuY, KOH, and phytic acid. The influences of electrolyte composition and concentration as well as the electrical control mode on coating properties were investigated. In addition, the cathode surface after MAO treatment was first characterized. The results show that the used electrolytes determine the coating composition and surface morphology through influencing the solution pH and conductivity. During MAO, copper is simultaneously involved in the formation of the MAO coating on the anode and the precipitation of metallic Cu on the cathode. The copper content in MAO coatings can be improved slightly by Na2CuY and significantly by KOH attributed to their influences on solution pH. Phytic acid is beneficial in developing MAO coatings, but at higher concentrations up to 4 g/L decreases the copper content. The copper in MAO coatings exists primarily in the form of Cu2O, while metallic copper particles in the range of 150-600 nm are deposited on the cathode through the reduction reaction. Compared with the constant current density mode, the fabricated MAO samples under the constant voltage mode contain a lower content of Cu and more evident microcracks.