Micro arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), which is applied to obtain hard oxide coatings on aluminum alloys. The coatings formed during the MAO process are superior to anodic oxide coatings on plenty of properties, such as strong coating-substrate adhesion, adequate thickness, high hardness, excellent corrosion and wear resistance. There are several parameters affecting the properties of MAO coating, including composition of the substrate and electrolyte, process time, applied voltage and current density, but few works study the influence of cathodic current on MAO coating.

This study investigated the effect of cathodic current density on the structures and properties of the coatings. The MAO coatings on 7075 Al alloys were obtained in aluminate-based electrolyte by using a bipolar pulsed power supply. The morphology, microstructure and compositions of the MAO coatings were characterized by using scanning electron microscope (SEM), X-ray diffraction (XRD), electron probe micro-analyzer (EPMA). The measurements of surface roughness, thickness, and microhardness were also conducted. Furthermore, the ball-on-disk tribometer and potentiodynamic polarization were used to evaluate the wear and corrosion resistance, respectively.

Experimental results showed that the ceramic coatings are mainly composed of α-Al₂O₃ and γ-Al₂O₃. The microstructural observations of the MAO coatings revealed that with increasing cathodic current, the presence of pancake-like structure and the size of micro-pores are significantly reduced, meaning the coatings became smoother and more compact. Moreover, due to the low porosity and the phase composition, the microhardness of the ceramic coatings can reach ~1400 HV. With increasing cathodic current, the MAO coatings are nearly pore-free and crack-free. Therefore, the coatings exhibit an obvious improvement in corrosion and wear resistance.