Properties of plasma electrolytic oxidation coatings formed on 2A97 Al-Cu-Li alloy using aluminate and phosphate electrolytes

The plasma electrolytic oxidation of 2A97 Al-Cu-Li alloy was carried out by pulsed bipolar current regime, in aluminate and phosphate electrolytes, respectively. The microstructure and phase composition of the produced coatings were analyzed by SEM and XRD, and the corrosion and wear properties of the coatings from the two electrolytes were evaluated by electrochemical polarization curves and dry sliding wear tests. The results show that pancake structures dominate the surface of the coatings from both electrolytes, and the coatings consist of an inner and an outer layer in their cross sections, and there are large amount of big pores between the two layers. The main phase compositions of the coatings are α, γ, δ-Al2O3, however, higher amount of α-Al2O3 is found in the coating from aluminate electrolyte. The coating from phosphate electrolyte exhibits higher corrosion resistance than a similar coating from aluminate electrolyte. The coating from the phosphate electrolyte shows lower coefficient of friction, possibly due to the incorporation of P in the coating. However, the coating from the aluminate electrolyte has higher wear resistance because more α-Al2O3 is found in the coating from aluminate electrolyte.