Characterisation of single microdischarges during plasma electrolytic oxidation of light metals

Plasma electrolytic oxidation (PEO) is a surface treatment for light metals (Al, Ti, Mg) offering enhanced corrosion and wear resistance through the formation of thick, adherent oxide-ceramic coatings. Unlike conventional anodising, PEO operates at higher voltages, inducing short-lived microdischarges, which play a central role in the generation of the coating. These short-lived microdischarges are influenced by process conditions like electrical parameters, substrate and electrolyte composition, and treatment time.

To gain a deeper understanding of these microdischarges, a single microdischarge setup was employed, using 1 mm diameter wire tips as anodes. Thereby, the wire is immersed in an electrolytic solution containing varying concentrations of potassium hydroxide (0.5–4 g/l), and in some cases, combined with sodium metasilicate (0.5–4 g/l). Diagnostic tools like high-speed imaging and optical emission spectroscopy are used to analyse discharge lifetimes, bubble dynamics, and plasma temperatures, while SEM analysis links these transient events to the coating structure.

The results indicate that microdischarge behaviour and surface morphology differ significantly across substrates (Al, Mg, Ti) based on the electrolyte type and concentration. In contrast to aluminium and titanium, microdischarge initiation on magnesium requires the presence of both KOH and Na₂SiO₃.