Synthesis and in vitro degradation evaluation of the nano-HA/MgF2 and DCPD/MgF2 composite coating on biodegradable Mg–Ca–Zn alloy

In the present study, nano-hydroxyapatite/magnesium fluoride (nano-HA/MgF2) and dicalcium phosphate dehydrate/magnesium fluoride (DCPD/MgF2) composite coating was synthesized via fluoride conversion coating process followed by an electrochemical deposition (ED) method on biodegradable magnesium (Mg) alloy. The surface morphology, crystalline structures, and compositions of coated specimens were characterized by X-ray diffraction (XRD), atomic-force microscopy (AFM), Fourier-transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The influence of the coatings on the corrosion behaviour of the specimens was evaluated by potentiodynamic polarization and immersion test in Kokubo solution at room temperature. AFM examination of the composite coating specimen indicated that root mean square roughness (RMS) of the nano-HA/MgF2 and DCPD/MgF2 composite coatings was approximately 395 nm and 468 nm respectively. However, fluoride treated and untreated showed lower RMS compared with the composite coated. The needle-like morphology of HA crystals had a diameter of 80 nm–150 nm and a length of about 7 μm. However, plate-like morphology of DCPD was relatively larger. Electrochemical tests exhibited a significant decline in corrosion current density from 365.2 to 5.23 μA/cm2 after composite coating of HA/MgF2 on Mg alloy. However, no significant reduction in corrosion rate was observed between the composite coated and fluoride treated specimens. Immersion tests also showed that HA/MgF2 and DCPD/MgF2 composite coatings reduced the hydrogen evolution and improved the nucleation site of apatite greater than that of the uncoated sample. However, DCPD/MgF2 showed lower corrosion resistance and hence composite coating of HA/MgF2 on Mg–Ca–Zn alloy is a promising candidate for biodegradable medical applications.