Mechanisms of plasma electrolytic oxidation of aluminum at the multi-hour timescales

We present certain novel insights into the processes that occur during the prolonged (several hours) plasma electrolytic oxidation (PEO) of aluminum. Scanning electron microscopy (SEM) equipped with the energy-dispersive spectroscopy option (EDS) was used to study morphologies of the PEO-formed ceramic coating/substrate (C/S) surfaces on aluminum. The morphologies of the oxide layers' cross-sections have also been examined. The plasma electron temperature was calculated from the spectral lines measured by the optical emission spectroscopy (OES). The obtained results demonstrate that during the one single discharge event, a completely new inner oxide layer is being formed at the bottom of the discharge channel. This layer is positioned between the metal substrate and the previous Al2O3 layers. It appears that this new oxide layer, together with the gas trapped in the channel becomes a barrier for the dielectric breakdown that is higher than the original coating layer itself.