Abstract Lubrication plays a vital role in flexible electronics and medical devices, yet achieving stable low friction under low-pressure hydrodynamic conditions remains challenging. Existing approaches often lead to a coefficient of friction (COF) that increases with sliding velocity, due to the instability of interfacial water layers. Inspired by ocular surface structures, we developed a bio-inspired lubricating hydrogel (BLH) with nanowire arrays that confine interfacial water. This confined water remains stable under pressure, allowing COF to stay low and nearly independent of velocity. Under eyelid pressure (1.3-7.0 kPa), BLH achieved an ultra-low COF (0.0079-0.028), representing a 34-80% reduction compared to flat hydrogels (FH) and approaching that of natural corneal surfaces (0.014-0.037). The low friction was sustained over prolonged durations (≈ 6,000 s) and maintained at blinking-relevant speeds (≈ 4 cm/s). Tests using pig-eyeball rubbing and endoscope-probe models further demonstrated the robustness of this strategy, highlighting its promise for low-hysteresis coatings, flexible electronics, and medical devices.
Abstract The global motivation for clean hydrogen energy has grown due to the need for additional sustainable energy sources. To address this demand, it is essential to minimize hydrogen loss during its production and transportation. Polymers are increasingly used to replace metals in components such as seals, valves, and gaskets. This transition is motivated by their superior tribological properties, corrosion resistance, and ability to operate without external lubrication. However, despite their excellent performance in air, these polymers have not been thoroughly tested in hydrogen environments. In this study, several polymer composites were developed using a vitrimer aromatic thermosetting copolyester (ATSP) and polytetrafluoroethylene (PTFE) matrices, and their tribological performance was evaluated in a specially designed tribometer under a 5% hydrogen atmosphere. Fluorine-rich transfer films were detected on the steel counterface, and the ATSP-matrix composite exhibited the lowest wear. These findings identify the materials studied as promising candidates for hydrogen-related tribological components. However, they should not be extrapolated directly to high-purity hydrogen, high pressure, elevated temperature, cyclic decompression, or long-duration service without additional validation.
Abstract The wear condition of the roller surface on the roll-forming end effector significantly affects the quality of automotive body molds and the service life of the corresponding equipment. To address these issues, this study proposes a biomimetic roller design strategy based on the microstructure of the pearl shell surface. A corresponding texture-morphology mapping model was established, followed by multi-scale mechanical and tribological performance analysis. Simulation results indicate that, under the same depth-to-width ratio conditions, the roller with arcuate grooves exhibits the lowest friction energy loss and the highest principal stress compared to rollers with vertical and rhombic grooves. Friction wear tests further validate the advantages of this structure, showing significantly lower coefficients of friction, wear depth, and wear mass than those of rollers with vertical and rhombic textures, demonstrating superior wear resistance. Based on this, the depth-to-width ratio structure of the arcuate striped roller was further optimized using the golden section method, determines a global optimal depth-to-width ratio of 0.775. Compared to ordinary rollers, the maximum principal stress along the rolling direction increased by 18.3% for the optimized roller, while rolling friction energy decreased by 70.28%. This study demonstrates that biomimetic textures can significantly improve the surface stress distribution and tribological performance of rollers, provide theoretical support and innovative solutions for enhancing the stability of automotive body manufacturing processes and the reliability of intelligent devices.
Abstract This study explored the achievement of macroscopic liquid superlubricity using a Ce-based emulsion at the steel/steel interface. The Ce-based emulsion was prepared with Poly-L-lysine (PLL), oleic acid (OA), and a cerium-based emulsifier. After an initial running-in period of approximately 300 s, a superlubricity state characterized by a minimum coefficient of friction (COF) of 0.001 and a maximum contact pressure up to 197 MPa was achieved. The superlubricity originated from the synergistic adsorption of OA, PLL, and the emulsifier, driven by the C=C/COOH groups in OA, the amino-rich chains of PLL, and the Ce3+ containing head of the emulsifier. This cooperative adsorption promoted the formation of a hierarchical tribofilm that effectively separated the sliding surfaces under boundary lubrication. The study demonstrated the potential of emulsion-based lubricants for achieving ultra-low friction and wear in mechanical engineering applications.
