In order to improve the high-temperature oxidation resistance of refractory high-entropy alloys (RHEAs), we used micro-arc oxidation (MAO) technology to prepare ceramic coatings on AlTiCrVZr alloy, and the effects of voltage on the microstructure and high-temperature oxidation resistance of the coatings were studied. In this paper, the MAO voltage was adjusted to 360 V, 390 V, 420 V, and 450 V. The microstructure, elements distribution, chemical composition, and surface roughness of the coatings were studied by scanning electron microscopy (SEM), energy dispersive (EDS), X-ray photoelectron spectroscopy (XPS), and white-light interferometry. The matrix alloy and MAO-coated samples were oxidized at 800 °C for 5 h and 20 h to study their high-temperature oxidation resistance. The results showed that as the voltage increased, the MAO coating gradually became smooth and dense, the surface roughness decreased, and the coating thickness increased. The substrate elements and solute ions in the electrolyte participated in the coating formation reaction, and the coating composition was dominated by Al₂O₃, TiO₂, Cr₂O₃, V₂O₅, ZrO₂, and SiO₂. Compared with the substrate alloy, the high-temperature oxidation resistance of the MAO-coated samples prepared at different voltages was improved after oxidation at 800 °C, and the coating prepared at 420 V showed the best high-temperature oxidation resistance after oxidation for 20 h. In short, MAO coatings can prevent the diffusion of O elements into the substrate and the volatilization of V₂O₅, which improves the high-temperature oxidation resistance of AlTiCrVZr RHEAs.

Micro-arc oxidation (MAO) ceramic coatings were prepared on TC4 titanium alloys by adding CuSO₄ to a (NaPO₃)₆ base solution. The microstructures of the MAO coatings were characterized by scanning electron microscopy (SEM), energy dispersive (EDS), and X-ray photoelectron spectroscopy (XPS). The corrosion resistance and wear resistance of these coatings were evaluated via hydrochloric acid immersion of weight deficit and friction tests. Those results indicated the presence of Cu in the MAO coating in the form of CuO and Cu₂O. Incorporation of CuSO₄ results in a thickness and roughness increase in the coating. The coating has a lower coefficient of friction (0.2) upon the addition of 4 g/L of CuSO₄. The antibacterial properties of the MAO coatings were maximized at 6 g/L of CuSO₄. However, the corrosion resistance of the copper-doped MAO coating did not exceed the undoped coating. This study shows that the addition of CuSO₄ to the electrolyte successfully prepared copper-containing micro-arc oxidation coatings, which improved the wear resistance and antibacterial properties of the coating.

Novel composite coatings were fabricated on 6061 aluminum alloy substrates by two steps combining micro-arc oxidation (MAO) plus electroless copper plating (Cu). Different MAO + Cu composite coatings were compared. Cu continuously and evenly covered an aluminum oxide surface during processing thus changing the surface topography. The adhesion of MAO + Cu composite coating was tested by the pull-out method. The best adhesion strength of the composite coating can reach industrial requirements. The effects of the composite coatings on friction were investigated using a ball-on-disc test method. It is found that copper in composite coatings plays a lubricating effect during the wear process under dry sliding. Furthermore, the mechanisms by which the observed advantages were produced are discussed.

In this paper, MAO-Cu/Cu-(HEA)N composite coatings on TC4 titanium alloy were prepared by combining micro arc oxidation (MAO) with magnetron sputtering (MS) to enhance the wear resistance and antibacterial ability of the substrate in simulated seawater. The number of micropores on the surface of the composite coatings decreased with increasing CuSO₄ concentration in the electrolyte, causing the surfaces to be flat and smooth. XPS and EDS analyses revealed that the MAO-Cu/Cu-(HEA)N composite coatings predominately contained TiO₂, Cu₂O, and (HEA)N. Moreover, the addition of CuSO₄ increased the growth rate of the MAO coatings. Comparatively, the MAO-Cu/Cu-(HEA)N composite coating with 5 g/L CuSO₄ showed superior wear resistance, reduced friction coefficient (approximately 0.2), and shallow and narrow grinding cracks were observed compared to the other coatings. Antibacterial experiments showed that the MAO-Cu/Cu-(HEA)N composite coatings had better bacterial killing effects than the TC4 substrate, which is of great significance to the antifouling abilities of titanium alloys in marine applications.