A mechanism for the growth of a plasma electrolytic oxide coating on Al

The growth of a plasma electrolytic oxide (PEO) coating on Al was widely considered as an ejection of molten alumina on the surface, induced by a localized breakdown of the solid coating. However, evidences presented here indicated no breakdown of the insulating coating in the PEO process, based on the X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrometry investigations. There are abundant channels in the coating layer from surface to substrate. However, the SEM observes indicating that the channels in the outer layer with a low-density distribution and a micron-sized diameter, the channels extending into the internal layer with a high-density distribution and a nano-sized diameter. In addition, a continuous thin amorphous alumina layer (AAL) was found between coating and substrate. An intermediate state of the AAL before transition to crystalline, with unusually thick character (∼2.7 μm), is reported for the first time, which presented a typical cross-sectional characteristic of the ionic migration processes. Based on these microstructure observations, a growth mechanism is proposed here to explain the structural features, in which the discharge ignited in the gas-filled channels passing through both the outer layer and the internal layer. The main voltage drop of applying the hole sample loaded on the AAL at a magnitude of ∼1V·nm-1 due to excellent conductivity of discharge plasmas in the channels. Under a high electrical field, the AAL was thickening, ultimately squeezing the coating expanding outward. Abundant moving micro-discharges and localized expansions could result in the growth of the whole coating.