Magnesium Alloy Surface Pretreatment And Its Effects On Preparation And Properties Of Calcium Phosphate Coatings

As a kind of medical metal materials,magnesium?Mg?and its alloys have been considered as a new generation of biodegradable implant materials to repair the hard tissue defects due to their superior mechanical properties matched to the natural bone,good degradability and biocompatibility.However,because of the existence of Cl^-in the human physiological environment,it is prone to occur pitting corrosion,resulting in the mechanical properties of magnesium and its alloys decreased too fast to provide long-term effective mechanical support.Therefore,surface modification has become the most commonly used method to improve the corrosion resistance and surface bioactivity of the magnesium alloys without affecting their mechanical properties.In this work,the CaP coating have been successfully fabricated on the pretreated AZ31 Mg alloy through the combination of chemical conversion method and microwave-assisted liquid deposition.The influence of HF and NaOH pretreatment on properties of Mg alloy substrate and subsequent CaP deposition coating was investigated.The influence of microwave temperature and time on the deposition progress and electrochemical performance of CaP coating on the HF pretreated Mg alloy was also studied.The growth mechanism of the coating was preliminarily explored.Finally,the long-term corrosion resistance,in vitro degradation process and mineralization ability of CaP coating on the HF pretreated magnesium alloys were studied by long-term immersion test.The results showed that the pretreatment layer provided temporary protection for the Mg alloy substrate during the preparation of deposition coating.The structure,composition and thickness of subsequent CaP coatings were obviously affected by the difference between pretreatment layers.As a result,the electrochemical corrosion resistance of the coated sample was different.After pretreated in the 20 wt.%HF solution at 37°C for 2 h,the MgF₂ pretreatment layer was formed.Compared with the NaOH pretreatment,the charge transfer resistance?R_t?of the sample after HF pretreatment was 15.22 k?.cm²,which enhanced the corrosion resistance of the substrate more effectively.In addition,during the process of microwave-assisted liquid deposition,the F^-came from MgF₂ pretreatment layer served as a doping ion to take part in the deposition of the CaP coating,resulting in fluorine doped hydroxyapatite?FHA?deposition with compact nanoneedle-like structure.The thickness of CaP coating prepared under 90°C for 10 min was almost 100?m and the coating contained the mentioned compact FHA as inner layer with prism-like DCPA deposition randomly distributed on its surface.The corrosion potential(E_corr)of the coated sample was-1.49V with the corrosion current densities(I_corr)decreased to 0.15?A/cm² and the R_t reached to 181.90 k?.cm²,indicating the corrosion resistance of the coating was obviously enhanced by the coating.The results of in vitro immersion test showed that the CaP coated magnesium with HF pretreatment possessed excellent long-term corrosion resistance.After immersion for 70 d,the original shape of the coating was kept with the R_t at 181.90 k?.cm² and I_corrorr at 1.81?A/cm².And the degradation rate of the coating was only 0.13 mg/?cm²·d??0.26 mm/y?,which basically met the requirements of degradable implant materials.During the immersion,the mineralization layers composed of different structures of apatite were successively deposited on the surface of the coated sample,demonstrating that the CaP coating on the MgF₂ layer had favorable surface bioactivity and in vitro mineralization ability.