Characteristics of a multi-component MgO-based bioceramic coating synthesized in-situ by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) has held great potential for the advancement of biodegradable implants, as it helps in developing porous bioceramic coatings on the surface of magnesium alloys. In this research work, MgO-based bioceramic coatings containing the Si, P, Ca, Na, and F elements have been successfully fabricated on an AZ31 magnesium alloy plate utilizing the PEO method. The characteristic current-voltage behavior of the samples during the process was surveyed in an electrolyte containing Ca(H2PO4)2, Na2SiO3·9H2O, Na3PO4·12H2O, NaF, and KOH with a pH of 12.5 and electrical conductivity of 20 mS/cm. The results revealed that applying a voltage of 350–400 V (that is 50–100 V higher than the breakdown limit) could greatly facilitate the synthesis of a PEO ceramic coating with fewer defects and more uniform morphology. The resulting coating was a compositionally graded bioceramic layer with a thickness in the range of 3.5 ± 0.4 to 6.0 ± 0.7 µm, comprising the above-mentioned elements as promising bioactive agents. The synthesized ceramic features were investigated in terms of the elemental distribution of components through the thickness, which indicated a gradual rise in the Si and P contents and, conversely, a decline in the F content towards the outer surface. The growth mechanism of the PEO coating has been discussed accordingly.