Preparation And Properties Of Hydroxyapatite-based Coating On The Surface Of AZ31B Magnesium Alloys

Compared with traditional implantable medical metal materials,magnesium alloys have unique advantages such as non-toxic,good biocompatibility,biodegradability,and avoiding secondary surgery.However,because of its fast degradation rate,it is easy to cause the failure of implant application.Therefore,controlling the corrosion rate of biodegradable magnesium alloy in vivo has become the main challenge for clinical application.So far,the main method of surface modification of magnesium alloy is to apply bioceramic coating.Hydroxyapatite,as a bioceramic coating,has good biological activity,resistance to degradation,and can interact with bone tissue.Hydroxyapatite can form a chemical bond and promotes the repair and re-growth of bone tissue,so it has become a widely used biological coating on the surface of alloys.In this study,in order to enhance the resistance to degradation and improve the bonding strength between it and bone tissue,two different fluorine-doped hydroxyapatite(FHAp)coatings were prepared on the surface of AZ31B magnesium alloy by different methods.The effects of preparation process parameters and fluorine ion doping on the morphology,phase composition and corrosion resistance of FHAp coatings were studied.Scanning electron microscopy(SEM),atomic force microscopy(AFM),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FTIR)and X-ray diffraction(XRD)were used to characterize and test.At the same time,the corrosion resistance,degradation behavior and cell compatibility of the coatings were evaluated by electrochemical impedance spectroscopy(EIS),polarization curve,in vitro simulated body fluid(SBF)immersion test and cell culture experiment.The FHAp/MAO composite coating was successfully prepared on the surface of AZ31B magnesium alloy through using the combination of micro-arc oxidation and hydrothermal treatment.First of all,the optimal conditions for the preparation of MAO coating were determined by controlling the process parameters of micro-arc oxidation:current 0.25 A,frequency 400 Hz,duty factor 75%,oxidation time 3 min.In the hydrothermal treatment process,with the increase of the amount of fluoride ion doping in the hydrothermal solution,the density of the coating gradually increased.When the F/Ca doping ratio was 0.2,the density of FHAp/MAO composite coating is maximum.And the coating is composed of closely connected nano-rod structures arranged in a highly radial pattern.The thickness of FHAp/MAO composite coating is about 20?m,and the coating is well bonded to the substrate.Electrochemical tests and SBF immersion experiments show that the composite coating has higher corrosion resistance and lower degradation rate than single HAp coating and blank substrate.In addition,in vitro cell culture experiments show that the FHAp/MAO composite coatings do not have cytotoxicity,and the surface microstructure of the coating is favorable for cell adhesion and spreading.The doping of fluoride ions further improves the activity of ALP and promotes the early differentiation of osteoblasts.A novel nanorod-like FHAp coating was successfully synthesized on the surface of AZ31B magnesium alloy by electrochemical deposition.In this study,the electrochemical deposition mode was first determined as a constant current mode.And then the electrochemical parameters were optimized to determine the deposition coating.The current density is 2.5 m A/m~2.It was found that alkali heat treatment can promote the conversion of the Ca-P coating to the HAp coating and allow the FHAp coating to continue to grow.In the electrochemical deposition process,with the increase of the amount of fluoride ion doping in the electrolyte,the density and uniformity of the coating also increase.When the F/Ca doping ratio is 0.2,the obtained FHAp coating has the highest density.And the coating is composed of nanorods arranged closely to each other.The diameter of nanorod is about 100 nm.The thickness of the FHAp coating is about 3?m,which combines well with the substrate and has a high average roughness.Electrochemical tests and SBF immersion experiments show that FHAp coating has higher corrosion resistance and lower degradation rate than pure HAp coating and magnesium substrate.In vitro cell culture experiments show that the surfaces of HAp coating and FHAp coating are conducive to early cell adhesion,growth and reproduction.Compared with HAp coating,FHAp coating has more cell adhesion and cell spreading,indicating that fluoride ion doping is beneficial to the proliferation and growth of cells.