Stark Broadening of the H­­­α and Hβ lines during plasma electrolytic oxidation with aluminium and titanium

Plasma electrolytic oxidation (PEO) is a novel technique to create a passivation layer on light metals like aluminium, titanium or magnesium. The process is characterised by anodic dielectric breakdowns in form of short living micro-discharges accompanied by gas evolution. To control the coating's morphology and composition, it is crucial to understand these micro-discharges and their dependence on substrate composition, deposition time and electrolyte concentration.

Here, optical emission spectroscopy (OES) is used to investigate the micro-discharges in a system designed to isolate single micro-discharges (SMD). The anode is reduced to a tip of a wire (1 mm diameter) and is surrounded by an insulating cladding and a glass tube filled with distilled water and 0.5-4 g/l KOH.

Stark-broadening of the Hα- and Hβ-lines is used to estimate the electron density during the PEO process. The Hβ-line, which is less influenced by the self-absorption effect, is more challenging due to a low signal to noise ratio and in case for Al anodes, it is overlapped by the AlO emission spectrum. Using a spectral-simulation, the AlO emission spectrum is determined allowing us to estimate the Stark-broadening of the Hβ-line as well as the rotational temperature of the AlO emission. The latter is found to be in the range of 3000 K. The resulting electron densities are up to half order of magnitude lower compared to the results for the Hα -line, depending on the importance of resonance broadening assumed in the calculation.