Research On Electropolishing And Oxidation Compound Surface Modifications Of NiTi Alloy

Due to its unique shape memory effect,superelasticity and good biocompatibility,NiTi alloy has been used more and more widely in the applications of biomedical field.However,the alloy contains more than 50 at%Ni,as well as a variety of toxic effects caused by the nickel dissolution in the physical environment,so it is necessary to carry out surface modifications to improve its corrosion resistance and biocompatibility.In this paper,the effects of electropolishing,electropolishing and oxidation compound treatment on the properties of NiTi alloy have been systematically studied,such as surface characterization,mechanical property,corrosion resistance and hemocompatibility,using scanning electron microscopy(SEM),electron probe microanalysis(EPMA),X-ray photoelectron spectroscopy(XPS) and X-ray diffraction(XRD).The results show that:The electrolyte composition is critical to the electropolishing of NiTi alloy.The optimium processing parameters of electropolishing are the current density of 1.0 A/cm²,electrolyte temperature of 0℃,electrode gap of 1.5cm and polishing time of 45s.Electropolishing can increase the TiO₂ content of NiTi surface,reduce the Ni content and significantly increase the Ti/Ni ratio.If the process parameters controlled unsuitably,the sample surface is prone to segregate,forming the granular compound enriched Ti and C.With the increasing oxidation temperature,the TiO₂ structure on the surface of NiTi alloy after compound treatment changes from anatase to rutile.The outmost layer of oxide structure is TiO₂,followed by TiO₂+Ni,Ni₃Ti and NiTi matrix.In comparision with oxidized sample that was preliminarily polished,the surface of compositely processed one is smoother and the roughness is lower.Electropolishing has little effect on the mechanical properties of NiTi alloy.The phase tranformation behaviors and mechanical properties of compositely treated NiTi samples are most related to oxidation temperature.After electropolishing and oxidation(500℃×0.5h) compound treatment,the A_f temperature of NiTi sample is closest to body temperature and it also has excellent superelasticity.No pitting occurred for the sample of electropolishing and oxidation(500℃×0.5h) compound treatment when the polarization potential reached 2.0V,showing better corrosion resistance.Compositely treated NiTi sample was hydrophobicity,the amount of adherent platelets on the surface is less.And their shapes are circular,almost without variation.Longer dynamic clotting time shows better anti-clotting properties.