Ultra-low Friction Coefficient Research Articles

Macro-Superlubricity Induced by Tribocatalysis of High-Entropy Ceramics.

Macroscale superlubricity has attracted considerable attention as a promising strategy to minimize frictional energy dissipation and achieve near-zero wear. However, realizing macroscale superlubricity with prolonged durability remains an immense challenge, particularly on engineering steels. Current superlubricants render steel surfaces susceptible to corrosion, causing severe wear and superlubrication failure. Herein, high-entropy ceramics (HEC) with catalytic properties are innovatively introduced to prevent corrosion of engineering steels and achieve macro-superlubricity through tribo-catalytic effect. Furthermore, this catalytically induced superlubricity system exhibits an ultra-low friction coefficient of 0.0037 under contact pressure up to 1.47 GPa, an ultra-long cycle lifetime of 1.25 × 106 cycles (corresponding sliding distance up to 5 km), and an extremely low wear rate of 3.032 × 10-10 mm3·N-1·m-1 on the HEC surface. Based on the experimental analysis and theoretical simulation, the in situ formed HEC nanocrystals reduce the Gibbs free energy of hydrolysis of PA molecules into inositol and phosphoric acid molecules in the lubricant. Notably, the hydrolysis products favorably contributed to the reduction of shear force in the lubrication system, which is essential for achieving macroscale superlubricity over a long time. This study provides a new perspective for designing robust superlubricity systems by harnessing the tribocatalytic effect of high-entropy materials.

Tuning Electronic Friction in Structural Superlubric Schottky Junctions.

Friction at sliding interfaces, even in the atomistically smooth limit, can proceed through many energy dissipation channels, such as phononic and electronic excitation. These processes are often entangled and difficult to distinguish, eliminate, and control, especially in the presence of wear. Structural superlubricity (SSL) is a wear-free state with ultralow friction that closes most of the dissipation channels, except for electronic friction, which raises a critical concern of how to effectively eliminate and control such a channel. In this work, we construct a Schottky junction between a microscale graphite flake and a doped silicon substrate in the SSL state to address the issue and achieve wide-range (by 6×), continuous, and reversible electronic friction tuning by changing the bias voltage. No wear or oxidation at the sliding interfaces was observed, and the ultralow friction coefficient indicated that electronic friction dominated the friction tuning. The mechanism of electronic friction is elucidated by perturbative finite element analysis, which shows that migration of the space-charge region leads to drift and diffusion of charge carriers at Schottky junctions, resulting in energy dissipation.

Enhanced tribological properties of sliding contacts through the synergistic effect of PTFE film and PSAIL 2280

PurposeThe purpose of this study is to investigate the effectiveness of a composite lubrication system combining polytetrafluoroethylene (PTFE) film and oil lubrication in steel–steel friction pairs.Design/methodology/approachA PTFE layer was sintered on the surface of a steel disk, and a lubricant with additives was applied to the surface of the steel disk. A friction and wear tester was used to evaluate the tribological properties and insulation capacity. Fourier transform infrared spectrometer was used to analyze the changes in the composition of the lubricant, and X-ray photoelectron spectroscopy was used to analyze the chemical composition of the worn surface.FindingsIt was found that incorporating the PTFE film with PSAIL 2280 significantly enhanced both the friction reduction and insulation capabilities at the electrical contact interface during sliding. The system consistently achieved ultra-low friction coefficients (COF < 0.01) under loads of 2–4 N and elucidated the underlying lubrication mechanisms.Originality/valueThis work not only confirm the potential of PTFE films in insulating electrical contact lubrication but also offer a viable approach for maintaining efficient and stable low-friction wear conditions.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-06-2024-0222/

Making Sticky-Slippery Switchable Fluorogels Through Self-Adaptive Bicontinuous Phase Separation.

