Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte

In the present work, four processes were carried out to produce ceramic coatings on aluminum substrate in two kinds of electrolytes (silicate and phosphate solution systems) using plasma electrolytic oxidation (PEO) technology. The voltage-time responses were recorded during different PEO processes. SEM/EDX and XRD were adopted to investigate the microstructure, elements distribution and phase composition of the coatings prepared in the two electrolyte systems. It is found that coatings produced in the silicate electrolyte have a more homogeneous morphology than those produced in the phosphate system. EDX analysis shows that silicon element tends to present primarily in the outer region of the coatings while phosphorus distributes uniformly throughout the coating thickness. According to the conventional anodic oxidation mechanism, a model is set up to explain the different characteristics of ceramic coatings fabricated in different electrolytes which is helpful to understand the growth mechanism of PEO coatings.