Influence of pulse frequency of plasma electrolytic oxidation on incorporation of CaO nanoparticles in the coating grown on AZ31 Mg alloy for biodegradable orthopedic implant applications

Magnesium alloys, such as AZ31, hold promise for biodegradable orthopedic implants due to their biocompatibility and properties similar to those of bone. However, their rapid degradation within the body presents a significant challenge. Plasma electrolytic oxidation (PEO) effectively enhances both corrosion resistance and bioactivity. This study investigated the impact of pulse frequency on the microstructure and composition of PEO coatings applied to the AZ31 Mg alloy, as well as the incorporation of CaO nanoparticles into the coating. The PEO coatings were fabricated in a phosphate-based electrolyte containing 10 g/l Na₃PO₄.12H₂O, 8 g/l NaF, and 2 g/l KOH with and without 3 g/l CaO nanoparticles as an additive. The coatings were produced using a constant voltage under bipolar pulsed waveforms at frequencies of 0.5, 1, 1.5, and 2 kHz. It was found that increasing the pulse frequency and adding CaO nanoparticles led to smaller and uniformly distributed pores, but with more microcracks as observed through a field emission scanning electron microscope. The energy-dispersive X-ray spectroscopy (EDS) results showed that the lowest frequency (0.5 kHz) caused more incorporation of the CaO nanoparticles due to longer pulse-on time, while increasing the frequency to 2 kHz led to a reduction of ≈ 77.4 % in CaO content. In contact with simulated body fluid (SBF) solution, all coatings showed irregular spherical hydroxyapatite clusters, indicating bioactivity, with a notably higher cluster density on those containing CaO nanoparticles. Electrochemical impedance spectroscopy displayed that the coating produced at 0.5 Hz with CaO nanoparticles showed the highest corrosion performance (with total resistance of ≈10.2 MΩ.cm²), attributed to its higher thickness, higher CaO content, and the formation of calcium phosphate compounds that seal pores effectively and boost protective performance after 14 days of immersion in SBF. These findings indicate that carefully selecting pulse frequency during the PEO process, particularly in the presence of CaO nanoparticles, presents a promising strategy for improving coatings for biodegradable orthopedic implants.