Phase transformation in plasma electrolytic oxidation coatings on 6061 aluminum alloy

• Published in: Surface & coatings technology Vol. 251; pp. 106 - 114
• Main Authors: Dehnavi, Vahid, Liu, Xing Yang, Luan, Ben Li, Shoesmith, David W., Rohani, Sohrab
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25-07-2014; Elsevier
• Subjects: Alpha alumina; Applied sciences; Cross-disciplinary physics: materials science; rheology; Electrical parameters; Exact sciences and technology; Gamma alumina; Materials science; Metallic coatings; Metals. Metallurgy; Phase transformation; Physics; Plasma electrolytic oxidation; Production techniques; Surface treatment; Surface treatments; Phase transformation; Gamma alumina; Electrical parameters; Plasma electrolytic oxidation; Alpha alumina; Plasma; Aluminium base alloys; Phase transformations; Surface treatments; Coatings; Electrodeposition; Oxidation; Alumina
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2014.04.010

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.
•Electrical parameters affected phase distribution and composition of the coatings.•Coatings were composed of γ-Al2O3, varying amounts of α-Al2O3 and a little mullite.•Increasing the pulse on-time enhanced γ→α-Al2O3 phase transformation.•Formation of mullite in some samples was linked to the micro-discharge behavior.•XRD depth profiling suggested that mullite is concentrated near the surface of coatings.