Effects of Electrolyte Compositions and Electrical Parameters on Micro-Arc Oxidation Coatings on 7075 Aluminum Alloy

Aluminum alloys are widely used in a variety of industries nowadays for their high strength-to-weight ratio, good formability, low density, and recyclability. However, their poor corrosion and wear resistance properties restrict their applications. This study investigated the effects of electrical parameters and electrolyte compositions on the microstructures of micro-arc oxidation (MAO) film on a 7075 Al alloy substrate. The morphology, microstructure, and compositions of the MAO coatings were characterized using a scanning electron microscope (SEM), X-ray diffraction (XRD), and an electron probe micro-analyzer (EPMA). Furthermore, measurements of microhardness, corrosion resistance, and wear resistance were also conducted. The cathodic current and duty ratio are proportional to film thickness, which consequently improves the wear and corrosion resistance. The microstructural observations of the aluminate-based coatings revealed that increasing cathodic current reduces the pancake-like structures, and a lot of small pores appear on the top of the coatings, which makes the surface smoother. Moreover, the aluminate-based coatings are mainly composed of α-Al2O3 and γ-Al2O3, while the silicate-based coatings mainly consist of γ-Al2O3 and a small amount of α-Al2O3 phase. Due to the phase compositions, the microhardness of the aluminate-based coatings can reach 1300~1500 HV and exhibit better wear resistance than silicate-based coatings.