Effect of pulse duty cycle on micro-plasma oxidation of aluminum alloy

The pulse power supply was applied to oxidation of an aluminum alloy by means of micro-plasma discharge in electrolyte solutions. The LY12 aluminum alloy was used as specimen in the form of a disc 20 mm in diameter and 6 mm in thickness. The electrolyte solutions consist of a sodium silicate or phosphate, etc., with pH value in the range of 7–12. The working voltage was 120–600 V. The current density was 10–60 A/dm2 and the temperature of the electrolyte solution ranged from 10 °C to 60 °C during oxidation. The properties of oxidation coating were characterized by surface and structural analytical techniques, i.e., scanning electron microscopy and X-ray diffraction analysis. It was ascertained that the thickness of the coating can be from a few micrometers up to more than 200 μm with a good adhesion between the coating and the substrate, and the coatings comprised a mixture of crystallized δ-Al2O3, γ-Al2O3, and α-Al2O3. The hardness of the coating was measured using a Vickers hardness tester with a 50 g load. The mean value of microhardness was HV 1200–2200 kgf/mm2.