Study On Properties And Stress Corrosion Behavior Of Strontium-containing Biomedical Magnesium Alloys

Magnesium alloys as biodegradable implant materials have received much research attention recently,because of their suitable mechanical properties and controllable corrosion rate.However,the interactions between the mechanical strength and the corrosion process(i.e.stress corrosion),which can lead to the early failure of the implant material,is hardly investigated.In this thesis,with the purpose of using biodegradable magnesium alloy as bone plate and cardiovascular scaffold,new strontium(Sr)containing biodegradable magnesium alloys – ZK40xSr(Mg-4 wt.%Zn-0.6 wt.%Zr-xSr,x=0,0.4,0.8,1.2,1.6 wt.%)were prepared.The effects of Sr on the microstructure,corrosion and stress corrosion cracking of the as-cast and forged ZK40 xSr alloys were investigated.The corrosion resistance and stress corrosion degradation properties of surface modified ZK40-0.4Sr alloys were studied(i.e.micro-arc oxidation(MAO)and micro-arc oxidation/poly(lactic-co-glycolic)acid(MAO+PLGA)composite coating on forged ZK40-0.4Sr).The results showed that Sr could refine the grain size and present as the Mgx ZnySrz phase in the grain boundaries of as-cast ZK40 xSr alloys.The refined grain improved the tensile strength and the elongation to failure rate,while the micro-galvanic corrosion formed between the boundary precipitates and the matrix,which led to the corrosion rate of the ZK40 xSr alloys increasing with the increase of Sr content.Moreover,the micro-galvanic corrosion along the strip or reticular grain boundary accelerated the radical expansion under mechanical stress.Namely,with the increase of Sr content,the stress corrosion sensitivity of the as-cast ZK40 xSr alloys increased.After forging deformation,the average grain size of the ZK40 xSr alloys decreased obviously from hundreds of microns to a few microns.The morphology of the grain boundaries changed from a strip or reticular structure to an island or ellipsoidal structure.The tensile strength and elongation to failure rate of the forged ZK40 x Sr alloys were improved significantly.The corrosion resistance of the forged samples was also improved.However,the refinement of the grains led to the increase of the absorption and propagation velocity of hydrogen atom in the forged ZK40 and ZK40-0.4Sr alloys,which caused brittleness and increased stress corrosion sensitivity.In the contrast,the uniformly distributed island or ellipsoidal second phase of the forged ZK40-1.2Sr and ZK40-1.6Sr alloys could partially block the propagation of hydrogen atoms in the matrix alloy,which reduced the stress corrosion sensitivity.MAO coatings and MAO+PLGA composite coatings were prepared on the forged ZK40-0.4Sr alloys.The results showed that the corrosion resistance of the forged ZK40-0.4Sr alloy was greatly improved.For instance,corrosion current density of the MAO+PLGA composite coated samples decreased by 3 orders of magnitude,and the electrochemical impedance performance wasimproved.The residual strength of stress corrosion test of the forged ZK40-0.4Sr bare metal under 0,20,40 and 60 MPa stress decreased with the increase of loading stress.The bare metal samples fractured after 11 d of stress corrosion test under 60 MPa.Due to the brittle nature of the MAO film,physiological solution could penetrate from the holes or cracks of MAO film into the matrix.It could cause the corrosion,especially the holes or cracks amplified under stress,deteriorate.So the MAO film could,to some extent,improve the corrosion resistance of alloy.The pores or micro-crack of MAO film could be sealed by PLGA coating which had a certain degree of plasticity and perfect corrosion resistance.The MAO+PLGA composite coating could still protect the substrate with or without stress in m-SBF.The residual tensile strength was 228.5 MPa and 209 MPa after 28 d with or without stress in m-SBF,respectivly.In this thesis,the biocompatibility and blood compatibility of the ZK40 x Sr bare metal and MAO,MAO+PLGA and MAO+PLGA+Vancomycin composite coatings were investigated.The results showed that the cytotoxicity of ZK40 xSr bare metal was level 2 and the hemolysis rate was higher than national standards.So the bare metal was not suitable to be used as an implant material directly.After surface modification by MAO,MAO+PLGA and MAO+PLGA+Vancomycin composite coatings,the cytotoxicity grade was level 1.In particular,there were many MC3T3-E1 cells adhesion,little platelet adhesion and hemolytic rate(2.95 %,3.91 %)of the MAO+PLGA and MAO+PLGA+Vancomycin composite coatings.It could meet the requirements of national standards and have excellent in vitro biocompatibility.