Influence of process parameters on electrolytic plasma discharging behaviour and aluminum oxide coating microstructure

In this study, a plasma electrolytic oxidation process (PEO) was used to produce oxide coatings on commercially pure aluminum (Al 1100) at two different current modes, pulsed unipolar and bipolar modes. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm-800 nm) was employed to investigate the PEO plasma. The emission spectra were recorded and plasma temperature profile versus processing time was constructed using line intensity ratios method. Scanning Electron Microscopy (SEM) with energy dispersive x-ray analysis (EDS) was used to study the coating microstructure and coating cross section. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings. The main effect came from the strongest discharges which were initiated at the interface between the substrate and the coating. Through manipulation of process parameters to control or reduce the strongest discharge, the density and quality of the coating layers could be modified. This work demonstrated that by adjusting the ratio of the positive to negative pulse currents as well as their timing in order to eliminate the strongest discharges, the quality of the coatings was considerably improved.