Effect of cathodic micro-discharges on oxide growth during plasma electrolytic oxidation (PEO)

• Published in: Surface & coatings technology 2015-05, Vol.269, p.131-137
• Main Authors: Nominé, A., Martin, J., Henrion, G., Belmonte, T.
• Format: Article
• Language: English
• Subjects: Anodic; Bias; Bipolar current pulse; Breakdown; Chemical and Process Engineering; Coatings; Electrolytes; Engineering Sciences; Magnesium; Materials; Micro-discharges; Oxidation; Oxides; Physics; Plasma electrolytic oxidation (PEO); Plasma Physics; Plasmas; Waveforms
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.01.076

Plasma electrolytic oxidation (PEO) of magnesium alloy is performed in NH4F and alkaline-silicate electrolytes within various waveforms of bipolar current pulses to investigate the role of the cathodic bias on both the growth mechanism of PEO layers and the behavior of micro-discharges. A particular attention is paid to cathodic micro-discharges and their ignition (or not) according to process conditions. Although cathodic micro-discharges have a detrimental effect on PEO layers, cathodic bias is necessary to improve the growth of oxide layers. Conditions that promote cathodic breakdown are discussed and a mechanism of layer cathodic breakdown is proposed. Cathodic micro-discharges, and to a greater extent the applied cathodic bias, influence the anodic breakdown coming immediately after, and therefore, the process behaviour during the anodic bias period. •A mechanism of cathodic breakdown of the PEO layer is proposed.•Cathodic micro-discharges are found to be detrimental to the oxide layer growth.•The mutual influence of cathodic and anodic biases is pointed out.•Size and lifetime of cathodic micro-discharges are in the range of 0.15–0.30mm2 and 100–150μs, respectively.