Developing gel materials with tunable frictional properties is crucial for applications in soft robotics, anti-fouling, and joint protection. However, achieving reversible switching between extreme sticky and slippery states remains a formidable challenge due to the opposing requirements for energy dissipation on gel surfaces. Herein, a self-adaptive bicontinuous fluorogel is introduced that decouples lubrication and adhesion at varying temperatures. The phase-separated fluorogel comprises a soft fluorinated lubricating phase and a stiff yet thermal-responsive load-bearing phase. At ambient temperature, the fluorogel exhibits a highly slippery surface owing to a low-energy-dissipating lubricating layer, demonstrating an ultralow friction coefficient of 0.004. Upon heating, the fluorogel transitions into a highly dissipating state via hydrogen bond dissociation, concurrently releasing adhesive dangling chains to make the surface highly sticky with an adhesion strength of ≈362kPa. This approach provides a promising foundation for creating advanced adaptive materials with on-demand self-adhesive and self-lubricating capabilities.

Carbon fiber/nano SiO2 reinforced polyelectrolyte-graft UHMWPE for water lubricated superlubricity

In this study, a new kind of UHMWPE composite with ultralow coefficient of friction (COF) and high mechanical properties was developed. An efficient method was established to graft hydrophilic monomer 3-sulfopropyl methacrylate potassium salt (SPMK) onto UHMWPE powder by thermal initiation radical polymerization. The COF of UHMWPE grafted SPMK sample was significantly reduced by 50 %. After filling carbon fiber and nano SiO2, the COF of UHMWPE composites can be reduced as low as 0.009 at 0.1 m/s. Besides, a 76.1 % decrease in wear rate of single UHMWPE was achieved. The outstanding tribological properties of UHMWPE composite was attributed to the synergistic effect of hydration lubrication and particle reinforcement. The prepared UHMWPE composite has great potential to realize water lubricated bearing.

Biomimetic Layered Hydrogel Coating for Enhanced Lubrication and Load-Bearing Capacity

Biomimetic hydrogel lubrication coatings with high wettability and low friction show great promise in tissue engineering, wound dressing, drug delivery, and intelligent sensing. Inspired by the hierarchical structure of natural cartilage, a layered hydrogel coating was constructed to functionalize rigid polyetheretherketone (PEEK). The layered hydrogel coating features a structural design comprising a top soft layer and a middle robust layer. The porous structure of the top soft hydrogel layer stores water molecules, providing surface lubrication, while the dense structure of the middle robust hydrogel layer offers load-bearing capacity. These synergistic effects of the gradient hydrogel layer endow the PEEK substrate with an ultra-low coefficient of friction (COF~0.010 at 5 N load), good load-bearing capacity (COF~0.031 at 10 N load), and excellent wear resistance (COF &lt; 0.05 at 5 N load after 20,000 sliding cycles). This study introduces a novel design paradigm for robust hydrogel coatings with exceptional lubricity, displaying the potential application in cartilage replacement materials.

Tribological behavior of h-BN/Al2O3 self-lubricating composites in extreme environments–Part Ⅰ: Lubricating behavior and ultra-low friction coefficient mechanism in air environments from room temperature to 1200 °C

The high-temperature tribological behavior of the h-BN/Al2O3 self-lubricating composite ceramics demonstrated a temperature-dependent variation, with an ultra-low friction coefficient about 0.09 being achieved at 1000 °C in an air environment when countered with Si3N4 pins. The molten B2O3 phase can effectively accumulate and mix the free h-BN phase at the friction interface to form a high-temperature adaptive similar solid-liquid tribo-film, thus providing ultra-low friction coefficients for composites in high-temperatures air environments. Meanwhile, the in-situ generated Al5BO9 phase can efficaciously enhance the strength and wear resistance of the friction interface, furnish mechanical support for the solid lubrication layer and preserve its structural integrity and stability during high-temperature friction.

Achieving ultra-low coefficient of friction of FeCoNiW multi-bonded composite coatings by Co-coated WC

It is of paramount importance to enhance the surface wear resistance and reduce the impact of corrosion caused by the marine environment in order to guarantee the propeller transmission components’ exterior protection. FeCoNiW multi-bonded Co-WC composite coatings were prepared with an ultralow friction coefficient (0.17) and wear rate (7.22 ×10−7 mm3⋅N−1⋅m−1) in this research. The uniform and fine hard phase skeleton, which is cobalt-rich, ensures that the coating exhibits reliable tribological characteristics. Furthermore, this coating exhibits high hardness (1000HV0.3 at 80 wt.%Co-WC) and sufficient low current density (7.485 ×10−6 A/cm2). The primary reason for these enhanced attributes is a reduction in the density of flaws in the coatings. Consequently, Co-WC-added composite coatings have considerable potential for utilization in drive shaft components and propeller applications.

