Mechanical and electrochemical properties of Al alloyed D8m-Ta5Si3 nanocrystalline coatings

In this work, the structural stability, mechanical properties and electronic structure of β-Ta5Si3 with different Al additions were first predicted by first-principles density functional calculations. The binary and ternary β-Ta5Si3 are thermodynamically and mechanically stable, but their thermodynamic stability slightly decreases with Al additions. The elastic constants, bulk modulus (B), shear modulus (G) and Young's modulus (E) of Al-alloyed β-Ta5Si3 were calculated. According to the criteria involving the bulk/shear modulus (B/G ratio) and Poisson's ratio, the D8m-structured ternary Ta5Si3, where two Si4a atoms are replaced by two Al atoms, shows improved ductility. Based on the theoretical analysis, the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating was deposited onto polished Ti-6Al-4V alloy substrates by double cathode glow discharge plasma method. The phase constitution, microstructure and composition of the coating were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The as-deposited coating was composed of nearly rounded D8m-β-Ta5Si3 grains with an average size of ∼4 nm and exhibited strong (002) and (400) preferred orientation. After Al alloying, the hardness and the compressive residual stress of the β-Ta5Si3 decreased, and the contact damage resistance and adhesion strength increased. The electrochemical corrosion properties of the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating were compared to the binary β-Ta5Si3 coating and Ti-6Al-4V in 3.5 wt% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. It was shown that the corrosion resistance of the β-Ta5Si3 was improved through the substitution of Al for Si.