Van der Waals heterostructures stacked by transition metal dichalcogenides and graphene provide a new opportunity for exploring superlubricity. However, the further reduction of friction is limited by the unavoidable charge transfer in the heterostructures. The dynamics of charge transfer occur at picosecond time scale, which cannot be detected by traditional friction instruments, making the friction mechanism of charge transfer unclear. Here, we investigate friction-induced charge transfer in WS₂/graphene heterostructures with ultrafast friction energy dissipation detecting technique. The observed friction exhibits a strong linear relationship with the dissipation rate of interlayer charge transfer. By modulating the band structure of heterostructures, the dissipation rate of interlayer charge transfer can be efficiently tuned from    to , resulting in a ~35% reduction in friction. This work gives the direct explanation of friction-induced charge transfer, which enables the high-performance micro-electro-mechanical systems and new insight into the origin of friction from the perspective of ultrafast electron dynamics.

In recent years, shipping-related CO₂ emissions have accounted for 3% of overall CO₂ emissions, and the corresponding direct economic losses have reached tens of billions of dollars. Reducing resistance during motion, as one of the effective countermeasures for saving energy and reducing greenhouse gas emissions produced by marine vehicles, has been widely studied by scholars. After billions of years of natural selection, various organisms, especially aquatic animals and plants, have evolved functional surfaces. Because the skin or surface of some aquatic organisms has low resistance or no adhesion, they can swim quickly in water and consume less energy. Since the last century, extensive studies have been conducted on animals and plants such as sharks, dolphins, and lotus leaves. This paper reviews the research progress on bionic drag reduction technologies inspired by typical animals and plants, including drag reduction by imitating shark skin, dolphin surfaces, jet function, lotus leaf surfaces, and mucus. We hope this review will be helpful for comprehensively understanding the research status of bionic drag reduction technology and developing more efficient drag reduction methods, which are highly important for saving energy and building marine environments.

Autonomous underwater vehicles (AUVs) have various applications in both military and civilian fields. A wider operation area and more complex tasks require better overall range performance of AUVs. However, until recently, there have been few unified criteria for evaluating the range performance of AUVs. In the present work, a unified range index, i.e., L^*, considering the cruising speed, the sailing distance, and the volume of an AUV, is proposed for the first time, which can overcome the shortcomings of previous criteria using merely one single parameter, and provide a uniform criterion for the overall range performance of various AUVs. After constructing the expression of the L^* index, the relevant data of 49 AUVs from 12 countries worldwide have been collected, and the characteristics of the L^* range index in different countries and different categories were compared and discussed. Furthermore, by analyzing the complex factors affecting the range index, methods to enhance the L^* range index value, such as efficiency enhancement and drag reduction, have been introduced and discussed. Under this condition, the work proposes a unified and scientific criterion for evaluating the range performance of AUVs for the first time, provides valuable theoretical insight for the development of AUVs with higher performance, and then arouses more attention to the application of the cutting-edge superlubricity technology to the field of underwater vehicles, which might greatly help to accelerate the coming of the era of the superlubricitive engineering.

