Formation And Growth Mechanism Of MAO Coating On Magnesium Alloy Based On The Current Control Mode

Micro-arc oxidation surface treatment technology (MAO) is able to in-situ formceramic coatings on the surfaces of aluminum, magnesium, titanium and their alloys by aplasma electrochemical method to enhance surface properties, such as strength, hardness,wear and corrosion resistance. Energy consumption of MAO process is constantly changing,so the rational allocation of power energy at different growth stage is not only beneficial tothe MAO coating growth, but also helpful for reducing energy consumption effectively.Based on the above research background, this paper firstly studied a single constantcurrent mode, and then put forward stepped decreasing current mode. MAO coatings wereformed on ZK60magnesium alloy samples in a dual electrolytes system composed ofsodium silicate and phosphate. Surface and cross-section morphology, and phase structureof MAO coatings were investigated using scanning electron microscopy, energy dispersivespectroscopy, confocal laser scanning microscopy and X-ray diffraction. Furthermore,immersion test and friction wear test were employed to evaluate corrosion and abrasionresistance, as well as surface roughness and hardness of the coatings. The growth processand formation mechanism were in-depth investigated, achieving a breakthrough in theresearch on the growth structure of MAO coating.The influence of the positive current in the single constant current mode on theforming process and properties of MAO coating was systematically investigated. When thecurrent was small (0.6A,0.8A,1.0A,1.2A), MAO process was stable resulting in a coatingwith good comprehensive performance. On the other hand, the MAO coatings produced bylarge currents (1.4A、1.6A、1.8A) showed poor corrosion resistance due to accumulation offused mass on the coating surface. Hence, small current is beneficial to the improvement ofcoating quality and saving energy.Then，the formation mechanism and film structure were investigated in the MAOprocess under the single constant current mode. First, according to the voltage–time curveand micro-arc discharge variation, the growth process of MAO was divided into five stages:(1) stage Ⅰ when anodic oxidation stage,(2) stage Ⅱ when the formation of micro-arcoxidation stage,(3) stage Ⅲ when rapid growth of micro-arc oxidation stage,(4) stage Ⅳwhen local growth of micro-arc oxidation stage, and (5) stage Ⅴ when film restorationstage. StageⅠis a arc process, while the MAO coating thickness is determined by stage Ⅱ and stage Ⅲ. In stage Ⅳ, local large spark discharge leads to defects of MAO coating, suchas ablation and dissolution. However, the defects could be repaired by the relatively lowvoltage in stage Ⅴ. Second, it was found that MAO coatings on both sides of ZK60samplegrew together in the initial of MAO process, but evolved asynchronously in the late ofMAO process, and finally consistent. For MAO coatings on both sides of the ZK60samplein MAO process, although the coating growth processes were different, the final propertieswere similar. The difference of surface elements and phases in every growth stage indicatedthat the elements and substances were transferring and changing continuously in thereaction process.Based on the above researches, two-step, three-step and four-step decreasing currentmodes were systematically investigated. The results showed that step decreasing currentmode could prolong the stage Ⅲ of MAO process, promoting the breakdown and growth ofMAO coating. Meanwhile, the step decreasing current showed a stable transition leading toa uniform and smooth MAO coating. Besides, the voltage of stage Ⅴ in step decreasingcurrent mode, which was higher than that in single constant current mode, could effectivelyrepair the MAO coating. MAO process was optimized by studying different steps andaccommodative amplitudes in step decreasing current modes. It was found that the two-stepdecreasing current mode (1.2-0.6A) was the optimal one, which resulted in a significantimprovement of MAO coating with enhanced hardness, corrosion and abrasion resistance,and surface quality. In addition, the applied current density was effectively reduced by thetwo-step decreasing current mode (1.2-0.6A), leading to saving energy.Finally, the properties of optimized MAO coating under two-step decreasing currentmode (1.2-0.6A) were tested by surface morphology, Vickers hardness, micro scratch,immersion, electrochemistry, friction and wear experiments, and heat-differential thermalanalysis. The results show that, Compared with the ZK60substrate, the hardness of MAOcoatings formed under optimized parameters is improved obviously which was about9times. It has a good mechanical properties, excellent corrosion and abrasion resistance,high insulation property and thermal stability.