Formation Mechanism And Abrasion Tests Of Micro-Arc Oxidation Ceramic Layers On Aluminum Alloy

Microarc oxidation treatment for aluminum alloys attracts more and more attentions from manufacture, and even it is placed into the acceptance specification of surface treatment for critical components in many seagoing vessels, automobile and other manufacturing enterprises, due to its environment protection feature of processing and layers with ceramic characteristic. However, because it is still indistinct to the mechanism about effect of solute elements in solution and pulse parameters in pulse electrical field environment on the formation and growth stage of microarc oxidation ceramic layers for aluminum alloys; there is no complete theoretical foundation for the development of processing curves about electrical parameters including pulse voltage, frequency, duty cycle, etc. varying with processing time during treatment in microarc oxidation solution systems. Firstly, although a great deal of research results indicate that inorganic salt, such as phosphate, silicate, carbonate, can be used as additives in microarc oxidation solutions, the ceramic layers obtained in these studies are mainly composed of alumina. It is believed that under the condition of microarc oxidation the dynamic of aluminum transforming to alumina only come from the oxidation reaction of aluminum atoms in molten microzones formed by microarc. In other words, aluminum atoms in molten microzones still take place oxidation reaction if only the applied electrical field can generate microarc on aluminum alloys surfaces in solution although there is no existence of incorporating solute elements. Therefore, understanding the action mechanism of solute elements plays an instructional role in the establishment of regulation for solution maintenance, because microarc does not generate without these solute elements in solution but adding them does not play a direct role in ceramic layer growth in theory. Secondly, as everyone known that microarc generates on aluminum alloy surface if only the applied voltage between stainless steel (cathode) and aluminum alloy (anode) in conducting solution is high enough. However, not only above 80% materials consumption cost of microarc oxidation treatment is electricity consumption, but also ceramic layers with coarse surface and loose microstructure are obtained if current is excessively strong. Thus, the investigation about the mechanism about effect of each pulse parameter on the formation and growth stage of alumina ceramic layers is beneficial to the optimization of microarc oxidation processing curves. In addition, although microarc oxidation can endow aluminum alloy surface with ceramic features, are there performance advantages in these ceramic layers in comparison with coatings formed by hard chromium plating and other techniques used nowadays?Based on above three aspects of theoretical problems, which restrict the application and development of microarc oxidation technique, in this study, microarc oxidation process was divided to two stages, namely, ceramic layer formation and growth stage; the impedance difference at film formation stage and compactness difference at layer growth stage during microarc oxidation were compared and analyzed in different solution system. These differences were led by variation of anions variety and concentration in electrolytic solution and pH value and so on. The microstructure and composition of ceramic layers were investigated by using XRD,SEM,EDS,XPS. The mechanism for effect of different anions on formation characteristics of oxide film at initial stage was quantified and discussed by measuring anodic polarization curves. The influence of pulse energy on phase composition and growth characterization of ceramic layers was investigated systematically. The results are as follows:Microarc oxidation process could be divided into two stages which are high-impedance film formation stage before microarc starting and ceramic layer growth stage after microarc starting. The formation of high-impedance oxide film on aluminum alloy surfaces was a prerequisite to microarc oxidation; the oxides could generate microarc discharging under the action of applied voltage. At the high-impedance oxide film formation stage, the rate of high-impedance film formation appeared significant differences due to the differences of polarization characteristics of aluminum alloy in different electrolytic solutions. The lower corrosion current, the higher corrosion potential, the easier formation of high-impedance oxide films when the electrochemical polarization carried out in electrolytic solutions. As a result, microarc oxidation in this electrolytic solution is ahead of that in other solution systems.The action of solute elements in electrolytic solution changed from improving the formation of high-impedance oxide film before microarc commenced to adjusting the solution conductivity after microarc commenced. It was found that growth rate of ceramic layers increased with the increasing of solution conductivity. However, the excessively high conductivity brought the formation of loose ceramic layers. There is no solute elements consumption during growth and thickening of ceramic layer.The formation of oxide films at initial stage before microarc discharging starting is different from that of normal anodic oxidation; the substance transmission of the former is primarily dependent on tunneling effect. After microarc discharge appearing, the growth and thickening of ceramic layers were mainly dependent on the procedure of direct combination of aluminum atoms via discharging channel with active oxygen ions generated in the environment of plasma, the formation of molten oxide, and then the formation of solid alumina ceramic by the quenching action of solutions. The oxygen atoms were primarily provided by OH" at the interface of aluminum alloys / solution or alumina / solution. As the continuous proceeding of oxidation, ionization equilibrium of H₂O was broken owing to the continuous consumption of H⁺ on cathode, which improved the ionization of H₂O. Therefore, the OH^- consumed during oxidation was recruited by that generated during ionization. On the other hand, the different adsorption characteristic of anions at the interface of aluminum alloy / solution and ceramic layer/ solution led to the different content of solute elements in ceramic layers. Although the difference did not influence the growth mechanism of ceramic layers, the ceramic layers with various properties could be prepared utilizing this characteristic.Phase composition of ceramic layers was primarily determined by the energy value of single pulse during microarc oxidation. The pulse energy value was controlled through adjusting voltage, current and pulse width during microarc oxidation. The larger pulse energy, the faster ceramic layer growth rate; and the metastable phaseγ-Al₂O₃ in ceramic layers gradually transformed to stable phaseα-Al₂O₃, but layer compactness decreased. Excessively high pulse energy in discharge channel caused the increase of discharge and gas pressures in channels. Consequently, molten alumina severely sprayed outward which resulted in the residual pores in discharge channels. It was the reason for the formation of loose layer in ceramic layers. Therefore, the microstructure and compactness of ceramic layers should be controlled by adjusting the magnitude of pulse energy at different stages during microarc oxidation.Large numbers of blind micropores which homogeneously distributed on microarc oxidation ceramic layers were beneficial to the formation of continuous oil films under the condition of lubricated sliding wear. In comparison with the results of wearing test of hard chromium plating and P-V-Cu wear resistant case iron, it was found that microarc oxidation treatment notably improved wear resistance of aluminum alloys. However, if the stress on contact points between ceramic layer surface and opposite wearing couple was much larger, microarc oxidation ceramic layer appeared local fracture, separation and formation of abrasive particles. As a result, wear-out-failure of microarc oxidation ceramic layers was accelerated.