Formation of ZrO2 in coating on Mg-3Â wt.%Al-1Â wt.%Zn alloy via plasma electrolytic oxidation: Phase and structure of zirconia

An investigation of the coating structure formed on Mg-3 wt.%Al-1 wt.%Zn alloy sample subjected to plasma electrolytic oxidation was examined by field-emission transmission electron microscopy. The plasma electrolytic oxidation process was conducted in a phosphoric acid electrolyte containing K2ZrF6 for 600 s. Microstructural observations showed that the coating consisting of MgO, MgF2, and ZrO2 phases was divided into three distinctive parts, the barrier, intermediate, and outer layers. Nanocrystalline MgO and MgF2 compounds were observed mainly in the barrier layer of ~ 1 μm thick near to the substrate. From the intermediate to outer layers, various ZrO2 polymorphs appeared due to the effects of the plasma arcing temperature on the phase transition of ZrO2 compounds during the plasma electrolytic oxidation process. In the outer layer, MgO compound grew in the form of a dendrite-like structure surrounded by cubic ZrO2.