Research Progress on Micro-arc Oxidation Colored Film Preparation Technology
With the development of ultra-thin, ultra-light, and miniaturization of electronic products, more and more alloy materials such as aluminum, magnesium and titanium were used in 3C fuselage structural parts. Electronic products, especially the casings of laptop computers and mobile phones, not only require high surface hardness and good wear resistance, but also require gorgeous and exquisite appearance, for which reason micro-arc oxidation colored films have become a research hotspot.In this paper, coloring methods of micro-arc oxidation coatings were firstly summarized, including changing the composition of electrolyte as adding inert colorant or transition metal salt and changing the composition of alloy as doping transition metal elements, etc. Among which, adding transition metal salts to the electrolyte was the most used, for which reason, a variety of works preparing micro-arc oxidation coating in black, brown, blue, and other colors in this way were reviewed subsequently. In the research of black and brown micro-arc oxidation coatings, the transition metal salts were converted into their corresponding oxides and then melted and deposited on the alloy matrix during the MAO process. The color of coatings came from their corresponding transition metal oxides.In the research of blue micro-arc oxidation coatings, although method was the same as that mentioned above, with the addition of transition metal salts, the reason for the colored coatings was disparate. The first research reported the matrix Al2O3 formed a bluish solid solution with Ti3+ iron added that colored the coating blue; another research reported the matrix Al2O3 combined with Co2+ to form the cobalt blue that colored the coatings blue.In the research of green micro-arc oxidation coatings, although the addition of different substances (Cr2O3 and K2Cr2O7) containing the same transition metal element Cr as colorants, the green coatings were both prepared and illustrated that there were different ways to color the coatings even with the same element.In the research of yellow micro-oxidation coatings, a great novel method was born that adding Na2SnO3 containing the element Sn that was in the same main group as Si to the basic Na2SiO3 electrolyte, and the SnO32− reacted in the same way as SiO32− in the micro-oxidation process that formed the yellow SnO2 coloring the coatings. Na2SnO3 also colored the coatings yellow despite the Sn element was not the transition metal element.During the above-mentioned coloring introduce, parameters that might affect the coating color were also briefly discussed according to the published research, including temperature, electrolyte composition and electrical parameters, etc. This article focused on an overview of the micro-arc oxidation coloring principle, coloring methods and currently studied color types, while also summarized the shortcomings of the micro-arc oxidation coloring film layer such as low glossiness, dull color and poor decorative effect.In the future, the development direction of micro-arc oxidation coloring films is to conduct in-depth research on the micro-arc oxidation coloring mechanism, improve the micro-arc oxidation electrolysis system, and explore the influence of process parameters to improve the decorativeness of the colored film and expand the color types of the micro-arc oxidation film until it can meet the application needs of more scenes.
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