Study On Preparation And Biodegradation Properties Of Hydroxyapatite Coating On Fluoride-treated Magnesium Alloy

In order to improve the corrosion resistance of magnesium alloy and enhance the biological activity,HA coating was prepared on HF-treated magnesium alloy AZ31 B by way of electrochemical deposition and hydrothermal deposition,respectively.The phase structure,chemical composition and micromorphologyof coatings were characterized by XRD,FTIR,EDS and SEM.Degradable properties of coatings were studied by electrochemical measurement technology and immersion test in Hank’s solution.HA coating were fabricated on the HF-treated magnesium by way of electrochemical deposition in electrolyte with Ca（NO₃）₂、NH₄H₂PO₄、NaNO₃ and H₂O₂ at 1 mA/cm² for 2h.MgF₂ conversion layer was formed on AZ31 alloy by HF-treatment.After soaking in Ca（OH）2,CaF₂ film with needle-like crystals about 10 μm in length were formed on the surface of MgF₂ layer.The micro/nano pores on CaF₂ film which have larger surface area are advantageous to the nucleation of calcium phosphate,and Ca-P coating formed is more homogeneous than that of on the surface without soaking in Ca（OH）₂.The Ca-P coating electro-deposited was composed of flake-like crystals of brushite（DCPD,CaHPO₄·2H₂O）with about 70 μm in length and 30 μm in width and converted to hydroxyapatite（HA）after alkaline-heat treatment.Ca-P/MgF₂ coating improved the corrosion potential of magnesium alloy and reduced the corrosion current density,although the corrosion resistance of Ca-P/MgF₂ coating was slightly decreased compared with MgF₂ coating.DCPD was dissolved after immersion for 5 d.After immersion for 12 d,some pit corrosion areas which were detected with Cl-were observed on the surface;While after immersion for 12 d,HA was relatively preserved,and the corrosion was not obvious.It indicated that HA is more stable and have better corrosion-resistance compare with DCPD.However,the inductance loop in EIS indicated that the pitting corrosion has been in the induction periods.HA coating with high purity and crystallinity on fluoride-treated magnesium was fabricated in an alkaline solution containing Ca-EDTA(C₁₀H₁₂N₂O₈Na₂Ca)and KH₂PO₄ by hydrothermal deposition.The results showed that the hydrothermal deposition on fluoride-treated magnesium had the characteristics of rapid nucleating,homogeneously distribution and fast-growing of crystal nucleus.pH value of the solution effects the morphology of crystals,which had the slice-like morphology at pH6 and needle-like at pH9.Compared with 0.05 M and 0.15 M,0.25 M concentration of the solution was advantageous to nucleating and growing of crystal.Degradable properties of coatings,which formed in solution of 0.25 M and pH9 after 4h hydrothermal treatmentat 90℃,were studied.The corrosion potential and break potential of AZ31 alloy were improved after hydrothermal deposition,and corrosion current density was reduced,implied the coating enhanced the corrosion resistance of the alloy.There was no bubble or flaking in the interface of coating and substrate during the immersion in Hank’s solution.The open circuit potential increased continuously up to 2400 s and the radius of capacitance were grows in first 3 days,implied the coating adsorb the ions and form a passive film in Hank’s solution at an early stage.With the extend of immersion time,tiny-pitting corrosion occurred on the surface after immersion for 5d,those pittings expanded and formed etching pit in 12d.