NiTi/Ti-Ag Gradient Alloy Structure Evolution And Mechanical And Antibacterial Properties

NiTi alloy of equiatomic powder not only has excellent mechanical properties,good corrosion resistance and biocompatibility,but also has unique shape memory effect and superelasticity.So,it was widely used in the medical field.However,it was found that the elastic modulus?⁵5 GPa?of NiTi alloy does not match the human bone elastic modulus?3²0 GPa?in clinical applications,and it is easy to produce stress-shielding effect at the bone tissue-implant interface after implantation,causing the material to loosen or break.At the same time,it belongs to the bio-inert material,it is difficult to form a bony bond with human tissues,which will reduce the life of the implant.Porosity of the material can reduce the elastic modulus and promote bone tissue ingrowth and vascularization,which can easily form stable biological fixation?bone consolidation?.However,the porous structure can lead to the strength of alloy decreased significantly and limit its application.In view of the above problems,this paper designs a NiTi/surface porous Ti-Ag gradient alloy.The middle of alloy is block alloy with a porous layer around it.NiTi alloy matrix retaining material s have excellent mechanical properties,corrosion resistance and biocompatibility,and the surface porous Ti-Ag layer can provide good biological activity and antib acterial properties for the material s,which improved and enhanced the problems in clinical application of NiTi alloy.On the basis of the previous work,NiTi/surface porous Ti-Ag gradient alloy was fabricated by spark plasma sintering?SPS?technology.The effects of sintering temperatures and Ag contents on microstructure,interface bonding,surface pore characteristics,mechanical properties,in vitro biological activity and antibacterial property of the alloy were investigated and the following conclusions were drawn:The NiTi/Surface porous Ti gradient alloy are consisted of NiTi,a-Ti,Ni,Ti₂Ni,Ni₃Ti mixed phase and gradually transformed into NiTi and a-Ti phase with increasing of sintering temperatures.Furthermore,we can observe a stable metallurgical bonding on the internal and external interface of the alloy.Meanwhile,the porosity and average pore size of surface porous layer are in a slowly decreasing trend.The matrix of NiTi/Surface porous Ti-Ag gradient alloy prepared at 950°C are mainly consisted of NiTi phase and a few complex phases of Ti₂Ni,and the surface porous layers are mainly consisted of a-Ti,Ti Ag and Ti₂Ag phases.Furthermore,we can observe a stable metallurgical bonding on the internal and external interface of the alloy.With increasing of Ag contents,the porosity and average pore size of surface porous layer did not vary significantly.With the increase of temperature,compressive strength of NiTi/surface porous Ti gradient alloy is significantly increased?from 179.8 MPa to 632.0 MPa?,but compressive elastic modulus of the alloy is changed not obvious?3.5 GPa to 9.0 GPa?.The mechanical properties of gradient alloys are closely related to the microstructure evolution,interface bonding,surface pore characteristics,and matrix density of the alloy.At low temperature,there are many complex phases in the gradient alloy,and the interface is defective and the matrix density is poor,so the strength of gradient alloy is low.As the temperature increases,the complex phase is reduced,the interface is well combined,and the density of the matrix is increased,so the intensity is greatly improved.With the increase of Ag content,compressive strength of NiTi/Surface porous Ti-Ag gradient alloy increased?431.6 MPa⁵53.1 MPa?,and the compressiv e elastic modulus was no change.The addition of Ag element and Ti element in the porous layer of the surface formed Ti Ag and Ti2Ag phase,which played the role of solid solution strengthening,so the compressive strength of the gradient alloy showed an increasing trend.The apatite-forming ability of gradient alloy is better than that of block alloy after soaking in SBF for 14 d,showing excellent in vitro biological activity.Meanwhile,the addition of Ag significantly improved the antibacterial properties of the gradient alloy and the higher the Ag content,the strong er the antibacterial performance.