Preparation and characterization of hydroxyapatite containing coating on AZ31 magnesium alloy by micro-arc oxidation

Magnesium alloys have received great attention as a new kind of biodegradable metallic implants. However, poor corrosion resistance limited its applications. In order to improve the corrosion resistance and bioactivity of magnesium alloys, hydroxyapatite containing coatings were grown on AZ31 magnesium alloy by microarc oxidation. The thickness, morphologies and component of the coatings were investigated by SEM, XRD and XPS. Further, the corrosion resistance and degradation rate were also investigated. The main crystal phases of the coatings were MgO and hydroxyapatite. The thickness of the coatings increased with increasing the NaOH concentration. The coating showed a porous structure, and volcano top-like pores could be observed when the NaOH concentration was higher than 0.2 mol/L. Compared with the AZ31 substrate, all the coatings showed to significantly improve the corrosion resistance and greatly reduce the degradation rate. Among all, the coated sample formed in 0.2 mol/L NaOH showed the best corrosion resistance and the lowest degradation rate. Furthermore, complete coatings performed higher bonding strength more than 50 MPa, which meets the requirement of an implant biomaterial. This study provides insight into the development of bioactive coatings on magnesium alloys for biomedical applications.