Research On Rotational Fretting Behaviors Of Two Solid Lubrication Coatings

Rotational fretting can be defined as the relative motion which induced by reciprocating rotation under the oscillatory vibratory environment. The rotational fretting damage is common in modern industrial or biomedical applications, such as ball-and-socket joint, wheel-axle of train, yoke-axle, hip joint and knee joint of human bodies and so on, finally, shortens the service life of parts. Solid lubricant coatings have found a wide industrial application as a main protective method to resist the fretting damage. However, researches on rotational fretting wear behaviors and damage mechanism have been rarely investigated.Two solid lubrication coatings (Bonded molybdenum disulfide coating and ion sulfide layer) and substrate (LZ50 axle steel) have been carried out on a new rotational fretting test system under ball-on-flat contact conditions with various normal loads and angular displacement amplitudes (θ). Dynamic analyses in combination with the microscopic examinations through optical microscop (OM), scanning electrical microscop (SEM), energy dispersive spectroscopy (EDX), and profilometer, have been performed to study the running behavior and damage mechanisms of rotational fretting for the substrate and two solid lubrication coatings, and a physical model on rotational fretting wear was built. Main conclusions are drawn as follows:1. The rotational fretting regimes of two solid lubrication coatings and substrate were strongly dependent on rotational angular displacement amplitudes and normal loads which resulted in partial slip regime or slip regime, but no mixed fretting regimes. Fretting regimes of the substrate were changed by the bonded molybdenum disulfide coating, i.e. SR shifted to lower angular displacement amplitudes; however, fretting regimes of the substrate were not changed by the ion sulfide layer.2. The friction coefficient of two solid lubrication coatings and substrate indicated that molybdenum disulfide coating have shown good friction-reducing effect in the whole of fretting regimes; ion sulfide layer had shown good friction-reducing effect in the partial slip regime and the initial stage of the slip regime, but in the steady stage of the slip regime, the friction coefficient was consistent with the substrate due to damaged.3. The damage of two solid lubrication coatings and substrate in the partial slip regime were very slights; in the slip regime, the damage mechanism of the ion sulfide layers and substrate were mainly delamination, abrasive wear and oxidation wear, but the damage mechanism of molybdenum disulfide coating were mainly delamination, tribo-oxidation; the "bulge" of wear scar of two solid lubrication coatings and substrate probably resulted from the accumulated plastic flow damage, but the duration of its emergence and existence was dependent on fretting parameter.4. Composite lubrications was consisted of the ion sulfide layer and the lubricants oil, which greater changed the fretting regimes of the substrate, andshown an excellent lubricating effect of reducing friction.