Preparation And Properties Of Composite Coating On AZ31 Magnesium Alloy

As lightweight metal materials,magnesium alloys exhibit excellent physical and chemical properties,and have been widely used in the automotive industry,light equipment,electronic products,biomedicine,etc.due to its low density,high strength and good biodegradability.However,the poor corrosion resistance of magnesium alloys limits their application.At present,surface modification is an effective method for increasing the corrosion resistance of magnesium alloys,besides improving purity and heat treatment.Surface treatments of magnesium alloys include electroless plating,chemical conversion,ion implantation,anodizing,micro-arc oxidation(MAO),vapor deposition,etc.Among them,MAO coating grows in situ on the surface of alloys,and its combination with the substrate is stable,which can effectively inhibit the corrosion of the substrate,but it still exists shortcomings.The compostite coating based on MAO treament not only solves the problem,but also give the magnesium alloys multiple functions to meet the application requirements.In this paper,AZ31 magnesium alloy was used as a substrate material.Two composite coatings with multiple functions,i.e.MAO/PEI/Ag composite coating and MAO/PLA/ ZnO composite coating,were formed on the surface of AZ31 magnesium alloy by using MAO and different technologies.The microstructure,composition and phase structure of the composite coatings were characterized by SEM,EDS,XRD,XPS,etc.The corrosion resistance was evaluated by electrochemical methods and the corresponding antibacterial activity was also evaluated with Staphylococcus aureus(S.aureus)as model bacteria.Then the biocompatibility of the composite coatings were investigated with the evualtion of cell viability and proliferation.The conclusions are as follows.(1)MAO/PEI/Ag composite coating was prepared on the surface of AZ31 magnesium alloy through MAO and self-assembly technology.The self-assembled coating of polyethyleneimine(PEI)and silver nanoparticles(Ag NPs)significantly improves the surface structure of the MAO coating.The corrosion current density of the composite coated sample in phosphate buffer solution(PBS)is about 3 orders of magnitude lower than that of the untreated magnesium alloy substrate.The survival rate of S.aureus on the surface is only 1.28%,which is 1/13 of the magnesium alloy substrate.The good corrosion resistance and antibacterial properties of the composite coating make it possible to be used to modify magnesium alloys as antibacterial materials.(2)MAO/PLA/ ZnO composite coating was prepared on the surface of AZ31 magnesium alloy by combining MAO and dip coating.The thickness of the composite coating is 37.46±1.02 nm.The micropores formed during MAO were sealed by polylactic acid(PLA)and zinc oxide( ZnO)nanoparticles with formation of an organic-inorganic coating,which significantly increases the surface density.The corrosion current density of the composite coated sample is reduced by about 4 orders of magnitude compared to that of bare magnesium alloy substrate.After soaking in simulated body fluid(SBF)for 14 days,the polymer on the surface of the coating was partially degraded,and the amount of hydrogen evolution is only 0.48 m L/cm~2,which is 3/65 of the magnesium alloy.After analysis,in addition to the contribution of PLA,the addition of ZnO nanoparticles also effectively hinders the corrosion of the substrate by the etching solution.(3)The antibacterial rate of MAO/PLA/ ZnO composite coated AZ31 is about 72.32%,which an increase of 31.60%compared to that of bare magnesium alloy substrate,indicating a good antibacterial property.The addition of ZnO nanoparticles results in enhancement of the hydrophilicity of coatings,and promotes the growth and proliferation of pre-cranial osteoblasts in mice in vitro.The enhanced cell compatibility of magnesium alloy with MAO/PLA/ ZnO composite coating will be benefit to its application as a bone implant material in clinical treatment.