Effects of anodic and cathodic current densities on microstructure, phase composition and properties of plasma electrolytic oxidation ceramic coatings on 6063 aluminum alloy

Ceramic coatings on 6063 aluminum alloy were prepared by plasma electrolytic oxidation (PEO) using pulsed bipolar power supply under different anodic and cathodic current densities. The scanning electron microscope (SEM), x-ray diffraction (XRD), potentiodynamic polarization curves analysis and tribometer were used to investigate the microstructure, phase composition, corrosion and wear resistance properties of PEO coatings. It is found that with the anodic and cathodic current densities increasing, the whole layer thickness increased while the rate of inner compact layer increased and then decreased. The proportion of α-Al₂O₃ phase in PEO coatings increased when anodic current density increased. However, higher cathodic current density would result in less α-Al₂O₃ phase in the coating. All these phenomena can be explained by the effects of anodic and cathodic current on PEO process. Higher anodic current density would enhance the discharge reaction and generate more heat and higher temperature, which are the essential conditions for γ-Al₂O₃ phase transforming to α-Al₂O₃ phase. But it also damages the ceramic coatings, causing large voids and microcracks in PEO coatings. Certain level of cathodic current can drive Al³⁺ losing in the electrolyte back to the oxide-electrolyte interface and inhibit the large size sparks, which favors the growth of ceramic coatings. However, too high cathodic current would inhibit the anodic process and heat generation, so that the rate of compact layer and proportion of α-Al₂O₃ phase decrease. Furthermore, higher rate of compact layer and larger proportion of α-Al₂O₃ phase in the ceramic coatings significantly improve the corrosion and wear resistances. It is possible to obtain the desired structure, composition and properties of PEO coatings by controlling anodic and cathodic current densities. (paper)