Growth and internal microstructure of micro-arc oxidized MgO-based nanocomposite coating

• Electrical resistance of coating in first minute is higher than next minutes.
• Electrical resistance of coating reduces with passing time.
• Samples need lower energy for MAO process than ordinary samples.
• No direct relationship between top coating's surface and internal porosities
• SMAT decreased volume and free surface of coating's internal porosities.

In this study, AZ31B magnesium alloy specimens were studied with both nanocrystallized and coarse-grained surfaces. Nanocrystallization was made by using the Surface Mechanical Attrition Treatment (SMAT). Then, micro-arc oxidation process was performed on both surface treated and untreated specimens in a phosphate-based suspension with and without titania nanoparticles. In this research, surface morphology of the obtained coatings was investigated by using the scanning electron microscope. In order to analyze the porosity of coatings, an image analysis software and a wettability test were used. To examine the effect of nanocrystallization on the necessary energy for micro-arc oxidation process, potential-time diagram of coating process was used. The results indicated that the required energy for coating process on the surface of nanocrystallized samples is lower than that of coarse-grained samples. Moreover, the results of wettability test indicated that there is no clear relationship between top-surface porosity and internal porosity of the coatings. Moreover, SMAT pre-process resulted in a decrease in volume and area of internal porosity of coatings.