Polyimide (PI) coatings are highly valued for their easy processing and exceptional mechanical qualities, which enable them to be applied in a variety of approaches. By integrating MXene, a two-dimensional material renowned for its low shear strength and excellent load-bearing capacity, into the PI matrix and subsequently using it as a coating on GCr15 surfaces, MXene/PI composites were produced. The tribological performance of these composites improved with increasing MXene content and then decreased. Under dry state and lubricated with PAO8, 0.25%MXene/PI composite demonstrated optimal tribological performance and achieved superlubricity, with its COF was only 0.002 and its wear rate was merely 1.92×10^-7 mm³·N^-1·m^-1. This improvement can be attributed to the even distribution of MXene in the PI matrix, particularly during frictional processes, as evidenced by SEM, DMA, and TEM analyses. Furthermore, the interaction between MXene and PI was confirmed through XPS analysis. These results not only establish the groundwork for developing high-performance PI coatings but also provide valuable insights for designing composite materials with superlubricity properties in engineering applications.

Humans rely on their fingers to sense and interact with external environment. Understanding the tribological behavior between finger skin and object surface is crucial for various fields, including tactile perception, product appearance design, and electronic skin research. Quantitatively describing finger frictional behavior is always challenging, given the complex structure of the finger. In this study, the texture and sliding direction dependence of finger skin friction was quantified based on explicit mathematic models. The proposed double-layer model of finger skin effectively described the nonlinear elastic response of skin and predicted the scaling-law of effective elastic modulus with contact radius. Additionally, the skin friction model on textured surface considering adhesion and deformation factors was established. It revealed that adhesive term dominated finger friction behavior in daily life, and suggested that object texture size mainly influenced friction-induced vibrations rather than the average friction force. Combined with digital image correlation (DIC) technique, the effect of sliding direction on finger friction was analyzed. It was found that the anisotropy in finger friction was governed by the finger’s ratchet pawl structure, which also contributes to enhanced stick-slip vibrations in the distal sliding direction. The proposed friction models can offer valuable insights into the underlying mechanism of skin friction under various operating conditions, and can provide quantitative guidance for effectively encoding friction into haptics.

Most studies of liquid lubricants were carried out at temperatures below 200 °C. However, the service temperature of lubricants for aerospace and aeroengine has reached above 300 °C. In order to investigate the friction mechanism and provide data for high temperature lubrication, the friction and wear properties of chlorophenyl silicone oil (CPSO)-lubricated M50 steel and Si₃N₄ friction pairs were investigated herein. Ball-on-disk experimental results show that the lubrication performance of CPSO varies significantly with temperature. Below 150 °C, coefficient of friction (COF) remains at 0.13–0.15 after the short running-in stage (600 s), while the COF in the running-in stage is 0.2–0.3. At 200 °C and above, the running-in time is much longer (1,200 s), and the initial instantaneous maximum COF can reach 0.5. Under this condition, the COF gradually decreases and finally stabilizes at around 0.16–0.17 afterwards. This phenomenon is mainly due to the different thickness of boundary adsorption film. More importantly, the wear rate of M50 steel increases significantly with the temperature, while the wear rate barely changes at temperatures above 200 °C. The anti-wear mechanism is explained as tribochemical reactions are more likely to occur between CPSO and steel surface with the increased temperature, generating the FeCl₂ protective film on the metal surface. Accordingly, FeCl₂ tribochemical film improves the lubrication and anti-wear capacity of the system. At high temperatures (200–350 °C), FeCl₂ film becomes thicker, and the contact region pressure becomes lower due to the larger wear scar size, so the wear rate growth of M50 steel is much smaller compared with that of low temperatures (22–150 °C). The main findings in this study demonstrate that CPSO lubricant has good anti-wear and lubrication capacity, which is capable of working under temperatures up to 350 °C.

The nature of solid–liquid interfaces is of great significance in lubrication. Remarkable advances have been made in lubrication based on hydration effects. However, a detailed molecular-level understanding is still lacking. Here, we investigated water molecule behaviors at the TiO₂–aqueous interfaces by the sum-frequency generation vibrational spectroscopy (SFG-VS) and atomic force microscope (AFM) to elucidate the fundamental role of solid–liquid interfaces in lubrication. Combined contributions of water structures and hydration effects were revealed, where water structures played the dominant role in lubrication for TiO₂ surfaces of varying hydrophilicity, while hydration effects dominated with the increasing of ion concentrations. Superior lubrication is observed on the initial TiO₂ surfaces with strongly H-bonded water molecules compared to the hydrophilic TiO₂ surfaces with more disordered water. The stable ordered water arrangement with strong hydrogen bonds and the shear plane occurring between the ordered water layer and subsequent water layer may play a significant role in achieving lower friction. More adsorbed hydrated molecules with the increasing ionic concentration perturb ordered water but lead to the enhancement of hydration effects, which is the main reason for the improved lubrication for both TiO₂. This work provides more insights into the detailed molecular-level understanding of the mechanism of hydration lubrication.