Abstract Twisted graphene bilayers exhibit rich physical phenomena under external fields, yet the coupled effect of electric fields and interlayer sliding on their charge distribution remains elusive. By combining first-principles calculations with a machine learning technique, we systematically investigate interlayer sliding-induced charge transfer and redistribution in both twisted and AB-stacked graphene bilayers under vertical electric fields. Unlike the homogeneous charge distribution observed in AB-stacked bilayers, the heterogeneous charge distribution in certain twisted graphene bilayers exhibits robust insensitivity to applied electric fields, even under strong fields up to 2 V/nm. This field insensitivity enables such twisted bilayers to maintain ultralow interlayer friction under extreme bias conditions. To quantitatively describe the coupled effect of interlayer sliding and electric fields on interlayer charge transfer and friction, we develop a charge-modified registry index (RICM) model for graphene bilayers. The revealed electric-field insensitivity in specific twisted configurations can be mainly attributed to the inherently small variation in the maximum registry index change (∆RICM) under bias, providing a simple descriptor for designing low-friction interfaces in electrically gated graphene devices.
Abstract When exposed to the electric current environment, the motor bearings in electric equipment often endure severe wear or arc erosion. Hence, liquid-like carbon dots (LCDs), an advanced conductive additive with great dispersibility and low-addition effectiveness, was designed and introduced into lithium-based grease to address the aforementioned current-carrying problem. LCDs could form a conductive network in grease to provide an effective electric current transport pathway, which improves the current-carrying tribological properties. Additionally, the ionic structure enables LCDs to easily adsorb on the substrate under current-carrying conditions and form a protective film. Thereby, this work not only provides some new insights into the current-carrying lubrication mechanism of conductive additives for grease, but also expands the potential application of LCDs in electric machinery.
Abstract The tribological behaviour of Ti-15-3 β-titanium alloy treated by integrating bulk treatment with surface functionalisation (IBTSF) was systematically investigated under unlubricated, oil-lubricated, deionised water and simulated seawater conditions. Reciprocating sliding tests were performed against a WC ball under loads of 10 N or 20 N for 10,000 cycles. The results demonstrate that the tribological performance strongly depends on both the IBTSF treatments and the test environments. Under unlubricated conditions, the catalytic ceramic conversion treatments (C3TAu32 and C3TAg32) exhibited significantly lower friction and wear than the conventional ceramic conversion treatment (C2T32), owing to the self-lubricating effect of Au/Ag and the formation of thicker oxide layers. Hence, under the most severe unlubricated sliding condition (20 N), C3TAu32 reduced the wear factor by 99% compared with C2T32. In contrast, C2T32 showed superior wear resistance in oil, deionised water and simulated seawater because of its dense oxide layer and strong oxide/substrate bonding, while C3TAg32 suffered from corrosive wear in simulated seawater. Thus, C2T32 exhibited a wear factor 90% lower than C3TAg32 under simulated seawater. These findings establish application-oriented guidelines for selecting IBTSF treatments under different service environments, with C3TAu32 recommended for unlubricated applications and C2T32 for oil-lubricated and marine environments. The possible wear mechanisms are discussed to advance scientific understanding.
Abstract Polar organic friction modifiers can form effective boundary-lubrication films, but their limited compatibility with non-polar oils restricts their delivery to sliding interfaces. In this study, LCC was selected as a model polar additive to evaluate the ability of AOT reverse micelles in n-dodecane to act as nanocarriers for oil-incompatible friction modifiers. Increasing the water-to-surfactant molar ratio, w, expanded the transparent solubilization range of the model additive and increased the hydrodynamic size of the reverse micellar aggregates. Neutron reflectometry showed that hydrated AOT aggregates formed an adsorbed layer at the FeOx/oil interface, and incorporation of the model additive increased the fitted adsorbed-layer thickness from 3.05 to 3.43 nm under the same fixed-SLD fitting condition. Ball-on-disk tests showed lower friction after additive incorporation, and XPS detected chlorine-containing LCC-derived species on the worn surfaces, indicating that LCC was delivered to and retained at the sliding interface. Molecular dynamics simulations were used to observe the structural response of additive-loaded reverse micelles under high-pressure shear. These results support a reverse-micelle-assisted delivery mechanism in which hydrated AOT aggregates solubilize polar additives in the bulk oil, adsorb at the interface, and release additive-derived species under sliding.
