Preparation,Entry Mechanism And Study On Properties Of The Micro-Arc Oxidation Coatings Containing Calcium,zirconium And Iron On Medical Magnesium Alloys

Magnesium is an essential element of human body.In addition,as a new type of degradable metal material,magnesium and magnesium alloys have obvious advantages in the repair and replacement of damaged tissue due to their low density,high specific strength,low elastic modulus,good biocompatibility and antibacterial property.As a biodegradable material,although magnesium alloys can avoid the secondary surgery,in vivo degradation rate is too fast.By changing the electrolyte composition and electrical parameters,the fabricated ceramic coatings directly on the surface of magnesium alloy by micro-arc oxidation?MAO?technology can not only effectively improve the corrosion resistance and biocompatibility of magnesium alloys,but also introduce some trace elements which are beneficial to human body.The corrosion resistance of MAO coatings developed on magnesium alloys can be effectively improved in solutions containing organic phosphorus electrolytes such as phytic acid or sodium phytate(Na₁₂Phy).In this article,Na₁₂Phy was selected as the basic electrolyte,and MAO coatings containing calcium,zirconium and iron elements were prepared by adding electrlytes containing calcium,zirconium and iron.This article mainly carried out by following three aspects:Firstly,in a base solution containing 6 g/L NaOH and 4 g/L Ca?H₂PO₄?₂,0,2,4,6 g/L Na₁₂Phy were separately added to prepare MAO coating on the surface of AZ31B magnesium alloys.The effects of Na₁₂Phy concentration on surface morphology,corrosion resistance,biocompatibility were studied.The changing regulation of organic electrolyte Na₁₂Phy in MAO process and the mechanism of calcium ion entering into MAO coatings were also discussed.The results show that Na₁₂Phy can improve the uniformity,thickness,degradation resistance,calcium and phosphorus amounts of MAO coatings.The fabricated MAO sample in the solution containing 6 g/L NaOH,4 g/L Ca?H₂PO₄?₂ and 4 g/L Na₁₂Phy achieves the excellent corrosion resistance,in vitro bioactivity and good biocompatibility.During MAO,Na₁₂Phy improves the calcium content of MAO coatings by its chelating action and is hydrolyzed into lower myo-inositol phosphate esters and inorganic phosphate radicals under the instantaneous temperature due to spark discharge.Secondly,in a solution containing NH₄HF₂,Na₁₂Phy and K₂ZrF₆,antibacterial coatings were successively fabricated by MAO on AZ31B magnesium alloy.The influences of processing parameters on corrosion resistance of anodic coatings and the entrance mechanism of fluoride and zirconium elements into MAO coatings are investigated by an orthogonal experiment of four factors with three levels.The results show that MAO coatings containing fluorine and zirconium can effectively improve the corrosion resistance of magnesium alloys.The corrosion resistance of the oxide film was affected indirectly by the coating thickness.Phosphate,fluoride and zirconium compete each other to take part in coating formation.Fluoride and zirconium enter into MAO coatings mainly by diffusion.The MAO coatings containing fluorine and zirconium have certain antibacterial properties,and the effect of zirconium on the antibacterial properties of the oxide film is greater than that of fluorine.Thirdly,in the basic solution containing 20 g/L HF,30 g/L H₃PO₄,35 g/L H₃BO₃,8 g/L phytic acid and ammonium salt,the iron-containing coating was prepared by one-step and two-step MAO,respectively.The properties such as surface morphology,structure and corrosion resistance of MAO coatings were studied by SEM,EDS,XRD,XPS and immersion experiments.The results indicate that with the increase of molysite concentration,the developed MAO coatings contain higher iron content with more uiniform micro pores,the increasing pore number and smaller pore size.The highest iron content in the oxide film was 4.85 wt.%.The oxide film is mainly composed of Mg,MgF₂,FeF₃ and Fe₂O₃.Besides,iron-containing MAO films can improve the corrosion resistance of the magnesium alloys.