Unveiling the mechanisms of coating formation during micro-arc oxidation of titanium in Na₂HPO₄ electrolyte

In the present work, the effect of the electric potential provided by the bipolar pulsed power supply during micro-arc oxidation of titanium in disodium hydrogen phosphate (Na₂HPO₄) electrolyte on the microstructure and chemical composition of the formed oxide coating was studied. The surface topography and microstructure of obtained coatings were investigated using scanning and transmission electron microscopy. In addition, the chemical analysis was performed with two complementary methods: energy-dispersive X-ray spectroscopy and more accurate but surface-sensitive X-ray photoelectron spectroscopy. These investigations revealed the formation mechanism of the titanium oxide coating, depending on whether an anodic or cathodic cycle is forced by the power supply. During the anodic cycle, the simultaneous interaction of negatively charged O²⁻ ions and HPO₄²⁻ anions with Ti⁴⁺ ions released from the substrate occurs. Introducing the cathodic cycle allows a small amount of Na to be incorporated into the coating material, mainly in the areas close to the porosity, which was not possible with the use of the direct current (DC) or unipolar pulsed power supply. Moreover, an increase in applied voltage increased rutile content and improved the crystallinity of the TiO₂-based MAO coating.