The polyionic liquid poly-PEGMA-r-METAC (PPM) with quaternary ammonium has been synthesized and evaluated as additive in aqueous lubricating fluids. The rheological behavior of aqueous lubricating fluids with PPM has been characterized to confirm PPM’s function as a viscosity modifier. The tribological behavior of aqueous lubricating fluids with PPM has been investigated on SRV-V and MTM testing machines. It was found that PPM has excellent viscosity-increasing, lubricating, and anti-wear properties as an additive for aqueous, which can be attributed to the ability of PPM to form the protective film and boundary tribofilm generated from complex tribochemical reaction on rubbing surface. The obtained PPM with dual functions of anti-corrosion additives and viscosity index improver can play an important role in diverse lubrication regimes.

Surface wettability plays a significant role in reducing solid–liquid frictional resistance, especially the superhydrophilic/hydrophilic interface because of its excellent thermodynamic stability. In this work, poly(acrylic acid)-poly(acrylamide) (PAA–PAM) hydrogel coatings with different thicknesses were prepared in situ by polydopamine (PDA)-UV assisted surface catalytically initiated radical polymerization. Fluid drag reduction performance of hydrogel surface was measured using a rotational rheometer by the plate–plate mode. The experimental results showed that the average drag reduction of hydrogel surface could reach up to about 56% in Couette flow, which was mainly due to the interfacial polymerization phenomenon that enhanced the ability of hydration layer to delay the momentum dissipation between fluid layers and the diffusion behavior of surface. The proposed drag reduction mechanism of hydrogel surface was expected to shed new light on hydrogel–liquid interface interaction and provide a new way for the development of steady-state drag reduction methods.

This study presents a nitrogen-doped microporous carbon nanospheres (N@MCNs) prepared by a facile polymerization–carbonization process using low-cost styrene. The N element in situ introduces polystyrene (PS) nanospheres via emulsion polymerization of styrene with cyanuric chloride as crosslinking agent, and then carbonization obtains N@MCNs. The as-prepared carbon nanospheres possess the complete spherical structure and adjustable nitrogen amount by controlling the relative proportion of tetrachloromethane and cyanuric chloride. The friction performance of N@MCNs as lubricating oil additives was surveyed utilizing the friction experiment of ball-disc structure. The results showed that N@MCNs exhibit superb reduction performance of friction and wear. When the addition of N@MCNs was 0.06 wt%, the friction coefficient of PAO-10 decreased from 0.188 to 0.105, and the wear volume reduced by 94.4%. The width and depth of wear marks of N@MCNs decreased by 49.2% and 94.5%, respectively. The carrying capacity of load was rocketed from 100 to 400 N concurrently. Through the analysis of the lubrication mechanism, the result manifested that the prepared N@MCNs enter clearance of the friction pair, transform the sliding friction into the mixed friction of sliding and rolling, and repair the contact surface through the repair effect. Furthermore, the tribochemical reaction between nanoparticles and friction pairs forms a protective film containing nitride and metal oxides, which can avert direct contact with the matrix and improve the tribological properties. This experiment showed that nitrogen-doped polystyrene-based carbon nanospheres prepared by in-situ doping are the promising materials for wear resistance and reducing friction. This preparing method can be ulteriorly expanded to multi-element co-permeable materials. Nitrogen and boron co-doped carbon nanospheres (B,N@MCNs) were prepared by mixed carbonization of N-enriched PS and boric acid, and exhibited high load carrying capacity and good tribological properties.

MXene possesses great potential in enriching the functionalities of hydrogels due to its unique metallic conductivity, high aspect ratio, near-infrared light (NIR light) responsiveness, and wide tunability, however, the poor compatibility of MXene with hydrogels limits further applications. In this work, we report a uniformly dispersed MXene-functionalized poly-N-isopropylacrylamide (PNIPAM)/poly-2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) double network hydrogel (M–DN hydrogel) that can achieve switchable friction regulation by using the NIR light. The dispersity of MXene in hydrogels was significantly improved by incorporating the chitosan (CS) polymer. This M–DN hydrogel showed much low coefficient of friction (COF) at 25 °C due to the presence of hydration layer on hydrogel surface. After illuminating with the NIR light, M–DN hydrogel with good photothermal effect rapidly raised the temperature to above the lower critical solution temperature (LCST), which led to an obvious increase of surface COF owing to the destruction of the hydration layer. In addition, M–DN friction control hydrogel showed good recyclability and controllability by tuning "on-off" of the NIR light. This work highlights the construction of functional MXene hydrogels for intelligent lubrication, which provides insight for interface sensing, controlled transmission, and flexible robotic arms.