Abstract In the aerospace sector, where extreme service conditions severely challenge the reliability and longevity of critical components, diamond-like carbon (DLC) coatings offer significant promise due to their superior tribological properties, chemical stability, and mechanical strength. However, their broader application is hindered by intrinsic limitations such as high residual stress, low fracture toughness, and strong environmental sensitivity of tribological behavior, which collectively restrict their high-performance deployment in aerospace systems. This review offers a thorough examination of recent advancements in the tribological behavior of DLC coatings under extreme environmental conditions. It begins with an examination of the coatings’ sensitivity to temperature, atmospheric composition, and humidity, along with the mechanistic principles underlying these environmental effects. Next, pivotal strategies for enhancing tribological performance are examined, encompassing layered structural design, compositional tuning, surface functionalization, and solid-liquid lubrication regulation, with an emphasis on their physicochemical mechanisms. The current status and challenges of DLC coating applications in critical aerospace components are then assessed. In response to the mounting demand for high reliability and multifunctionality in extreme aerospace environments, the review outlines several prospective research directions: optimization of micro/nano-scale architectures and interfaces, development of self-healing DLC functional interfaces, construction of multifunctional composite DLC systems, and implementation of intelligent, environmentally adaptive surface engineering strategies. Together, these endeavors aspire to establish a theoretical and technological foundation for the reliable, long-term service of aerospace systems under harsh conditions.
Abstract Transitions in tribologically induced deformation and wear are well established for metals and polymers, through scratch-based deformation maps, yet a comparable framework for elastomeric materials is still lacking. In particular, the relationship between frictional response and deformation mode transitions in rubber remains insufficiently established. In this study, single-asperity scratch-based testing was employed to systematically investigate the relationship between friction, penetration depth and deformation behaviour in four elastomer compounds under dry and lubricated conditions. Four distinct deformation modes, namely elastic deformation, ploughing, tearing and cutting were identified and linked, not only to visual damage but also to characteristic frictional response signatures under both constant and increasing penetration depth. Scratch-based tests were conducted in dry and lubricated conditions. Elastic deformation is characterised by low friction and a small deviation in the amplitude of the frictional force along a single scratch. For ploughing, higher friction is measured along a slightly higher deviation of the amplitude of the frictional force. The tearing mode shows high friction, though, the amplitude of the frictional force over the scratch length still remains low. The cutting mode exhibits both a high frictional force and a large amplitude variations in this signal, primarily due to the stick slip phenomena, as commonly known for rubber materials. Lubricating the samples decreased the coefficient of friction and increased the penetration depth required to generate a transition in deformation mode of the material. A deformation mode map was developed to illustrate the different modes based on the relation between the penetration depth and the interfacial shear strength. This deformation map contributes to a better understanding of rubber deformation under varying tribological circumstances. By capturing the initial deformation behaviour, the deformation map could provide valuable insight into the mechanisms driving wear evolution.
Abstract Lubricating greases are widely used in bearings, gears, and other heavily loaded machine elements, but conventional lithium- and calcium-soap greases based on mineral oils and metal-containing additives raise increasing concerns regarding toxicity, persistence, and regulatory compliance. This review summarises recent progress in green greases, defined as greases that employ renewable or environmentally acceptable base oils, thickeners, and additives to reduce environmental impact while maintaining functional performance. After briefly outlining the composition and operating principles of traditional greases, we discuss the physicochemical properties, advantages, and limitations of bio-based base oils, with emphasis on vegetable and synthetic esters and on ionic liquids used either as base fluids or co-base oils. We review renewable and biodegradable thickener systems, including modified soaps, polysaccharides, lignin-related structures and mineral or hybrid networks, and relate their molecular structure and gel microstructure to rheology, oil retention and mechanical stability. Particular attention is paid to oleogels, hydrogels and ionogels as emerging gel technologies that enable semi-solid green lubricants. Natural antioxidants, bio-based friction modifiers and environmentally acceptable anti-wear additives are surveyed, and representative formulations are compared with commercial reference greases in terms of friction, wear and other key performance indicators. In addition, green synthesis routes for bio-based gelators and mild, energy-saving processing conditions for grease manufacture are discussed in the context of green-chemistry principles and industrial scalability. On this basis, the environmental performance, application domains and market prospects of green greases are analysed, and the main trade-offs between performance, cost and sustainability are identified. Finally, the remaining challenges, especially high-temperature reliability, long-term stability, standardisation and large-scale implementation, are discussed, and directions are outlined for the rational design and broader adoption of next-generation green greases.
