Insight into continuous growth mechanisms of ZrO2-Al2O3 composite PEO coatings with superior cavitation erosion resistance

The ZrO2-Al2O3 composite coatings were fabricated on TiB2/2024Al composite by the plasma electrolytic oxidation (PEO) treatment with either monoclinic (m-ZrO2) or tetragonal zirconia (t-ZrO2) nanoparticles (NPs) addition. Such PEO coatings showed a double layer morphology due to the limited physical motion ability of ZrO2 colloidal particles into coatings. t-ZrO2 remained as the main phase in ZrO2-Al2O3 PEO composite coatings whether t-ZrO2 or m-ZrO2 NPs were added. Based on the microstructure analyses by focus ion beam plus scanning electron microscopy and transmission electron microscopy, ZrO2 showed netlike morphology in the outer layer with NPs partially melted at the mesh boundary and lines-like morphology fully melted at the inner mesh in the coating. A prominent morphological transition of ZrO2 was clearly observed at the interface between outer layer and inner layer. Therefore, the continuous growth mechanisms of ZrO2-Al2O3 composite PEO coatings were proposed in terms of morphological evolution process. Furthermore, electrochemical measurements and cavitation erosion tests were conducted to study the comprehensive surface properties (corrosion and impact resistance) of ZrO2-Al2O3 composite coatings. As a result, the ZrO2-Al2O3 composite PEO coatings with m-ZrO2 addition exhibited the greatly reduced corrosion rate, and superior cavitation erosion resistance. Accordingly, the underlying mechanisms for these improved properties were intensively discussed.