Synovial fluid is made up of various biomacromolecules, including hyaluronic acid, aggrecans, lubricins, and phosphatidylcholine lipid, which are assembled onto the surface of articular cartilage in a gel state. Among them, brush-like biomacromolecules or assemblies have a vital effect on human joint lubrication. Inspired by this, the combination of brush-like molecular structures and gel-like assembly may be an efficient approach for the synthesis of biomimetic lubricating matters. Learning from the lubrication system of human joints, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes grafted poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-co-AA)) microgels, abbreviated as MBs-g-MGs, were synthesized as one kind of biomimetic lubricating additives. It is worth noting that this bionic strategy considered both molecular structure and assembled form, which enabled this hairy microgel to achieve low friction in aqueous medium. Meanwhile, the effective lubrication was still achieved when using MBs-g-MGs at high temperature, indicating that this microgel maintains a good lubricating effect over a wide range of temperature. In addition, this kind of microgel possessed good biocompatibility, which laid the foundation for potential biomedical applications. Looking beyond, these biomimetic microgels may provide an effective lubricating effect for water-based sliding interfaces, especially in biomedical systems.

At present, more and more diseases are associated with the lubrication dysfunction, which requires a systematic study of the complex lubrication behavior of tissues and organs in human body. Natural biomacromolecular lubricants are essential for maintaining ultra-low coefficients of friction between sliding biological interfaces. However, when the surface lubrication performance of tissues or organs destroys heavily, it will bring friction/shear damage for sliding contact interfaces. Therefore, the application of exogenous biological lubricating materials to improve the lubrication situation of damaged tissue or organ interfaces has attracted extensive attention of researchers. In this review, based on a simple summary of lubrication mechanism at sliding biological interface, we systematically introduce the research progress of several kinds of representatively biolubrication materials, including eye drops, tissue anti-adhesion agents, joint lubricants, and medical device lubricants. Meanwhile, the lubrication mechanism and individual advantage and shortcoming for each of these synthetic exogenous lubricated materials are clarified. Correspondingly, the important lubrication application functionality of these biolubricant materials in typically medical surgery scenes, such as dry eye syndrome, tissue adhesion, arthritis, and interventional medical devices, is discussed. Finally, we look forward to the future development direction of artificial biolubricant materials.

A series of new halogen-free dicationic ionic liquids (ILs) with different alkyl chain lengths were prepared, and the relationship between the alkyl chain length, physicochemical and tribological properties of ILs, and their role as neat lubricant for steel-steel friction pairs, was investigated. Evaluation of stability during hydrolysis and copper strip corrosion test results show that synthetic ILs are stable and not corrosive to metal contacts, due to the halogen-free anions. The friction and wear test results indicate that ILs with long alkyl chains have excellent friction-reducing and anti-wear properties, especially at high temperatures. Based on the surface three-dimensional (3D) profiles, electrical contact resistance, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and the X-ray photoelectron spectrometry (XPS) analysis of the worn surfaces of steel discs, we can conclude that the efficiency of ILs is due to the formation of high quality tribofilms that consist of both tribochemical reaction and ordered absorption films.

To enhance the lubricating and extreme pressure (EP) performance of base oils, two types of oil-soluble ionic liquids (ILs) with similar anion albeit dissimilar cations were synthesized. The physical properties of the prepared ILs were measured. The anticorrosion properties of ILs were assessed by conducting corrosion tests on steel discs and copper strips, which revealed the remarkable anticorrosion properties of the ILs in comparison with those of the commercial additive zinc dialkyldithiophosphate (ZDDP). The tribological properties of the two ILs as additives for poly-α-olefin-10 (PAO10) with various mass concentrations were investigated. The tribological test results indicate that these ILs as additives are capable of reducing friction and wear of sliding contacts remarkably as well as enhance the EP performance of blank PAO10. Under similar test conditions, these IL additives exhibit higher lubricating and anti-wear (AW) performances than those of ZDDP based additive package in PAO10. Subsequently, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) were conducted to study the lubricating mechanism of the two ILs. The results indicate that the formation of tribochemical film plays the most crucial role in enhancing the lubricating and AW behavior of the mixture lubricants.

Friction exists wherever relative motion occurs and is the main source of energy consumption. Lubrication plays a significant role in improving fuel efficiency, reducing emissions, and prolonging the service life of machines. Surface interactions between two moving solid surfaces or the flow of a fluid (and/or environment) on a solid surface are the primary causes of friction. Apart from the mechanical design of moving parts, surface physicochemistry is of crucial importance to lubrication. This review deals with the frontier research on controlling friction and lubrication, highlights the importance of physicochemistry aspects, and enumerates the state-of-the-art chemistry solutions to tribological issues. It aims at inspiring talented young scientists from different fields to make significant contributions to the area.