Preparation And Properties Of Black Micro-arc Oxidation Coatings On Aluminium Alloys

Aluminium alloys are widely used in lightweight manufacturing industry due to their advantages of low density, high strength to weight ratio, easy processing and high conductivity. However, their shortcomings, such as poor corrosion resistance,easy aging and monotonous color, have seriously restricted their extensive applications in some special areas. Employing plasma electrolytic oxidation（PEO）technique by using different coloring ions can deposit different ceramic alumina coatings on Al alloys, which greatly improves their corrosive and aging properties.The black coatings, which have high light and infrared radiation absorptance, are widely used in aerospace, automobile, optical devices and architectural decoration industries. But, black ceramic coating has poorer adhesion. Aiming at improving the adhesion property of black ceramic coatings on aluminium alloys, the following studies were carried out in this work.Firstly, the coloring rule of different coloring ions(Co²⁺, Cu²⁺, WO₄^2-, VO₃^-) in the plasma electrolytic oxidation process and the adhesion property of coatings formed by those ions were studied. Different colored ceramic coatings were prepared by using same molar concentration and negative voltage in the invariableness current modal. The microstructure and composition of the coatings were measured by scanning electron microscopy（SEM） and energy dispersive spectrometry（EDS）. The adhesion strength of the coatings was studied by thermal shock test. The results show that the concentration of coloring ions have a great influence on the color and the adhesion strength of the coatings. With the increase of the concentration of coloring ions, the colors of the ceramic coatings become deeper and the adhesion strength of the black ceramic coating become weaker formed in the same condition.Secondly, the black ceramic coatings were prepared on Al alloys by plasma electrolytic oxidation（PEO） in silicate electrolyte containing NH₄VO₃. The optical property and adhesion strength of the coatings were studied by spectrophotometer and thermal shock test. The forming process of the coatings was investigated by scanning electron microscopy（SEM） and energy dispersive spectrometry（EDS）. The results show that the black ceramic coatings have high light absorptance, but the surface layer is easy to peel off. The forming process of black PEO coatings can be divided into two phases. At first, the oxidation of substrate plays a predominant role, forming a thin layer of alumina which appears gray and adheres well to the substrate. And then,the compound deposition in the electrolyte plays a leading role. The color of coating is further deepening, and the delamination is occurred with the previous coating,which leading to deterioration of adhesion.Thirdly, the adhesion strength of the black ceramic coating was enhanced by adjusting the negative voltage and optimizing electrolyte during the PEO process. The microstructure and composition of the coatings were measured by scanning electron microscopy（SEM） and energy dispersive spectrometry（EDS）. The results show that with the improvement of the content of Na₃PO₄, the adhesion strength of the black ceramic coating improves notably; the increase of negative voltage can improve the adhesion strength of the black ceramic coating and make the color of black coating become lighter; Periodical increase of negative voltage can improve the adhesion strength of the black ceramic coating and keep the black ceramic coating has high light absorptance.