Investigation of morphology and composition of plasma electrolytic oxidation coatings in systems of Na2SiO3–NaOH and (NaPO3)6–NaOH

Individual and sequential plasma electrolytic oxidation (PEO) processes were employed to investigate the morphology and composition of plasma electrolytic oxidation coatings grown on aluminum in the silicate and phosphate solution systems. The instantaneous variation of voltage–time responses during different processes were recorded. The results show that under a certain current density, more energy is need in the phosphate solution systems than in the silicate ones, which results in more cracks and fractures in the coatings prepared in phosphate solution. X-ray diffractometer (XRD) patterns show that more energy input is propitious to forming α-Al2O3 phase. Analysis also shows that silicon tend to present primarily in the outer region of the coating while phosphor primarily in the inner region of the coating.