The interlayer friction behavior of two-dimensional transition metal dichalcogenides (TMDCs) as crucial solid lubricants has attracted extensive attention in the field of tribology. In this study, the interlayer friction is measured by laterally pushing the MoTe₂ powder on the MoTe₂ substrate with the atomic force microscope (AFM) tip, and density functional theory (DFT) simulations are used to rationalize the experimental results. The experimental results indicate that the friction coefficient of the 1T'-MoTe₂/1T'-MoTe₂ interface is 2.025 × 10⁻⁴, which is lower than that of the 2H-MoTe₂/2H-MoTe₂ interface (3.086 × 10⁻⁴), while the friction coefficient of the 1T'-MoTe₂/2H-MoTe₂ interface is the lowest at 6.875 × 10⁻⁵. The lower interfacial friction of 1T'-MoTe₂/1T'-MoTe₂ compared to 2H-MoTe₂/2H-MoTe₂ interface can be explained by considering the relative magnitudes of the ideal average shear strengths and maximum shear strengths based on the interlayer potential energy. Additionally, the smallest interlayer friction observed at the 1T'-MoTe₂/2H-MoTe₂ heterojunction is attributed to the weak interlayer electrostatic interaction and reduction in potential energy corrugation caused by the incommensurate contact. This work suggests that MoTe₂ has comparable interlayer friction properties to MoS₂ and is expected to reduce interlayer friction in the future by inducing the 2H-1T' phase transition.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have layered structures with excellent tribological properties. Since the energy difference between hexagonal-molybdenum ditelluride (2H-MoTe₂) and distorted octahedral-molybdenum ditelluride (1T′-MoTe₂) is very small among the transition metal dichalcogenides (TMDCs), MoTe₂ becomes one of the most promising candidates for phase engineering. In our experiment, we found that the friction force and friction coefficient (COF) of 2H-MoTe₂ were an order of magnitude smaller than those of 1T′-MoTe₂ by the atomic force microscope (AFM) experiments. The friction difference between 1T′-MoTe₂ and 2H-MoTe₂ was further verified in molecular dynamics (MD) simulations. The density functional theory (DFT) calculations suggest that the friction contrast is related to the difference in sliding energy barrier of the potential energy surface (PES) for a tip sliding across the surface. The PES obtained from the DFT calculation indicates that the maximum energy barrier and the minimum energy path (MEP) energy barrier of 2H-MoTe₂ are both smaller than those of 1T′-MoTe₂, which means that less energy needs to be dissipated during the sliding process. The difference in energy barrier of the PES could be ascribed to its larger interlayer spacing and weaker Mo–Te interatomic interactions within the layers of 2H-MoTe₂ than those of 1T′-MoTe₂. The obvious friction difference between 1T′-MoTe₂ and 2H-MoTe₂ not only provides a new non-destructive means to detect the phase transition by the AFM, but also provides a possibility to tune friction by controlling the phase transition, which has the potential to be applied in extreme environments such as space lubrication.

The issues regarding energy dissipation and component damage caused by the interface friction between a friction pair attract enormous attention to friction reduction. The key-enabling technique to realize friction reduction is the use of lubricants. The lubricants smooth the contact interfaces, achieving an ultralow friction contact, which is called superslippery or superlubricity. At present, superslippery and superlubricity are two isolated research topics. There is a lack of unified definition on superslippery and superlubricity from the viewpoint of tribology. Herein, this review aims at exploring the differences and relations between superslippery and superlubricity from their origin and application scenarios. Meanwhile, the challenges for developing superslippery surface and superlubricity surface are discussed. In addition, perspectives on the interactive development of these two surfaces are presented. We hope that our discussion can provide guidance for designing superslippery or superlubricity surfaces by using varies drag-reduction technologies.

Superlubricity has been developing very rapidly in recent years as a new and important area in tribology. Many new phenomena and materials, as well as some new mechanisms in both liquid and solid superlubricity have been obtained. In liquid superlubricity, tens of new kinds of liquids with superlubricity have been found (e.g., water-based liquids, oil-based lubricants, and liquids combined with additives of two-dimensional (2D) materials that exhibit very good superlubricity properties under high pressure). In the field of solid superlubricity, more materials with superlubricity have been observed, including graphene-to-graphene surfaces, highly oriented pyrolytic graphite to graphene surfaces, and heterostructure surfaces where a friction coefficient as low as 0.00004 has been obtained. However, superlubricity is still under laboratory research. What is the future of superlubricity? What is the barrier restricting superlubricity from industrial applications? How do we transfer superlubricity from scientific research to industrial application? These questions and application fields of superlubricity in near future have been analyzed, and the concept of "superlubricitive engineering" has been proposed in the present work.

The layered double hydroxide (LDH) is a kind of natural mineral, which can also be manually prepared. It has been practically applied in various fields due to its unique crystal structure and diversity of composition, size, and morphology. In this work, LDHs with different chemical compositions (Co²⁺, Mg²⁺, Zn²⁺, and Ni²⁺) and topographical features (flower-like, spherical, and plate-like) were successfully prepared by controlling the reaction conditions. Then, they were mechanically dispersed into base grease and their tribological properties were evaluated by a ball-on-disk tester under a contact pressure of 2.47 GPa. It was found that the variation of morphology, instead of chemical composition, had great influence on the tribological performance. The "flower-like" LDH sample with high specific surface area (139 m²/g) was demonstrated to show the best performance. With 1 wt% additive, the wear volume was only about 0.2% of that lubricated by base grease. The tribofilm with unique microscopic structure and uniform composition was derived from tribochemical reaction between LDH additives and sliding solid surfaces, effectively improving tribological properties of the lubrication system. This work provided the guidance for optimizing lubricant additives and held great potential in future applications.