Optical emission spectroscopy of plasma electrolytic oxidation process on 7075 aluminum alloy

Plasma electrolytic oxidation (PEO) on 7075 Al-Zn-Mg-Cu alloy was performed to produce the ceramic coatings in silicate electrolyte at constant voltage. The plasma electron temperature, electron density and atomic ionization degree in plasma zone were evaluated by analyzing the spectral lines of optical emission spectroscopy (OES), and the high spike peaks on plasma temperature profiles were emphatically discussed. The average electron temperature in plasma zone was about 3000 K-15,000 K, and the electron density was about 4.95 × 1021 m− 3-1.65 × 1022 m− 3, meanwhile the atomic ionization degree of Al was less than 10− 3, while the temperature inside the alloy is below 120 °C. It was believed that the high spike peaks on plasma temperature profiles appeared in the later stage of PEO process resulted from the calculation deviation of plasma temperature from weak OES spectral line intensities. The generation of these spike peaks depended on the spark density and illumination intensity rather than the appearance of large discharge sparks, which was different from the previous viewpoint.