Modifications of an electrolytic aluminum oxide film under the treatment with microdischarges during plasma electrolytic oxidation, a self-organized dielectric barrier discharge (DBD) and a DBD-like plasma jet

Abstract

A key to the understanding of mechanisms during plasma electrolytic oxidation (PEO) is the interaction between microdischarges and an amorphous oxide film. The PEO microdischarges, which are randomly distributed on the surface of a treated lightweight metal substrate (Al, Ti, Mg), cause material extraction and support the formation of hard and dense crystalline oxide films. Characterization of these microdischarges is a complicated task under PEO conditions, because of the stochastically temporal and spatial behavior as well as the small dimension of the microdischarges. Microdischarges at atmospheric pressure conditions can leave similar erosion traces on metallic films (Al, Ti) as PEO microdischarges on oxide films, and possibly can support a better understanding of the plasma-solid-interactions as well as microdischarge characteristics during PEO. A porous aluminum oxide film is deposited on aluminum substrates by pre-anodizing at a voltage of 250 V and is treated afterwards with a relative short (duration of 1 min) PEO process at a voltage of about 500 V or filamentary dielectric barrier discharges, namely a self-organized Dielectric Barrier Discharge (DBD) and a DBD-like plasma jet operated both with a He/N2 (95%/5%) gas flow. The gas temperature at DBD plasma conditions, measured using the rotational distribution in the emission spectra of molecular nitrogen, is low and amounts to about 400 K. Erosion traces on the surface of the oxide film caused by PEO and plasma spots of both atmospheric pressure discharges are studied by scanning electron microscopy and energy dispersed x-ray spectroscopy. Form and dimensions of erosion traces and established modifications of the material composition generated by the treatment with these DBD microdischarges under atmospheric pressure conditions are similar to those ones generated by the PEO process. Hence, a similar mechanism of these processes is supposed. For stronger evidences of the assumed PEO mechanism additional experimental studies are needed.