Comparative Tribological Efficiency of Hard Coatings on Titanium Alloy Across Lubrication Regimes

Abstract

The tribological performance of diamond-like carbon (DLC) coatings on Ti-6Al-4V was evaluated under dry air and 0.9% NaCl at a 1 N normal load using ball-on-disk tests (3000 cycles) with alumina counterfaces. Uncoated Ti-6Al-4V exhibits high friction and wear; therefore, the influence of saline lubrication, relative to dry conditions, on the DLC response was quantified. Steady-state coefficients of friction (COF; mean ± SD, n = 3) decreased from 0.38 ± 0.03 (dry) to 0.19 ± 0.02 (0.9% NaCl; p < 0.001), accompanied by shallower wear tracks and carbon-rich debris consistent with tribofilm stabilization. The ball-specific wear rate (k) likewise decreased from 4.60×10⁻⁵ to 2.23×10⁻⁵ mm³/(N·m) (~51% reduction). These findings highlight the coupling between aqueous lubrication and DLC tribofilm stability and provide guidance for components operating in saline-like environments.