Investigating the Effect of Silicon Content on Microstructure and Properties of Electrolytic Oxidation Plasma on Aluminum- Silicon Alloys

The use of an environmentally friendly method called plasma electrolytic oxidation (PEO) on aluminum and its alloys to create a coating with desirable properties is very important. One of the parameters affecting of PEO process as well as the performance of the prepared coatings is the composition of the substrate. In this study, the effect of increasing the percentage silicon of substrate on the plasma electrolytic oxidation process with bipolar pulse current in a silicate-based electrolytic bath was investigated. The results showed that at constant current, the rate of increase of positive voltage decreases with increasing the amount of silicon and the values of positive voltage approach each other with the onset of oxidation. Scanning electron microscopy was used to evaluate the morphology and structure of the coating and X-ray diffraction test was used for phase detection. With increasing the percentage silicon of substrate from 1 to 13% by weight, in addition to reducing the thickness and roughness of the coatings, the average coating speed also decreased from 1 to 0.43μm / min. As the Si content of the substrate increases, volcano-shaped morphology dominated over pancake morphology in the growth of the coating and a porous surface with scattered micro-pores in its structure was observed. The results showed coatings mainly containing mixtures of γ-Al2O3, η-Al2O3, δ-Al2O3, SiO2 and a small amount of mullite and some amorphous phases.