Abstract
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, Pmax 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.
