Recent friction force microscopy (FFM) observations of sliding friction on a cleaved NaCl surface submerged in ethanol have revealed characteristic stick-slip patterns at atomic resolution. To elucidate the relationship between the motion of ethanol molecules and macroscopic friction/lubrication, we employ a computational FFM model that couples interfacial atomic motion with the dynamics of the FFM cantilever. This model enables us to study the fundamental mechanisms of friction and lubrication in a liquid environment across a wide range of scales. Our results identify a layered ethanol tribofilm on the NaCl surface; while a slip plane between these layers leads to stick-slip motion, an ethanol-rich environment transitions the system toward fluid lubrication.

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DC motors with contact points operate through a mechanism in which fixed and rotating electrodes maintain sliding electrical contact. The tribo-film formed on the rotating electrode surface is known to exert a significant influence on motor performance. In this study, films formed on the electrical contact surfaces of DC motors subjected to actual operational testing were examined. The study encompassed an analysis of the graphite crystal structure on the film surface, as well as morphological and elemental analyses of both the film surface and its cross-section. Comparative evaluations were performed on films collected from motors operated for varying durations up to the end of their service life. The results revealed no substantial changes in film morphology or in the crystalline structure of graphite over test time. However, pronounced differences were observed between the curved and slit regions of the rotating electrode. In particular, the slit region exhibited a markedly lower degree of crystallinity. The film thickness in the curved region increased gradually during the initial operation time and stabilized at approximately 300-400 nm, whereas the film in the slit region was about twice as thick. These findings indicate that distinct film formation processes occur depending on local discharge effects.

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