Effects of electrolytes on the growth behavior, microstructure and tribological properties of plasma electrolytic oxidation coatings on a ZA27 alloy

In this study, plasma electrolytic oxidation (PEO) was successfully applied on a ZA27 alloy to produce continuous and dense ceramic coatings. The effects of the electrolytes (silicate, aluminate and aluminate/borate composite solutions) on the coating growth behavior, microstructure, phase compositions, and tribological properties were investigated. ZnAl2O4, Al2O3, and CuAlO2 can be found in all the three coatings. In addition, the coating produced with silicate solution contains ZnSiO4. During the process of coating, the Al element preferred to react with electrolytes to form the ceramic coatings. The addition of borate solution can accelerate the coating growth rate and be beneficial to produce a denser coating. Coatings produced with silicate and aluminate/borate electrolyte have a structure with porous outlayer, dense layer, and inner barrier layer, whereas the coating produced with aluminate electrolyte only shows a dense monolayer structure. The coating produced with aluminate/borate composite electrolyte achieved the best wear performance, lowest friction coefficient, and highest wear resistance among the three coatings under both low and high normal loads.