Due to its innate instability, the degradation of black phosphorus (BP) with oxygen and moisture was considered the obstacle for its application in ambient conditions. Here, a friction force reduced by about 50% at the degraded area of the BP nanosheets was expressly observed using atomic force microscopy due to the produced phosphorus oxides during degradation. Energy-dispersive spectrometer mapping analyses corroborated the localized concentration of oxygen on the degraded BP flake surface where friction reduction was observed. Water absorption was discovered to be essential for the degraded characteristic as well as the friction reduction behavior of BP sheets. The combination of water molecules as well as the resulting chemical groups (P-OH bonds) that are formed on the oxidized surface may account for the friction reduction of degraded BP flakes. It is indicated that, besides its layered structure, the ambient degradation of BP significantly favors its lubrication behavior.

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO2)/silicon (Si) substrate, generally considered as a solid–solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO2/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO2 interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO2 interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Understanding the nanoscale friction properties of 2D materials and further manipulating their friction behaviors is of great significance for the development of various micro/nanodevices. Recent studies, taking advantage of the close relationship between friction and surface charges, use an external out‐of‐plane electric field to control the interfacial friction. Nevertheless, friction increases with the application of the out‐of‐plane electric field in most cases. Here, an in‐plane potential gradient is applied for the investigation of the contribution of electric charges to friction on the surfaces of 2D materials. Experimental results show that the friction between an atomic force microscope tip and the flakes of 2D materials decreases with the application of the in‐plane potential gradient, and the higher the potential gradient, the greater the friction decrease. By comparing the in situ atomic‐level stick‐slip maps before and after the application of the in‐plane potential gradient, it is proposed that the promotion of low friction dissipative motion during the stick‐slip process owing to the presence of the potential gradient gives rise to the friction reduction. These results not only help to reveal the origin of friction, but also provide a novel way to manipulate friction through an electrically‐controlled sliding process.

No abstract

Polytetrafluoroethylene (PTFE) has shown an outstanding lubricity as a solid lubricant, but its application is limited due to its low-mechanical strength and high-wear rate. In this study, core-shell nanoparticles were synthesized using PTFE as the core and polymethylmethacrylate (PMMA) as the shell. The formed core-shell nanocomposites by leveraging the core-shell nanoparticles as basic structural units exhibit remarkable enhancement on uniformity, tensile strength, and wear resistance, compared to mechanically mixed composites with the same composition. Our experiments demonstrated the following results: (1) Owing to the excellent uniformity, the maximum tensile strength of core-shell nanocomposites was 62 MPa, three times higher than that of mechanically mixed composites. (2) The composite matrix formed by PMMA shell had better reinforcement and protection effect on inner PTFE phase, resulting in a reduced wear rate of 0.3 × 10 −5 mm 3 /(N m), one order of magnitude lower than that of mechanically mixed composites. (3) The friction coefficient and interfacial mechanical properties of the core-shell nanocomposites at different temperatures have been systematically studied to get insights into lubrication mechanisms. It is proved that the temperature can decrease the modulus and increase the interfacial adhesion as well as the loss tangent of the core-shell nanocomposites, thus affecting the lubrication properties in multiple ways.