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

Alumina coatings were prepared by plasma electrolytic oxidation (PEO) on Al alloys using NaAlO2 electrolytes with added Al(NO3)3 to enhance the mechanical properties of the coatings. Electrolytes (0.1 M NaAlO2) with small amounts of Al(NO3)3 ranging from 1.0 to 5.3 mM were employed at a fixed current density (10 A/dm2). With the added Al(NO3)3, the onset of spark discharges was delayed, which is associated with the dissolution and oxidation of Al alloys in the nitrates. X-ray diffraction results showed that the obtained coatings consisted of both the α-Al2O3 and γ-Al2O3 phases. The maximum α-Al2O3 content of the coatings was obtained with the addition of 1.6 mM Al(NO3)3 to the electrolytes. The hardness of the coatings was significantly enhanced by increasing their α-Al2O3 content, which could be achieved by adding suitable amounts of Al(NO3)3 to the electrolytes.