Effect of heating cycles on phase transformation in ceramic coatings produced by Plasma Electrolytic Oxidation of Al6061 alloy

The introduction of plasma electrolytic oxidation (PEO) as a surface finishing technique has enabled a range of hard, dense oxide coatings on aluminum, magnesium, titanium and other lightweight alloy substrates. The development of PEO technology over the last decade has provided coatings with controlled appearance, hardness, corrosion resistance and other tribological properties to benefit a broad base of industrial sectors. Phase composition and ratio are the principal factors that determine the mechanical or tribological performance of PEO coatings. Controlling the phase composition and ratio in the treated layer is, therefore, an important and practical issue in PEO. In the case of aluminum alloys, α-Al2O3 is the hardest phase and as a result is favoured in wear resistance applications. Research shows that solidification of liquid alumina subjected to considerable undercooling results in the formation of γ-Al2O3 rather than α-Al2O3 because of its lower critical free energy for nucleation. Some researchers have studied phase distribution in PEO coatings on aluminum substrate but controlling phase transformation in order to create favourable phases suitable for specific applications like wear resistance is an area that has not received enough attention. In this study the effect of different heating cycles produced by manipulating electrical parameters (frequency and duty cycle) on γ  α-alumina phase transformation in PEO ceramic coatings on Al6061 alloy was investigated. The results showed that ceramic coatings mainly consisted of γ-Al2O3. A small amount of α-Al2O3 with varying ratios was also observed in the samples with long ton time during a cycle. The results showed that α-Al2O3 and γ-Al2O3 ratio in the coatings can be altered using different pulse parameters. This would make it possible to produce α-alumina through controlled heating cycles by proper choice of frequency and duty cycle either in a single step during coating growth or as a separate heat treatment process after the coating has been formed.

Keywords:

Plasma electrolytic oxidation , phase transformation , alumina coating