First-Principles Calculation And Biological Properties Of ?-?Nb?1-x?Ag_x?₅Si₃

The effect of Ag addition on the structural stability,meachanical properties and thermodynamic properties of?-Nb₅Si₃ were systematically studied by using the first-principles calculations based on density functional theory.The calculated formation enthalpies shows that all the?-(Nb_1-xAg_x)₅Si₃compounds are thermodynamic stable due to their negative enthalpies,and Ag atoms prefer Nb^4c site rather than the Nb^16l site.With increasing concentration of Ag,the elastic moduli and theoretical Vickers hardness of the?-(Nb_1-xAg_x)₅Si₃ compounds decrease,but they become more ductile according to estimation from Pugh's rule?B/G?,Poisson's ratio,Cauchy pressure,Peierls stress and stress-strain responses.Different anisotropic indexes and factors are calculated to characterize the elastic anisotropy,and the results have shown that Ag addition slightly rises the mechanical anisotropies of?-(Nb_1-xAg_x)₅Si₃ compounds.Ag alloying increases the linear thermal expansion?LCTE?coefficients of the?-(Nb_1-xAg_x)₅Si₃ compounds along the c and a axes directions,with a decrease of thermal expansion anisotropy.The analysis of density of States and differential charge densities shows that the main reason for the improvement of toughness is that the strength of Nb-Si covalent bond decreases with the addition of alloying element Ag.Depending on the calculation results,the?-Nb₅Si₃,?-(Nb_0.95Ag_0.05)₅Si₃ and?-(Nb_0.9Ag_0.1)₅Si₃nanocrystalline coatings were deposited onto mirror-polished titanium alloy substrates by a double cathode glow discharge plasma method.The XRD results show that all coatings are consisted of only tetragonal?-Nb₅Si₃ phase.The cross-section SEM images show that all the coatings with a uniform thickness of 12¹5?m,exhibit a dense and largely homogeneous microstructure and are strongly attached to the substrate,free of any visible interfacial delamination or voids.TEM images indicate that the?-(Nb_1-xAg_x)₅Si₃?x=0,0.05 and 0.1?coatings are composed of spherical grains with grain size at nanometer level.Electrochemical tests were employed to analysis the corrosion behavior of the three coatings and substrate in simulated body fluid?Ringer's solution?.The electrochemical experiment results show that:the three as-deposited coatings can remarkly enhance the corrosion resistance of the Ti-6Al-4V alloy in simulated body fluid,and their corrosion resistance appear almost independent with the soaking time.The results of antimicrobial experiment showed that both the substrate and the?-Nb₅Si₃ coating had no antibacterial properties and the bacteria could grow and reproduce normally.The Ag-deposited?-(Nb_1-xAg_x)₅Si₃ coatings had a good inhibitrory effect on the growth of bacteria,and the bactericidal rate was almost 100%after 24 hours.The biocompatibility study showed that the three nanocrystalline coatings were soaked in Hank's solution for 15 days,and the apatite layer was formed on both of the surfaces,which was beneffical to the coating materials'rapid osseointegration in the living body.Cytotoxicity test results showed that?-(Nb_0.9Ag_0.1)₅Si₃coatings had a slight inhibitory effect on cell growth,and the three coatings had no cytotoxicity.Therefore,the?-(Nb_1-xAg_x)₅Si₃ coating has good mechanical properties,antibacterial properties and biocompatibility,and has no biological toxicity to human cells.It can provide help to improve the defects of titanium alloy human transplantation materials.