Preparation And Characterization Of Zinc-Copper Alloy With Tunable Degradation

Zinc(Zn)and its alloys have been considered as potential metallic materials for biodegradable medical devices due to their desirable in vivo degradation rates and good biocompatibility.After completing specific functions,the biodegradable Zn-based implants are continuously degraded in the body fluid environment and are gradually absorbed or discharged.The degradation diversity of different implant sites and distinct healing cycles of injured tissues place demands on the tunable degradation of Zn-based alloys.In this paper,the controlled degradation of Zn and its alloys in physiological environment is concerned.The medical Zn alloys with excellent comprehensive properties and graded degradation are prepared through alloy screening,second phase regulation and surface modification.Firstly,the key factors affecting the degradation behavior of pure Zn in simulated body fluid(SBF)were explored by electrochemical and immersion tests.And a Zn-Cu-Li alloy with excellent comprehensive performance was designed,which was prepared by vacuum smelting and rolling technologies.Then,the surface of the alloy was modified by Cu ion implantation technology,and the application potential of the surface modified alloy was comprehensively evaluated by means of microstructure analysis,mechanical tensile test,immersion test,in vitro cell culture and antibacterial test.Finally,the effects of the surface gradient structure induced by Cu ion implantation on the degradation process of the prepared alloy was analyzed by micro-or nano-scale characterization technology.The results showed that the corrosion resistance of corrosion product layer was an important factor to determine the degradation rate of Zn in SBF,but localized corrosion significantly accelerated the degradation of Zn and led to higher failure risk.The degradation products of pure Zn had multilayer structure,among which ZnO,Zn3(PO4)2,Zn5(OH)8Cl2,Zn5(CO3)2(OH)6 and Ca/P phases possessed good corrosion resistance.The degradation mode of pure Zn in SBF gradually changed from uniform degradation accompanied by intergranular corrosion to localized degradation which originated from grain boundaries and preferentially extended to the interior of {0001} oriented Zn grains.Cu alloying improved the antibacterial ability,electrochemical stability and corrosion resistant of degradation products of Zn matrix,but the formation of CuZn4 second phase induced galvanic corrosion.The continuous layered CuZn4 corrosionresistant barrier regulated by rolling process could avoid severe localized corrosion of pure Zn.Meanwhile,the Cu element was conducive to maintaining plasticity and the trace Li element could significantly improve the strength of Zn.Therefore,the novel Zn-4Cu-0.02Li alloy had excellent mechanical properties.The yield strength,tensile strength and elongation at fracture of the alloy were 256 MPa,342 MPa and 39.8%,respectively.Ion implantation technology induced a surface modified layer with gradient structure without altering the original size of the sample.The stable degradation rate of the modified layer within 30 days was reduced by 50%compared with that of the substrate area unaffected by implantation,and the graded degradation of the alloy from "slow to fast" was realized.The surface modified layer consisted of three gradient regions from outside to inside:a 70-nm element implanted layer,a 600-nm nanocrystalline layer,and a 2-μm columnar crystal layer.Cu ion implantation significantly enhanced the inhibitory effect of S.aureus,but did not remarkably affect the viability of human umbilical vein endothelial cells(HUVECs)and mouse embryonic osteoblasts(MC3T3).The microstructure evolution induced by the long-range effect of ion implantation was the key to control the degradation rate and degradation mode of Zn alloy.The degradation rate of different gradient areas of the surface modified layer was in the order of element implanted layer>columnar crystal layer>nanocrystalline layer from fast to slow.The outermost element implanted layer had a large number of structural defects affected by the ion bombardment on the surface,resulting in a much higher degradation rate than the substrate.The grain refinement of nanocrystalline layer was beneficial to the formation of corrosion-resistant products,and had the strongest corrosion resistance.The degradation rate of columnar crystal layer was affected by defects,local strain accumulation,texture,grain size and amorphization of corrosion products.The uniform strain,the uniform distribution of Cu elements,the poor orientation of small grains and the CuO enrichment at phase interface induced by Cu ion implantation all promoted the uniform degradation of the alloy.Therefore,the "tunable degradation" of Zn alloy could be achieved by adjusting the thickness of each zone of the surface modified layer,which had great application potential.