In this study, aluminum oxide was deposited on an Al-alloy substrate to produce hard ceramic coatings using a Plasma Electrolytic Oxidation (PEO) process working at atmospheric pressure. Two different operation modes were used, namely a DC power mode and pulsed DC power mode with different frequencies. Optical Emission Spectroscopy (OES) was employed to study the species, electron temperature and densities of the plasma. The morphology, composition, and microstructure of the coatings on the Al substrate were investigated using Scanning Electron Microscopy (SEM) with energy dispersive X-ray (EDX) analysis, and X-ray diffraction. At the early stage of the PEO process the plasma electron temperature increased which shows the same trend as the output voltage. Aluminum emission line intensities (which are related to the spark behavior during the discharge) were higher for the pulsed DC mode than that from DC mode, causing different surface morphologies. It was also found that pulsed DC mode enhanced the coating growth during the early discharge stage, due to the strong ejection of aluminum from the substrate-oxide interface during the plasma discharges. The coating characteristics and OES analysis thus led to a better understanding of the ceramic coating growth behavior as influenced by the power operation modes.

*Corresponding author. Tel: 1-519-253- 3000, ext. 4148; Fax: 1-519-971-7007; E-mail address: xnie@uwindsor.ca (X. Nie)