Enhanced hydrophobic properties, wear and corrosion resistance of plasma electrolyte oxidation coatings on AZ31B magnesium alloys with addition of TiO2 nanoparticles

Nanocomposite coatings were fabricated on AZ31B magnesium alloy by means of plasma electrolyte oxidation (PEO) with the addition of TiO₂ nanoparticles into the alkaline electrolyte, followed by low surface energy modification in the ethanol solution of lauric acid. The dual-layered nanocomposite PEO coatings composed of MgO, Mg₂SiO₄, anatase-TiO₂, rutile-TiO₂, and Mg₂TiO₄ were formed, and the TiO₂ nanoparticles incorporated into the PEO coatings via both inertia and reactive modes. Besides, the penetration and adsorption of TiO₂ nanoparticles created a distribution gradient in the coating thickness direction. The wear rate of the PEO coatings with TiO₂ nanoparticles in 4 g/L was approximately 82% lower than that of the bare substrate and approximately 39% lower than that of the PEO coatings without TiO₂ addition, respectively. The enhancement in wear resistance was due to the synergistic effect of the lower coefficient of friction (COF), the higher microhardness and the denser microstructure with finer micropores and less microcracks on the coatings than that of the TiO₂-free coatings. Besides, the hydrophobic property was enhanced for the low surface energy modified PEO coatings with the addition of TiO₂ nanoparticles in the electrolyte. Moreover, a significant enhancement in the corrosion resistance was achieved for the PEO coating with the addition of TiO₂ nanoparticles in 4 g/L. It exhibited a higher corrosion inhibition efficiency and a lower corrosion rate than that in other concentrations and without TiO₂ addition. The improvement in anti-corrosion property was originated from the formation of phases including TiO₂ and Mg₂TiO₄ in the coating, and denser microstructure with better hydrophobic properties for the PEO coatings with TiO₂ nanoparticles in 4 g/L compared with that in other concentrations. Accordingly, the hydrophobic, wear-resistant and corrosion-resistant PEO coatings with TiO₂ nanoparticles exhibited significant advantages in the field of Mg alloys protection.