The Effects of Sodium Tungstate on the Characteristics of Microarc Oxidation Coating on Ti6Al4V

Microarc oxidized coatings on Ti6Al4V alloy are produced in a basic Na2SiO3-(NaPO3)6-NaAlO2 electrolyte with different concentrations of sodium tungstate. The effects of sodium tungstate on the characteristics of the coating are investigated through current–time response, microstructure, composition, hardness, bonding strength and antifriction measurements. The coating is mainly composed of Al2TiO5, TiO2, Al2O3 and amorphous substances regardless of the concentration of sodium tungstate. The sodium tungstate is transformed into trace amounts of WO3, which significantly promotes the growth of the coatings. The bonding strength and antifriction performance of the coatings are enhanced with the concentration of sodium tungstate less than 4 g/L, whereas excessive sodium tungstate results in a coarse outer layer with more microdefects, which deteriorates the wear resistance of the coating. Based on the results of scratch and wear test with energy-dispersive x-ray spectroscopy of the worn surface and debris, the optimal concentration of sodium tungstate doped into the electrolyte is identified, and this concentration yields superior tribological property. Excellent wear resistance is achieved by combining a high bonding strength and hardness with a less defective outer layer.