Research On Process And Coating Formation Mechanism In Plasma Electrolytic Oxidation Of ZL101

Cast Al-Si alloys are widely used due to their good mechanical properties and castablity. However, the application of Al-Si alloys is frequently restricted by their low hardness and poor tribological properties. By use of plasma electrolytic oxidation (PEO), the ceramic oxide layers, with high insulating properties, hardness, wear resistance, corrosion resistance anti-oxidizing, allows to produce on Al-Si alloys. It is very significant for Al-Si alloy to improve the performance of surface and extend the application.The effect of electrolyte and electric parameter on constitute microstructure and properties of PEO coatings on ZL101 alloy was investigated in this paper. The chemical reaction in PEO process and the formation mechanism of PEO coating were studied by analyzing initial PEO coating. The following results were obtained.The concentration of KOH and Na2SiO3 in the electrolyte has great effect to the PEO process of ZL101 alloy. With increasing the KOH concentration, the break voltage of PEO process descends the PEO reaction progresses easily. When KOH concentration was more than 3g/L, the PEO reaction becomes drastic. This is a great disadvantage to growth of the PEO coatings. As the concentration of Na2SiO3 in electrolyte increases, the current and voltage have a tendency to increase. The rational concentration of KOH and Na2SiO3 in electrolyte is 3g/L and 15g/L respectively.The results of EDX and XRD show that, the chemical phase of PEO coatings of ZL101 alloy isγ-Al2O3,α-Al2O3, SiO2 and mullite(3Al2O3·2SiO2). With the prolonging of PEO processing time, the content of mullite increases and theγ-Al2O3 transforms to theα-Al2O3 gradually. The results of image analysis of PEO coatings show that, in the constant voltage of 300V, the coating is thinner and has many holes with small diameter. The holes have uniform distribution over coatings surface. But, in the constant voltage of 400V, the coatings are thicker and have few holes with large diameter. The holes have not uniform distribution over coatings surface.The corrosion resistance of ZL101 alloy in the solution of NaCl and HCl, is remarkably improved by PEO coatings. The optimal corrosion resistance is obtained at the condition of KOH concentration of 3g/L and current density of 12A/dm2 The PEO coatings also improves the anti-oxidizing of ZL101 in the high temperature. By the experiment with the 550℃/8h, the coatings don't blister and break off.In the initial stage of PEO, the coatings form in the two phase ofα(Al) andβ(Si) at the same time. The two phases has a great deal of holes like the volcano. Along with the processing time, the covering area of coatings extends and the amount of hole reduces. The PEO process undergoes the anode oxidation, the oxide film breakdown and plasma discharging, and takes place a series of complicated chemical and electrochemical reaction under the condition of plasma discharging. The ceramic coatings containing Al2O3 and SiO2 formed on surface of ZL101 alloy.