Phase transformation in plasma electrolytic oxidation coatings on 6061 aluminum alloy

Oxide coatings were produced on a 6061 aluminum alloy using a pulsed unipolar plasma electrolytic oxidation (PEO) process. The effect of electrical parameters including pulse frequency, duty cycle and current density on phase formation in the coatings was revealed using conventional and glancing angle X-ray diffraction. The results show that PEO coatings are mainly composed of γ-Al2O3. Depending on the electrical parameters employed, the coatings can also contain α-Al2O3 and mullite with varying concentrations. Higher current densities and higher duty cycle were found to favor the formation of mullite. Under the experimental conditions used, the ratio of the integrated XRD peaks for α- and γ-Al2O3 varied from 0 to about 0.6, indicating that the relative content of α-Al2O3 in the PEO coatings varied over a wide range. Longer pulse on-times and higher current densities promoted the gamma to alpha-alumina phase transformation. Depth profiling of PEO ceramic coatings using glancing angle XRD with different incident beam angles revealed that mullite was more concentrated in the top surface of the coatings. No significant variation in α-Al2O3 concentration across the coatings could be concluded in this study, unlike the results of some other studies.