Abstract Oil-based lubrication is widely used in industrial robots and specialized unmanned aerial vehicles (UAVs), yet frictional heating and elevated-temperature operation often accelerate lubricant depletion and lubrication failure; converting such adverse thermal stimuli into adaptive self-lubrication remains a key challenge. Inspired by the friction-stimulated, mucus-mediated self-lubrication of earthworm skin, a dynamic covalently cross-linked oleogel was fabricated by integrating heterogeneous gradient-wettability materials with a metastable lubricant material matching effect. The oleogel shows robust interfacial adhesion (8.56 MPa) and toughness (8.7 MJ m-3) and operates as a thermoresponsive wettability switch that releases an earthworm-like polymeric “mucus” lubricant. Temperature-induced sol-gel transition of the network, combined with capillary adsorption in surface and internal micro-nano pores, enables more than ten reversible cycles of lubricant locking, exudation, and recovery, thereby establishing an intelligent self-lubrication mode based on an effective conversion from a hydrophilic to a hydrophobic interfacial state. Under this thermosensitive switching, the oleogel adaptively transitions between solid-solid and solid-liquid contact; at room temperature the low-viscosity state affords a moderate coefficient of friction (μ = 0.123), whereas thermal activation yields a low-friction self-lubricating state (μ = 0.055). This work provides theoretical guidance and technical support for smart lubrication of robotic joints and for polymer designs that accommodate multiple friction regimes.
Abstract Magnetic fluid sealing (MFS), with advantages such as zero leakage and long service life, has become the core solution for dynamic seals in extreme environments like nuclear energy and aerospace. However, the high-energy irradiation environment where nuclear industry equipment is located is prone to cause the deterioration and alter the frictional behavior of MFS, which restricts its engineering application. Based on the author's years of research, this paper systematically reviews the irradiation evolution laws and related mechanisms of three core components, namely magnetic fluid (MF), permanent magnets, and lubricants, as well as their overall performance in MFS. Research shows that the agglomeration of magnetic particles (MPs), the desorption of surfactants, and the degradation of base carrier fluid are the core factors leading to the deterioration of MF performance. There are two explanations for the irradiation demagnetization of permanent magnets: the thermal spike effect and the defect pinning effect. Under irradiation, lubricants exhibit degradation behaviors such as abnormal viscosity, increased acid value, gas release, non-crystallization and oxidation. Meanwhile, the existing research has limitations such as mainly analyzing single components, lacking multi-phase coupling mechanisms, and insufficient correlation with overall sealing performance. Based on this, the research team of authors has invented a series of MFS that can withstand radiation dose up to 10MGy, and successfully applying them to nuclear industry equipment. In the future, efforts should be focused on breaking through in areas such as the coupling mechanism of component performance, long-term assessment of dynamic irradiation, modification of irradiation-resistant materials, and quantitative prediction models for frictional properties and sealing lifespan. This paper reveals the evolution mechanism of the tribological behavior of MFS system under nuclear irradiation conditions. A research framework for MFS in nuclear energy equipment has been established, providing theoretical references for material selection, structural optimization and reliability improvement of MFS.
Abstract To realize the adaptive regulation of friction performance for core mechanical components under a wide temperature range, an integrated antifriction and wear-resistant composite structure combining coating, texture and lubricant was proposed and prepared. Arrayed circular micropore textures were fabricated on CSS-42L high-temperature bearing steel with a nickel-based hard coating, and SnAg soft metal lubricant doped with negative thermal expansion agent ZrW₂O₈ was filled into the textures. Cellular automaton models were established to simulate the diffusion process of the lubricant, and combined with systematic tribological experiments, the frictional behavior and adaptive regulation mechanism at different temperatures were systematically analyzed. Simulation results based on cellular automata models show that with increasing temperature, the proportion of medium and high-concentration regions of SnAg lubricant increases gradually and reaches 75.28% at 350 °C, confirming the favorable diffusion behavior at elevated temperatures. Tribological tests reveal that the 95Sn5Ag-filled coating and the 95Sn5Ag-10 wt%ZrW₂O₈-filled coating exhibit significantly superior tribological properties compared with the pure Ni-based coating across a wide temperature range. At 350 °C, the average friction coefficients of the 95Sn5Ag-filled coating and the 95Sn5Ag-10 wt%ZrW₂O₈-filled coating are 0.42 and 0.44, reduced by 35.4% and 39.7% compared with room temperature, and the wear rates decrease to 1.3×10⁻⁷ and 2.0×10⁻⁷ mm³N⁻¹m⁻¹. The lubricating layer thicknesses reach 4.48 μm and 4.18 μm, increased by 164% and 80%. The 95Sn5Ag-10 wt%ZrW₂O₈-filled coating presents more stable adaptive lubrication performance. The unique "inhibition at high temperatures and promotion at low temperatures" regulation mechanism of ZrW₂O₈ effectively enhances the wide-temperature adaptability. The synergistic effect of coating, texture and lubricant enables excellent antifriction and self-lubrication, which provides an efficient and feasible strategy for the active design and performance optimization of wear-resistant and lubricating functions of mechanical components.