The remarkable mechanical adaptability of arapaima (Arapaima gigas) scales has made them an important subject of study. However, no research has been conducted into their tribological properties, which are crucial for the protectability and flexibility of arapaimas. In this study, by combining morphological characterizations, friction experiments, and theoretical analyses, the relationship between the surface morphology and tribological properties of arapaima scales is determined. These results indicate that arapaima scales exhibit varying surface morphologies in different regions. More specifically, the exposed regions of scales feature grooves and a circulus, whereas the covered regions exhibit bumps. The specific surface morphology of arapaima scales produces varying tribological properties across different regions and sliding directions. The unique tribological properties of arapaima scales influence the forces received from predator attacks and neighboring scales, directly influencing the arapaima’s protective capabilities. This study provides new insights into the mechanisms of natural flexible dermal armors, and it has potential applications in personal protective systems.

Polyether-etherketone (PEEK) is a corrosion-resistant material that has been widely used in aqueous lubrication. However, its anti-wear performance must be improved for its application in the industry. In this study, to improve the anti-wear performance of PEEK for aqueous boundary lubrication, PEEK/MoS₂ composites were prepared by ball-milling and spark plasma sintering processes. A competitive MoS₂ mechanism between the low shear strength property and the role of promoting wear debris generation influences the anti-wear performance of PEEK/MoS₂ composites. Experiments demonstrated that the coefficients of friction (COF) and wear rate of PEEK composite with 0.25 wt% MoS₂ were significantly reduced 68% and 94%, respectively. Furthermore, this was the first time that a PEEK composite could achieve a COF of less than 0.05 in aqueous boundary lubrication. Its anti-wear performance was verified to be better than that of PEEK/carbon fiber (CF) and Thordon composites. The PEEK/MoS₂ composite may be a potential material for underwater equipment because of its outstanding anti-wear performance in aqueous boundary lubrication.

Atomic and close-to-atomic scale manufacturing (ACSM) aims to provide techniques for manufacturing in various fields, such as circuit manufacturing, high energy physics equipment, and medical devices and materials. The realization of atomic scale material manipulation depending on the theoretical system of classical mechanics faces great challenges. Understanding and using intermolecular and surface forces are the basis for better designing of ACSM. Transformation of atoms based on scanning tunneling microscopy or atomic force microscopy (AFM) is an essential process to regulate intermolecular interactions. Self-assemble process is a thermodynamic process involving complex intermolecular forces. The competition of these interaction determines structure assembly and packing geometry. For typical nanomachining processes including AFM nanomachining and chemical mechanical polishing, the coupling of chemistry and stress (tribochemistry) assists in the removal of surface atoms. Furthermore, based on the principle of triboelectrochemistry, we expect a further reduction of the potential barrier, and a potential application in high-efficiency atoms removal and fabricating functional coating. Future fundamental research is proposed for achieving high-efficiency and high-accuracy manufacturing with the aiding of external field. This review highlights the significant contribution of intermolecular and surface forces to ACSM, and may accelerate its progress in the in-depth investigation of fundamentals.

Fluid viscosity is ubiquitous property and is of practical importance in intelligent fluids, industrial lubrication, and pipeline fluid transportation. Recently, there has been a surging interest in viscosity regulation. Here, we have developed a group of photorheological fluids by utilizing azobenzene polymers with a light-induced microstructure transformation. In this work, a photosensitive polymer with 4,4'-bis-hydroxyazobenzene as the main chain was designed and synthesized as a pivotal functional material. The sufficiently large structural difference under ultraviolet and near-infrared light makes it possible to regulate the viscosity of a polyethylene glycol solution. The viscosity of the photosensitive rheological fluids under ultraviolet light radiation is found to be up to 45.1% higher than that under near-infrared light radiation. To explore this intelligent lubricating technology, the friction regulation of ceramic sliding bearings was investigated utilizing photosensitive rheological fluids. Reversible friction regulation with a ratio of up to 3.77 has been achieved by the alternative irradiation of near-infrared and ultraviolet light, which can be attributed to the differences in mechanical properties and molecular structures under ultraviolet and near-infrared light according to both simulations and experiments. Such photorheological fluids will have promising applications in controllable lubrication, intelligent rheological fluids, and photosensitive dampers.

Bio-inspired reversible adhesion has significant potential in many fields requiring flexible grasping and manipulation, such as precision manufacturing, flexible electronics, and intelligent robotics. Despite extensive efforts for adhesive synthesis with a high adhesion strength at the interface, an effective strategy to actively tune the adhesion capacity between a strong attachment and an easy detachment spanning a wide range of scales has been lagged. Herein, we report a novel soft-hard-soft sandwiched composite design to achieve a stable, repeatable, and reversible strong adhesion with an easily scalable performance for a large area ranging from ~1.5 to 150 cm² and a high load ranging from ~20 to 700 N. Theoretical studies indicate that this design can enhance the uniform loading for attachment by restraining the lateral shrinkage in the natural state, while facilitate a flexible peeling for detachment by causing stress concentration in the bending state, yielding an adhesion switching ratio of ~54 and a switching time of less than ~0.2 s. This design is further integrated into versatile grippers, climbing robots, and human climbing grippers, demonstrating its robust scalability for a reversible strong adhesion. This biomimetic design bridges microscopic interfacial interactions with macroscopic controllable applications, providing a universal and feasible paradigm for adhesion design and control.