Diamond-like carbon (DLC) film has been developed as an extremely effective lubricant to reduce energy dissipation; however, most films should undergo running-in to achieve a super-low friction state. In this study, the running-in behaviors of an H-DLC/Al₂O₃ pair were investigated through a controllable single-asperity contact study using an atomic force microscope. This study presents direct evidence that illustrates the role of transfer layer formation and oxide layer removal in the friction reduction during running-in. After 200 sliding cycles, a thin transfer layer was formed on the Al₂O₃ tip. Compared with a clean tip, this modified tip showed a significantly lower adhesion force and friction force on the original H-DLC film, which confirmed the contribution of the transfer layer formation in the friction reduction during running-in. It was also found that the friction coefficient of the H-DLC/Al₂O₃ pair decreased linearly as the oxygen concentration of the H-DLC substrate surface decreased. This phenomenon can be explained by a change in the contact surface from an oxygen termination with strong hydrogen bond interactions to a hydrogen termination with weak van der Waals interactions. These results provide new insights that quantitatively reveal the running-in mechanism at the nanoscale, which may help with the design optimization of DLC films for different environmental applications.

Surficial water adsorption and interfacial water condensation as natural phenomena that can alter the contact status of the solid interface and tribological performances are crucial in all length scales, i.e., from earthquakes to skating at the macroscale level and even to micro/nano-electromechanical systems (M/NEMS) at the microscale/nanoscale level. Interfacial water exhibits diverse structure and properties from bulk water because of its further interaction with solid surfaces. In this paper, the evolutions of the molecular configuration of the adsorbed water layer depending on solid surface chemistry (wettability) and structure, environmental conditions (i.e., relative humidity and temperature), and experimental parameters (i.e., sliding speed and normal load) and their impacts on tribological performances, such as adhesion, friction, and wear, are systematically reviewed. Based on these factors, interfacial water can increase or reduce adhesion and friction as well as facilitate or suppress the tribochemical wear depending on the water condensation kinetics at the interface as well as the thickness and structure of the involved interfacial water.

This study achieved water-based superlubricity with the lubrication of H₃PO₄ solution in vacuum (highest vacuum degree <10^-4 torr) for the first time by performing a pre-running process in air before running in vacuum. The stable water-based superlubricity was sustainable in vacuum (0.02 torr) for 14 h until the test was stopped by the user for non-experimental factor. A further analysis suggested that the superlubricity may be attributed to the phosphoric acid-water network formed in air, which can efficiently lock water molecules in the liquid lubricating film even in vacuum owing to the strong hydrogen bond interaction. Such capability to lock water is strongly affected by the strength of hydrogen bond and environmental conditions. The realization of water-based superlubricity with H₃PO₄ solution in vacuum can lead to its application in space environment.

Using an atomic force microscope, the running-in process of a single crystalline silicon wafer coated with native oxide layer (Si–SiO_x) against a SiO₂ microsphere was investigated under various normal loads and displacement amplitudes in ambient air. As the number of sliding cycles increased, both the friction force F_t of the Si–SiO_x/SiO₂ pair and the wear rate of the silicon surface showed sharp drops during the initial 50 cycles and then leveled off in the remaining cycles. The sharp drop in F_t appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si–SiO_x/SiO₂ pair. During the running-in process, the contact area of the Si–SiO_x/SiO₂ pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO₂ tip, which caused the reduction of friction force and the wear rate of the Si–SiO_x/SiO₂ pair. A phenomenological model is proposed to explain the running-in process of the Si–SiO_x/SiO₂ pair in ambient air. The results may help us understand the mechanism of the running-in process of the Si–SiO_x/SiO₂ pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems (MEMS).