Atomically thin two-dimensional (2D) materials are excellent candidates for utilization as a solid lubricant or additive at all length scales from macro-scale mechanical devices to micro/ nano-electromechanical systems (MEMS/NEMS). In such applications, wear resistance of ultrathin 2D materials is critical for sustained lubrication performance. Here, we investigated the wear of fluorinated graphene (FG) nanosheets deposited on silicon surfaces using atomic force microscopy (AFM) and discovered that the wear resistance of FG improves as the FG thickness decreases from 4.2 to 0.8 nm (corresponding to seven layers to single layer) and the surface energy of the substrate underneath the FG nanosheets increases. On the basis of density function theory (DFT) calculations, the negative correlation of wear resistance to FG thickness and the positive correlation to substrate surface energy could be explained with the degree of interfacial charge transfer between FG and substrate which affects the strength of FG adhesion to the substrate.

Mechanochemical wear has attracted much attention due to its critical role in micro/nanodevice applications, reliable microscopy, and ultraprecision manufacturing. As a process of stress-associated chemical reactions, mechanochemical wear strongly depends on temperature; however, the impact mechanism is not fully understood at any length scale. Here, we reported different water-temperature dependence of mechanochemical wear on two typical single crystal silicon (Si) surfaces, involving oxide-covered Si partially terminated with Si–OH groups and oxide-free Si fully terminated with Si–H groups. As the water temperature increased from 10 to 80 °C, the mechanochemical wear of the oxide-covered Si underwent a process from no obvious surface damage to significant material removal but that occurring at all temperatures decreased gradually on the oxide-free Si surface. The opposite temperature-dependence was found to have a strong relation to the growth or degeneration of the Si–OH surfacial groups. The mechanochemical wear on the both Si surfaces decreased with the Si–OH coverage rising, which facilitated the growth of strongly hydrogen-bonded ordered water and then suppressed the chemical reaction between the sliding interfaces. These results can provide new insight into the mechanism of the surrounding temperature affecting the reliable micro/nanodevices, manufacturing, and microscopy.

The mechanical performance and surface friction of graphene oxide (GO) were found to inversely depend on the number of layers. Here, we demonstrate the non-monotonic layer-dependence of the nanowear resistance of GO nanosheets deposited on a native silicon oxide substrate. As the thickness of GO increases from ∼0.9 nm to ∼14.5 nm, the nanowear resistance initially demonstrated a decreasing and then an increasing tendency with a critical number of layers of 4 (∼3.6 nm in thickness). This experimental tendency corresponds to a change of the underlying wear mode from the overall removal to progressive layer-by-layer removal. The phenomenon of overall removal disappeared as GO was deposited on an H-DLC substrate with a low surface energy, while the nanowear resistance of thicker GO layers was always higher. Combined with density functional theory calculations, the wear resistance of few-layer GO was found to correlate with the substrate's surface energy. This can be traced back to substrate-dependent adhesive strengths of GO, which correlated with the GO thickness originating from differences in the interfacial charge transfer. Our study proposes a strategy to improve the antiwear properties of 2D layered materials by tuning their own thickness and/or the interfacial interaction with the underlying substrate.

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to A dist, A force, and A bond, respectively) are not equivalent at all, while we interestingly clarify that only A dist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only A bond is proportional to the friction force indicating that A bond is the dominant one for determining materials’ frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction.

Because of its self-lubricating qualities, porous polyimide (PPI) has been used as a crucial cage material for space bearings; however, overcoming the failure followed by blackening of PPI remains a challenge. Herein, we developed a surface smoothening approach to optimize the tribological performance of PPI in an oil-impregnated state. Pre-polishing using sandpaper and then with Al2O3 paste enables the fabrication of nanoscale rough PPI surfaces with significantly reduced friction and surface wear and complete suppression of the blackening phenomenon. Compared with polishing using sandpaper or Al2O3 paste, the compound polishing method created a substantially smoother surface. It exposed more polytetrafluoroethylene at the outermost surface, improving the liquid and solid lubrication properties of the contact interface under a mixed lubrication state. The further analysis indicated that decreasing the size of PPI surface pores during pre-polishing using sandpaper was crucial in resisting the adsorption of residual polishing particles.