Soft Sparking Discharge Mechanism of Micro-Arc Oxidation Occurring on Titanium Alloys in Different Electrolytes

Soft sparking during micro-arc oxidation can form a ceramic coating with high hardness and high bond strength on titanium alloy while avoiding the continuous strong micro-arc that can damage the substrate properties and the integrity of the coating. Existing studies have reported that the soft spark discharge is significantly influenced by the electrolyte anions, and the detailed mechanism of its influence remains unclear. Therefore, we considered four monolithic electrolytes, namely Na2B4O7, NaF, Na3PO4, and Na2SiO3, for the bipolar pulsed micro-arc oxidation (MAO) treatment of the Ti6Al4V alloy to investigate the mechanism of the soft sparking discharge and the affections of different electrolytes on the soft sparking discharge. The results showed that soft spark discharges were observed in both Na2SiO3 and Na3PO4 electrolytes while not in Na2B4O7 and NaF electrolytes. We attributed this situation to the fact that the deposition of Si and P elements in the coating changed the structure and passivation ability of the coating and affected the rate of ion transport and electron tunneling in the coating, resulting in forming a thick and dense, soft spark MAO inner layer. Additionally, the soft sparking discharge facilitated particle deposition and did not destroy the structure of the initial film layer, and also had no significant effect on the corrosion resistance.