Study On Properties And Surface Modification Of Biodegradable Mg-2Zn-xRE(Gd,Nd,Y)-0.5Zr Alloys

The main factors limiting Mg alloys wide applications are the high degradation rate and relatively low mechanical properties of the Mg.Alloying is one of the most effective methods to improve the corrosion resistance and mechanical properties of the Mg alloys,especially the rare earth elements(REEs).REEs could enhance the corrosion resistance of Mg alloys by formation of some special second phases and REEs could also improve the mechanical properties of the Mg alloys through second phase strengthening and grain refinement strengthening.In addition,surface modification could not only improve the corrosion resistance but also biocompatibility and some biofunctions.The purpose of this thesis was to explore a new type of rare earth-containing Mg alloys with good mechanical properties and corrosion resistance,and to further improve the corrosion resistance,wear resistance and other biological properties of the alloy by means of surface modification.Some meaningful results have been obtained and many attempts have been done for the ultimate application of rare earth'contaning Mg alloys in orthopaedics.Effects of REEs(Gd,Nd and Y)and their content on the microstructure,mechanical properties and corrosion resistance of as-cast Mg-2Zn-0.5Zr alloy were studied.The results showed that all the three elements exhibited good second phase strengthening and grain refinement strengthening.Mg-2Zn-0.5Nd-0.5Zr had the best plasticity(21%)and Mg-2Zn-1 Y-0.5Zr had the highest strength(UTS=220 MPa).Proper addition of Gd,Nd and Y can improve the corrosion resistance of the alloy.When the content of REEs was high,the second phase increased and lead to severe galvanic corrosion,therefore,the corrosion resistance of the alloy decreased.The as-cast Mg-2Zn-1Gd-0.5Zr had the best corrosion resistance.The mechanical properties and corrosion resistance of three kinds of extruded alloys(Mg-2Zn-1Gd-0.5Zr,Mg-2Zn-0.5Nd-0.5Zr and Mg-2Zn-0.5Y-0.5Zr)were studied.The results showed that Mg-2Zn-0.5 Y-0.5Zr alloy had the highest tensile strength(300 MPa)and yield strength(270 MPa).Mg-2Zn-0.5Nd-0.5Zr alloy had the best plasticity(25%).The extruded Mg-2Zn-1Gd-0.5Zr alloy had the best corrosion resistance with the corrosion rate of 0.45 mm/y in Hank's solution.Solution treatment could decrease the yield strength of the alloys.But it could significantly improve the corrosion resistance of the alloys.The elongation of Mg-2Zn-0.5Nd-0.5Zr alloy(27%)increased obviously after 500 ?×5 h treatment.Mg-2Zn-1Gd-0.5Zr alloy had the best corrosion resistance after 510?×5 h solution treatment.The degradation rate of Mg-2Zn-0.5Nd-0.5Zr alloy and Mg-2Zn-0.5Y-0.5Zr alloy could be significantly reduced after 500?×5 h solution treatment.The studies on the effects of three coatings(MAO coating,Sr-P coating and Ca-P coating)on the corrosion resistance and wear property of Mg-2Zn-1Gd-0.5Zr alloy showed that Gd element could increase the compactness of the coatings and stabilize the coatings in the corrosion products of MAO and Sr-P coatings.Therefore,the corrosion resistance of the alloy can be obviously improved.On the contrary,the corrosion resistance of Ca-P coatings was poor because the deposits were coarse and loose,and they were easy to fall off during immersion.But Ca-P coating had good wear resistance,which was mainly due to the rod-like disordered distribution of deposits in the Ca-P coating,which could significantly hinder the friction and wear process and improved the wear resistance of the coating.The surface of MAO coating was smooth and flat.The whole surface coating was integrated and it was not easily disintegrated.Therefore,MAO coating showed good wear property.For Sr-P coatings,the particles in the coatings were small and the coating was thin,therefore,the coating could easily to be destroyed during wear.The studies on the in vitro degradation and antibacterial property of a copper-containing micro-arc oxidation coating on Mg-2Zn-1Gd-0.5Zr alloy showed that the addition of copper could further improve the corrosion resistance of MAO coating.This was mainly because copper could stabilize the coating,and the hydrolysis of copper ion could promote the deposition of Ca,P on the coating surface.Therefore,the protective effect was improved.The addition of copper could make MAO coating possess good antibacterial property even with lower degradation rate.After 12 h co-culture,the antibacterial rate of the Cu+MAO coating could reach more than 96%.Cytotoxicity test showed that Cu+MAO coating had good biocompatibility and the addition of Cu did not reduce the biocompatibility of the coating.