Friction Fade‑Out of DLC Films in Hydrogen Environments via Tribocatalytic Formation of sp¹‑Hybridized Polyenyne Tribofilms

Friction fade-out (FFO) is a superlubricity phenomenon observed when a catalytic ZrO₂ pin slides against a hydrogenated diamond-like carbon (a-C:H) film in H₂ environments containing alcohol vapor.FFO achieves ultralow friction coefficients near 10⁻⁴ under high loads (63.7 N, P max 2.6 GPa).However, the tribocatalytic mechanism and carbon-bonding structures remain incompletely understood.This study investigated FFO in H₂ environments containing ethanol-derived decomposition gases (C₂H₄, CH₄, C₃H₆, CO₂) and analyzed the resulting tribofilms to infer their formation pathways.Stable FFO was observed with C₂H₄ at relatively low loads (19.6 N, 1.8 GPa), whereas CO₂ addition increased the load capacity to 49.0 N (2.4 GPa).Depth-resolved XPS suggested that ethanol-derived tribofilms were dominated by sp¹-hybridized carbon, with subsurface fractions reaching 70-78 at.%.These sp¹ structures are inferred to transform into softer sp²-sp³ bonds at the sliding interface through hydrogenation.TOF-SIMS suggested that sp¹-hybridized tribofilms likely consist of linear polyenyne polymers [-(C≡C)₄CH=C(CH₃)-] n , while C₂H₄ is presumed to yield non-polymerized polyyne monomers H(C≡C)₄H.CO₂ addition may promote sp² cross-linking, thereby enhancing tribofilm stiffness and load capacity.These findings support a tribo-induced topochemical polymerization forming sp¹-hybridized tribofilm.The results provide new insight into the potential for sustained superlubricity under high-load, oil-free conditions.