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.

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Phase transformation in plasma electrolytic oxidation coatings on 6061 aluminum alloy

Fabrication and characterization of hydrophobic microarc oxidation/poly-lactic acid duplex coating on biodegradable Mg-Ca alloy for corrosion protection

Study plasma electrolytic oxidation process and characterization of coatings formed in an alumina nanoparticle suspension

In this study, micro arc oxidation (MAO) process was applied to 6082 aluminum alloy in an alkaline-based electrolyte with and without the addition of carbon nanotubes (CNTs) to the electrolyte. After oxidation, the coatings were investigated by a series of structural, morphological, and tribological tests to understand the effect of CNTs addition to the base electrolyte. X-ray diffraction analyses revealed that an oxide layer composed of α-Al 2 O 3 and γ-Al 2 O 3 in varying intensities was formed on the surface, and the addition of CNTs to the electrolyte may contribute to the formation of more α-Al 2 O 3 after MAO for longer oxidation times. High-resolution SEM micrographs showed that CNTs were successfully incorporated into the pores of the alumina coating generated during the discharging stage of the MAO process. The addition of CNTs to the base electrolyte induced a rougher surface accompanied by a decrement in the coating thickness as compared to the sample treated in the base electrolyte. The surface hardness and dry sliding wear resistance of 6082 aluminum alloy increased by performing the MAO process in the base electrolyte, as expected, due to the formation of an alumina coating at the surface, and the addition of CNTs to the electrolyte resulted in a further increment in the coating hardness and dry sliding wear resistance.

Chen He

Papers

Effect of ultrasonic on microstructure and growth characteristics of micro-arc oxidation ceramic coatings on 6061 aluminum alloy