Tailoring the Surface Performance of Near-β Ti-13Nb-13Zr Alloy Through TaN/Au Bilayer Coatings: Electrochemical Stability, Tribocorrosion Mechanisms and Biocompatibility

β-Type titanium alloys mitigate stress shielding through their low elastic modulus, but their limited wear resistance restricts their use in articulating surfaces. This study evaluated the effect of a 100 nm Au interlayer on the structural, electrochemical, tribocorrosion and biological behaviour of TaN coatings deposited by magnetron sputtering on aged Ti-13Nb-13Zr. Coatings were characterised by XRD, SEM and AFM and assessed by potentiodynamic polarisation, electrochemical impedance spectroscopy and ASTM G119 tribocorrosion tests in Ringer’s lactate at 37 °C, ICP-MS, and MTT and LDH assays with MG-63 cells. Both coatings were columnar and dominated by δ-TaN; the interlayer attenuated the ε-TaN and γ-Ta2N phases and increased the crystallite size from 24 ± 3 to 31 ± 2 nm. The TaN/Au system shifted the corrosion potential from −210 ± 18 to +35 ± 10 mV vs. Ag/AgCl and lowered the corrosion current density 23-fold. Under tribocorrosion, the friction coefficient fell from 0.28 to 0.17, the wear volume fell 7.2-fold and the synergistic fraction fell from 35% to 14%. Ti release fell from 1.85 to 0.16 µg·cm−2 and 72 h cell viability rose from 82 ± 5% to 96 ± 3%. A nanometric noble interlayer is therefore a promising strategy for articulating surfaces of β-titanium alloys.