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

Abstract

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.

Effects of Er2(SO4)3 doping on characteristics of MAO coating

“…The resulting molten material then repeatedly condensed and accumulated in the discharge holes, eventually forming ‘crater-shaped’ sinter disks. 8,21,22…”

“…The resulting molten material then repeatedly condensed and accumulated in the discharge holes, eventually forming 'crater-shaped' sinter disks. 8,21,22 Based on Figure 2(c, e, g, i), the dimension of the discharge micropores and sintered disks on the surface gradually shrank and the quantity of micropores dramatically increased with the addition of Er 2 (SO 4 ) 3 . This was due to the increasing voltage caused by the addition of Er 2 (SO 4 ) 3 provided energy for MAO process.…”

Hardness Distribution and Growth Behavior of Micro-Arc Oxide Ceramic Film with Positive and Negative Pulse Coordination

“…The duration of most discharge spots does not exceed ten microseconds. This phase indicates the surface passivation process and the initial formation of the alumina film [14,15].…”

“…The duration of most discharge spots does not exceed ten microseconds. This phase indicates the surface passivation process and the initial formation of the alumina film [14,15]. Stage II spans from 1 min to 21 min, during which the rate of voltage rise for both positive and negative electrodes diminish to less than one-fifth of that observed in the preceding stage.…”

Micro-Arc Oxidation in Titanium and Its Alloys: Development and Potential of Implants

“…This can be addressed by raising the cathodic current, leading to the adoption of alternating current and pulse power sources. There is ongoing research into soft spark discharge states, which emit less heat, reducing excessive and damaging discharges [75]. This fosters uniform electrode reactions, lowers energy consumption, and promotes coatings rich in crystalline phases.…”