In-situ synthesized MgO-based ceramic coatings on a novel Mg–Ca material: Morphological response to the PEO electrolyte

MgO-based ceramic layers were in-situ synthesized on a novel Mg-based substrate via plasma electrolytic oxidation (PEO). The effects of electrolyte composition on the morphology of the layers were investigated through preparing three different ion arrangements, including common silicate-based and phosphate-based electrolytes as well as a modified Ca-containing phosphate-based electrolyte. Characterization methods including scanning electron microscopy and EDS elemental analysis were used to investigate the morphological features of the coatings, at the surfaces and cross-sections. Results indicated that the silicate-based electrolyte yields morphologically inhomogeneous coating with small porosities, while using the phosphate-based electrolytes may result in improved thickness and modified porosities. However, both of the coatings prepared via common electrolytes present Ca-containing heterogeneities, called nuggets. Using the modified phosphate-based electrolyte, it was indicated that the addition of Ca²⁺ ions not only removes any elemental/morphological heterogeneities in the PEO coating but also provides a ceramic layer with homogenous thickness distribution. However, it was confirmed that the PEO conditions should be changed to obtain the optimum morphological characteristics in coatings synthesized via Ca-contained electrolyte.