| Micro arc oxidation(MAO) is a surface modification technique which can be employed on valve materials including aluminum, magnesium, titanium and their alloys. The whole process is simple and environmentally friendly coupled with coating merits of good adhesion, high hardness, excellent wear and corrosion resistance, while the discharging phenomena during the MAO process are still hard to explain and few models can be built to direct the discharging model. Not only coating efficiency of MAO method deserves a further development, but also tip discharging and area effects which limit the application of MAO should also be restricted. To this end, the present work studied the discharging process carefully, especially the submerged discharging behavior in order to apply the technology reasonably and avoid its shortage.MAO dischaargeing and coating growth process was studied. The existence of micro molten pool which could lead the phenomenon of submerged discharging was elucidated by conducting experiments. Furthermore, the successively appearance of electrochemical discharging and plasma discharging was also demonstrated. The study which focused on both electrochemical discharging and plasma discharging indicated that the behavior of micro pore gas discharging is the essence of discharging and coating formation, and the contradiction between gas discharging and the ability of energy transmission of electrolyte cause the tip discharging and area effect. Based on this, we proposed scanning MAO processing method.Scanning MAO system was built. The power supply and servo system which are provided with shortcut protection and electric arc control were invented through analyzing discharging characteristics and discharging gap. A pulse power supply which is composed of PMW chopping wave and full-bridge inverter circuit was invented and human-computer interface, an auxiliary servo system containing the functions of detecting, controlling and power amplification and circulating system with inner flushing structure were designed. The discharging behavior is limited to the narrow space between the anode and pipe cathode and the current density is increased to hundred times as much as traditional MAO increasing the discharging efficiency and eliminating the tip discharging and area effects.Scanning MAO technical were studied. Based on the analysis of MAO discharging and coating growth aforementioned, the effects of parameters including electrodes-voltage, pulse frequency, duty ratio, condenses of the electrolyte and the disciplines of coating formation were studied.In addition, the special characteristics of scanning MAO method such as discharging interval, the number of scanning, speed and overlap rate were determined and their influences on coating performance were discussed.The discharging method and coating formation characteristics of scanning MAO process was studied. The voltage-current response of scanning MAO method is different from that of conventional MAO process: the voltage increases spontaneously to the stable discharging area after being charged without experiencing the passivation stage of anodizing of traditional MAO. However, the region outside the discharging region suffers the process of anodizing and passivation which demonstrates the intrinsic reason of higher efficiency of scanning MAO method. The discharging area on anode is constrained to the projection area of cathode. The growth of the coating at discharging area is as soon as the electrodes are charged, and then the whole process will conduct by moving the cathode, which is different from the coating growth process of conventional MAO technique. The values of scanning is a critical parameter for scanning MAO method, although it only has a mild influence the surface quality, while the coating thickness at the transition area is significantly affected by the overlap rate.The surface and cross-section morphologies and coating properties of scanning MAO were studied, which focused on micro structure, hardness, roughness and wear and corrosion resistance. The SEM results indicate that the coatings which are treated with single scanning are comparably thin with small and uniformly distributed pores on the top surface, while the multi-layer structure is obviously after multi scanning. The multi-layer structure is consisted of compact layer and porous layer, where the compact layer takes up around two thirds of the whole coating thickness and the number of the micro pore is few but with large diameter. The XRD results illustrated that the percentage of α-Al2O3 is enhanced through multi scanning. Also the laser confocal results exhibited that the roughness of the coating is uniform. The corrosion test demonstrated that the corrosion resistance of coating treated by unidirectional pulse power is better than the coating prepared by bidirectional power. The value of friction coefficient is smaller under conditions of low speed, high speed, low loading and high loading, while that of multi scanning is more stable indicating a better wear resistance. Through the systematic theoretical and experimental study of coating formation mechanism and properties of scanning MAO method, it could be concluded that scanning MAO method is effective. |
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