Effect of Na₂WO₄ in Electrolyte on Microstructure and Tribological Behavior of Micro-arc Oxidation Coatings on Ti₂AlNb Alloy

Micro-arc oxidation (MAO) ceramic coatings were prepared on Ti₂AlNb alloy in silicate/phosphate electrolytes with different concentrations of Na₂WO₄. The influence of Na₂WO₄ on the coating growth process, coating structure and composition was analyzed by SEM, XRD and XPS. The tribological behavior of MAO coatings was evaluated by the ball-disc wear test. The results show that the growth rate of MAO coating in electrolyte without Na₂WO₄ is only 0.08<i>μ</i>m/min, meanwhile, the coating is loose and rough, and "networks" connecting with big pores exist on the coating surface.The main phase compositions of this coating are rutile TiO₂, anatase TiO₂, Al₂O₃, and Nb₂O₅. The addition of Na₂WO₄ in the electrolyte shortens the time before sparking of Ti₂AlNb alloy, increases the growth rate of the coating, improves the uniformity of coating and meanwhile, a small amount of WO₃ is introduced in the coating. Besides, MAO coatings formed in the participation of Na₂WO₄ have better wear resistance. Severe abrasive wear occurs when the test is made on Ti₂AlNb alloy with Si₃N₄, the friction coefficient reaches 0.5-0.7. Both the friction coefficient and wear rate decrease obviously when Ti₂AlNb is treated by MAO. The friction coefficient and wear rate of MAO coating prepared in the electrolyte with 4g/L Na₂WO₄ are 0.24 and 6.2&#215;10^-4mm³/(N&#183;m), respectively. Only "fish scales" caused by fatigue wear appears on the coating surface.