Study On The Tribocorrosion Behavior Of MAO Coating On Magnesium Alloy

As a degradable metal implant material,magnesium alloy has an elastic modulus that matches natural bone,good biocompatibility and biosafety,which is widely used in orthopedics and oral implants.In comparison,it shows unparalleled advantages over other metal implants.However,magnesium alloy has low hardness and poor corrosion resistance.It will not only cause local abrasion due to human movement or chewing by tooth,but also will be severely corroded by the attack of corrosive ions in the human environment.The synergy of the two will cause a greater degree of degradation of the implant material,and then prematurely lose its due mechanical properties and other related functions.Therefore,it is crucial to adopt appropriate surface modification techniques to improve the tribo-corrosion behavior of medical magnesium alloys.Micro-arc oxidation(MAO)technology can prepare ceramic coating with high hardness,good wear/corrosion resistance,which is widely used in the surface reinforcement of magnesium alloys.In this thesis,the MAO coating was prepared on the surface of AZ91 magnesium alloy in silicate-based electrolyte containing ZnO nanoparticles(ZnO NPs)additives.The effects of positive current density,MAO treatment time and ZnO NPs additives on the microstructure,composition,hardness,roughness,corrosion resistance and of MAO coating were studied.The research on the influence of different factors on the tribocorrosion behavior of the coating is focused,and the related mechanisms are discussed.The results show that with the increase of positive current density and MAO treatment time,the pores and cracks of the coating are increased,but the number of micropores is significantly reduced and the thickness,hardness,roughness of the coating are increased significantly.With the addition of ZnO NPs,the coating becomes denser,and the number and size of the pores and surface roughness are reduced considerably.Analysis of EDS,XRD and XPS shows that the coating mainly contains Mg O,Mg₂Si O₄,Mg,ZnO,and a small amount of Mg₃(PO₄)₂.Meanwhile,it is also found that the increase of current density and MAO treatment time is conducive to the formation of the Mg₂Si O₄ phase.Electrochemical corrosion experiments show that MAO technology significantly improves the corrosion resistance of magnesium alloys.With the increase of positive current density,the coating's corrosion resistance is increased first and then is decreased.With the increase of MAO treatment time,the corrosion resistance of the coating is10 min>2 min>5 min>15 min.After increasing the concentration of ZnO NPs additives,the corrosion resistance of the coating is improved.The tribocorrosion experiment shows that the magnesium alloy matrix suffered severe abrasive wear after tribocorrosion,accompanied by the appearance of a bit of corrosion pits,and the degradation product was Mg(OH)₂.MAO treatment significantly improved its anti-tribocorrosion.When the positive current density is small or the MAO treatment time is short,the coating wears out easily,and there are local corrosion pits.With the increase of the positive current density or the prolongation of time,the tribocorrosion behavior of the coating has been enhanced.With the increase in the concentration of ZnO NPs,the anti-friction and wear resistance effects of the coating make better significantly.The wear mechanism is mainly fatigue wear,the surface damage is relatively light,and no corrosion pits are found.