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 ZrO pin slides against a hydrogenated diamond-like carbon (a-C:H) film in H environments containing alcohol vapor.FFO achieves ultralow friction coefficients near 10 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 H environments containing ethanol-derived decomposition gases (CH, CH, CH, CO) and analyzed the resulting tribofilms to infer their formation pathways.Stable FFO was observed with CH at relatively low loads (19.6 N, 1.8 GPa), whereas CO addition increased the load capacity to 49.0 N (2.4 GPa).Depth-resolved XPS suggested that ethanol-derived tribofilms were dominated by sp-hybridized carbon, with subsurface fractions reaching 70-78 at.%.These sp structures are inferred to transform into softer sp-sp bonds at the sliding interface through hydrogenation.TOF-SIMS suggested that sp-hybridized tribofilms likely consist of linear polyenyne polymers [-(CC)CH=C(CH)-] n , while CH is presumed to yield non-polymerized polyyne monomers H(CC)H.CO addition may promote sp cross-linking, thereby enhancing tribofilm stiffness and load capacity.These findings support a tribo-induced topochemical polymerization forming sp-hybridized tribofilm.The results provide new insight into the potential for sustained superlubricity under high-load, oil-free conditions.