Abstract
Friction fade‑out (FFO) is a remarkable superlubricity in which friction coefficients approach 10⁻⁴ when a catalytic ZrO₂ pin slides against a hydrogenated DLC film in an H₂ environment containing ppm‑level ethanol vapor. This study developed a flywheel‑type tribotester to investigate FFO in pivot bearings operating at high rotational speeds under circumferential line‑contact conditions. The test bearing consisted of a 7.93‑mm ZrO₂ ball and a hydrogenated DLC‑coated SUJ2 cup. Experiments were conducted at 100–770 rpm (sliding speeds: 21–160 mm/s) under a maximum load of 73.5 N (Pmax = 0.22 GPa) in an H₂ environment with ethanol vapor. After the run‑in process, FFO was established; increasing rotational speed slightly increased the friction coefficient to 10⁻³, whereas reducing speed restored the FFO state. This reversible behavior indicates that FFO is highly sensitive to the spatial overlap of tribofilms formed on the ZrO₂ ball and the DLC‑coated cup. The appearance of FFO at low Pmax suggests that FFO is not governed solely by contact pressure but is strongly influenced by sliding speed. Post‑test observations confirmed uniform tribofilm formation and negligible wear. These findings demonstrate that FFO can be sustained under high‑speed rotation even at low line‑contact pressures, highlighting its potential for oil‑free, long‑life bearing systems.
