Tangential fretting wear of amorphous carbon films: Evolution of fretting regimes and wear mechanisms

While amorphous carbon-based films are recognized for their efficacy in mitigating fretting wear, owing to the fact that tribopairs demonstrate divergent tribological responses under different fretting states, it remains essential to explore their friction-mitigation mechanisms across distinct fretting regimes and elucidate the evolutionary patterns of these regimes. This effort is critical to gaining a thorough and systematic insights into the fretting characteristics of amorphous carbon-based films. The running conditions fretting map was constructed via friction force-displacement curves, and the evolutionary relationship between the two fretting regimes was explored. Additionally, the wear mechanisms and friction-mitigation mechanisms under these two regimes were systematically investigated via advanced characterization techniques, including FIB-TEM, SEM, Raman spectroscopy, and XPS. Results show that increasing normal load shifts fretting regime toward partial slip regime, leading to a decreased friction coefficient and increased wear volume, dissipated energy, and tangential stiffness. Increasing displacement amplitude drives the fretting regime to evolve toward the slip regime, resulting in increased friction coefficient, wear volume, and dissipated energy, along with decreased tangential stiffness. Notably, an amorphous-nanocrystalline composite structure, in which iron oxides are encapsulated by graphitized carbon film, forms on the surface of counterpart balls in the slip regime. This structure exerts a pivotal effect on mitigating the friction coefficient and fretting wear. Furthermore, this work advances the fundamental understanding of the mechanisms governing the tangential fretting wear of DLC films, and offers valuable design guidance and a robust theoretical basis for alleviating fretting damage.