Self-lubricating fabric composites significantly enhance the tribological performance of joint bearings by minimizing friction and wear on both the inner and outer rings, thereby substantially extending their operating life. These characteristics make them excellent candidates for bearing liner materials. However, conventional polytetrafluoroethylene (PTFE)/Nomex phenolic-based fabric composites exhibit limitations in high-temperature environments, necessitating the development of more thermally stable alternatives. To address this challenge, this study introduced a novel poly-p-phenylene benzobisoxazole (PBO)/PTFEu2013polyimide (PI)-based self-lubricating fabric composite and proposed a PBO/PTFEu2013PI@M50 tribo-pair system suitable for high-temperature working conditions, utilizing M50 bearing steel as the counterpart material. Compared with Nomex fiber-reinforced self-lubricating fabric composites, PBO/PTFEu201340PI (fabric composites with a resin mass fraction of 40%) demonstrated significantly superior high strength and high-temperature tribological performance. Notably, even at 300 u00B0C, it maintained an elastic modulus of approximately 15u202FGPa and a tensile strength of approximately 335u202FMPa while achieving a low friction coefficient of 0.023 and an impressively low wear rate of 0.83u00D710u22126 mm3/(Nu00B7m). The superior properties of the PBO/PTFEu2013PI@M50 tribo-pair system stemmed from the exceptional heat resistance and mechanical stability of the composite at high temperatures. Under the combined effects of u201Cthermalu2013mechanicalu2013chemicalu201D interactions, the fiber-reinforced composite material formed a dense, uniform, and strongly stable transfer film on the surface of the M50 steel ring. Detailed analysis revealed that the stability of the film was due primarily to the viscoelastic transition of PI and PTFE at high temperatures, coupled with their strong tribo-chemical reactions with the steel ring. Given its outstanding performance, the PBO/PTFEu2013PI@M50 tribo-pair system holds considerable promise for advanced engineering applications.
Lubrication failure of moving parts at extremely cryogenic temperatures poses a major challenge for advancements in space exploration, superconductivity, and other technologies. This study systematically investigates the tribological behavior of hydrogenated amorphous carbon (a-C:H) films in vacuum from u2212200 to 25 u00B0C. Notably, as the temperature decreases, the friction coefficient and the wear life of the a-C:H films exhibit an abnormal increase. At u2212200 u00B0C, the wear life exhibits a remarkable enhancement of at least two orders of magnitude. Introducing in situ mass spectrometry and cryogenic micro/nano indentation, the dynamic monitoring of interface damage, hydrogen passivation, and hardness evolution was conducted during the friction process. The work indicated that cryogenic temperatures significantly reduce the damage of a-C:H films, leading to changes in the synergistic lubrication involving hydrogen passivation, graphitization, and transfer films, resulting in high friction and low wear. This is fundamentally attributed to cryogenic temperatures altering the interfacial activity, which is the key factor in activating the synergistic lubrication of the above mechanisms. Crucially, with a suitable interfacial activity at u221275 u00B0C, a-C:H films can achieve an ultralow friction coefficient of ~0.015 and a wear rate of ~10u22128 mm3/(Nu00B7m). This work provides critical insights and establishes a foundation for deploying a-C:H films for cryogenic applications.
Abstract Thin film coatings are widely applied to extend the service life of hot metal forming dies by improving wear resistance, reducing friction, and mitigating thermal and chemical damage. Nitride coatings deposited by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods have demonstrated the potential to address critical challenges at the die–workpiece interface. This work reviews the designs and applications of nitride coatings to hot metal forming dies. It begins with the fundamentals and characteristics of monolithic, binary, and laminated coatings, which aim to enhance tribological performance at the interface. It then explores the application of nitride coatings used in hot forming processes of steels, aluminum alloys, and other metal alloys. Finally, the review concludes with key highlights and recommendations for coating selection and future research to improve the tribological performance of hot metal forming dies.
