Kinetic features of wear-resistant coating growth by plasma electrolytic oxidation

The kinetic features of coating growth on titanium alloy Ti–6A1–4V by plasma electrolytic oxidation (PEO) are investigated. It was observed that the coating growth rate decreases significantly with the increasing of the PEO time treatment. At the same time, large longitudinal pores (cracks) were formed; thus, the major part of the coating is separated from the substrate. Coating growth is realized by the two mechanisms: (a) migration-diffusional and (b) thermochemical treatment of the deposited aluminum hydroxide (electrolysis) and oxidation of the substrate at the bottom of the coating pores. It presumably occurs due to the saturation of the electrolyte in the pores with titanium hydroxide and the relatively low intensity of the microdischarges, which is insufficient to eject the substance from the channels for ‘pancake’ formation. This increases the amount of the aluminum titanate (TiAl₂O₅) in the coating. The latter presumably forms due to a eutectic reaction in the coating or due to the exothermic reactions of titanium dioxide formation. The TiAl₂O₅-based coating containing at least 10% aluminum oxide α-Al₂O₃ (more than 20% in the outer layer) and 11% titanium dioxide (TiO₂) increases the wear resistance of the titanium alloy at least six times.