Effects of addition of Al(NO3)3 to electrolytes on alumina coatings by plasma electrolytic oxidation

► Alumina coatings were synthesized by PEO using Al(NO3)3-added electrolytes. ► Al(NO3)3-added electrolytes yield higher α-Al2O3 contents in the coatings. ► Alumina coatings with high α-Al2O3 contents lead to high hardness.