Environmental effects on superlubricity of hydrogenated diamond-like carbon: Understanding tribochemical kinetics in O₂ and H₂O environments

Hydrogenated diamond-like carbon (H-DLC) exhibits superlubricity in inert conditions, making it ideal for protective coating in machines, but its superlubricity is often lost in humid air. Knowing reaction mechanisms of such oxidation processes would be a prerequisite to incorporate environment tolerance into the H-DLC chemistry for superlubricious performance in ambient air. But mechanistic understanding has been hampered by air-oxidation of the tribo-tested H-DLC surface during the sample transfer for ex-situ analyses. As an alternative approach, this study derived a Langmuir-type kinetics model to analyze tribochemical processes occurring at the H-DLC surface in oxidative environments. The model unveiled very distinct environmental effects of O₂ and H₂O on tribochemistry of H-DLC. The O₂-oxidized H-DLC surface retains low friction after the run-in period (μ≈0.057) but is susceptible to frictional wear (wear rate ≈ 32 μm³/(N·mm)). In contrast, the H₂O-oxidized surface is susceptible to the adsorption of water which acts like boundary lubricant layers protecting the surface from wear (∼3.4 μm³/(N·mm)) but with relatively high friction after the run-in period (μ≈0.22). The oxidation probability of H-DLC by O₂ or H₂O (∼10⁻⁴ Torr⁻¹ s⁻¹) is found to be comparable to the reaction probability of hydrocarbons on catalytically-active metals (∼10⁻³ Torr⁻¹ s⁻¹).