Theoretical Calculation And Experiment Research On Properties Of Nb-Alloyed Ti₅Si₃-Based Nanocomposite Coatings

The effect of Nb alloying on the properties of Ti5Si3were studied by experiment and first–principles calculation in this paper. The calculated results show that (Ti1-xNbx)5Si3(x=0,0.0333,0.0667,0.167) satisfy the conditions of thermodynamic stability and mechanical stability, but as thecontent of Nb increases, the (Ti1-xNbx)5Si3tends to be unstable thermodynamically; the addition of Nblowers the shear modulus (G) and Young，s modulus (E)，accompanied by a little change of bulkmodulus(B). The Nb alloying improve the toughness of Ti5Si3for the empirical constant of B/G,Poisson’s ratio and Cauchy pressure which are used to predict the toughness of material increased.The reason for the improved toughness of (Ti1-xNbx)5Si3can be explained from the perspective ofelectronic structure：the bonding direction of Ti4datoms along the c axis modified according to thecharge density difference and the distance of (001) is lager after niobium addition，both of the twofactors reduce the CTE(c),then the toughness of (Ti1-xNbx)5Si3is improved. The addition of Nbweaken the bonding between the Ti(Ti6g)3d and Si3p electrons which can be deduced from DOS andcharge density difference analysis，and this phenomenon is related to the improved toughness of(Ti1-xNbx)5Si3.The nanaocrystalline (Ti1-xNbx)5Si3(x=0,0.04,0.1,0.2) coatings have been deposited ontoTi6Al4V alloy successfully using the double cathode glow discharge technique. The XRD resultsshow that the nanocrystalline (Ti1-xNbx)5Si3coatings are consisted of single hexagonal D88-structuredTi5Si3phase. The cross-sectional SEM morphology indicates that the coatings are10μm thick andwell bonded to the substrate. The TEM image reveals closely agglomerated cauliflower-likecolonies, each about180nm in diameter, composed of many smaller crystallites withan average grain size of~15nm. Nanoindentation tests show that the coatings largely enhance thehardness of TC4，the addition of Nb lowers hardness and elastic modulus of Ti5Si3. Scratch tests showthe adhesion strength between the coatings and the substrate is enough for engineering application.The electrochemical tests before and after immersion in5wt%H2SO4solution show that the additionof Nb enhances the corrosion resistance of Ti5Si3. XPS analysis reveals that the passive layers formedon the Nb-containing Ti5Si3nanocrystalline coatings are highly enriched in SiO2, since SiO2exhibits ahigher dielectric characteristic and chemical stability than TiO2。The Mott–Schottky analysis indicatesthat Nb alloying can markedly reduce the donor densities of the passive film, lower the the flat bandpotential and promote the formation of a thicker passive film. The electrochemical tests in simulated physiological media (37℃Ringer’s solution) show that the coatings enhance the corrosion resistanceof TC4，and Nb alloying also enhances the corrosion resistance of Ti5Si3. The analysis shows that Nballoying can lower the residual stress level in the films, and tailor the structural,electronic and chemical properties of the passive layers.(Ti1-xNbx)5Si3will be promisingfor biomedical applications.For the nitride on the (Ti1-xNbx)5Si3coating during the preparation process, the effects of Nbaddition on the properties of TiSiN coatings are systematic studied. The result indicate that TiNbSiNcoatings have superior mechanical and corrosion resistance properties, and TiNbSiN is expected to bea value system for the surface modification of titanium.