Two-step plasma electrolytic oxidation of Ti–15V–3Al–3Cr–3Sn for wear-resistant and adhesive coating

Ti–15V–3Al–3Cr–3Sn (Ti–15–3) is one of the important practical titanium alloys with high cold deformability and high mechanical strength, but its wear resistance is poor. This paper reports the formation of wear-resistant and adhesive ceramic coatings on Ti–15–3 by two-step plasma electrolytic oxidation (PEO). The PEO of Ti–15–3 has been carried out first in alkaline aluminate electrolyte to form a wear-resistant oxide layer and then in acid electrolyte containing both phosphoric acid and sulfuric acid to improve adhesion of the coating. The coating formed in the alkaline aluminate electrolyte is more than 10 μm thick, and highly crystalline. The main phase is Al2TiO5. This coating shows high wear resistance, but is not adherent to substrate due to the development of a number of voids and pores in the oxide layer close to the substrate. A new oxide layer with amorphous structure is formed next to the substrate in the subsequent PEO in the acid electrolyte, during which the voids are filled with a new oxide formed in the acid electrolyte, reducing the porosity. As a consequence, the adhesion of the coating is markedly improved without deteriorating the high wear resistance.