Simple and effective: Superlubricity behaviour of the G-quadruplex hydrogel with high selectivity for K+ ions based on the ocular environment

Hydrogels have been the subject of significant research in the field of friction due to their exceptional lubricating properties. In this study, the G-quadruplex hydrogel with high selectivity for K+ ions was formed by introducing a mixture of G, 2-formylphenylboronic acid, and polyethylene glycol diamine into simulated artificial tears solution with high transparency, and an ultra-low coefficient of friction (COF) of about 0.004 was obtained based on the simulated ocular environment, thus achieving macroscopic superlubricity. In friction pairs simulating the ocular environment, to assess the frictional performance of the G-quadruplex hydrogel as both a lubricant and a friction pair based on the simulated ocular environment, we conducted experiments considering various factors such as concentration, sliding speed, and stress. Through these experiments, it was found that superlubricity was achieved when the G-quadruplex hydrogel was applied as lubricant or friction pair. This effect was attributed to the three-dimensional network structure and hydrophilicity of the hydrogel, which facilitated the formation of a highly bearing and flowing hydration layer, promoting macroscopic superlubricity. Compared to the G-quadruplex hydrogel with low concentration, the high concentration hydrogel (75 mM) exhibited increased mechanical strength and robustness in superlubricity. Combined with biocompatibility experiments, our synthesized G-quadruplex hydrogel has excellent biocompatibility and offers a novel approach to achieve superlubricity in ocular drug delivery.

Achieving the ultra-low friction and wear rate of PEEK-PTFE composites by Ti3C2Tx MXene reinforcement

In this work, a series of poly(ether-ether-ketone) (PEEK) and poly(tetra-fluoroethylene) (PTFE) composites reinforced by Ti3C2Tx MXene are fabricated, and the mechanical as well as tribological properties are investigated. The MXene forms a strong bonding with both PEEK and PTFE, thus improving the compressive strength by 28.37 % when compared to the undoped composite. Meanwhile, the MXene selectively adheres PTFE on the ZrO2 counterpart and facilitates the formation of transfer tribofilm. Eventually, the PEEK(Ti3C2Tx)-PTFE composites reach an ultra-low coefficient of friction (COF) of 0.065 ± 0.003 accompanied by a decrement in wear rate of 40 %. The promising performances of PEEK-PTFE composites by MXene reinforcement indicate an efficient way towards self-lubricating composites.

Nickel phosphorous trisulfide: A ternary 2D material with an ultra-low coefficient of friction

Ultra-low friction is crucial for the anti-friction, anti-wear, and long-life operation of nanodevices. However, very few two-dimensional materials can achieve ultra-low friction, and they have some limitations in their applications. Therefore, exploring novel materials with ultra-low friction properties is greatly significant. The emergence of ternary two-dimensional materials has opened new opportunities for nanoscale ultra-low friction. This study introduced nickel phosphorous trisulfide (NiPS3, referred to as NPS), a novel two-dimensional ternary material capable of achieving ultralow friction in a vacuum, into the large nanotribology family. Large-size and high-quality NPS crystals with up to 14 mm × 6 mm × 0.3 mm dimensions were grown using the chemical vapor transport method. The NPS nanosheets were obtained using mechanical exfoliation. The dependence of the NPS nanotribology on layer, velocity, and angle was systematically investigated using lateral force microscopy. Interestingly, the coefficient of friction (COF) of NPS with multilayers was decreased to about 0.0045 under 0.005 Pa vacuum condition (with load up to 767.8 nN), achieving the ultra-low friction state. The analysis of the frictional dissipation energy and adhesive forces showed that NPS with multilayers had minimum frictional dissipation energy and adhesive forces since the interlayer interactions were weak and the meniscus force was excluded under vacuum conditions. This study on the nanoscale friction of a ternary two-dimensional material lays a foundation for exploring the nanoscale friction and friction origin of other two-dimensional materials in the future.

Extremely enhanced friction and wear performance of hydrogen-free DLC film at elevated temperatures via Si doping

Hydrogen-free diamond-like carbon (DLC) films have garnered significant attention as a highly efficient solid lubrication material, but poor high-temperature lubrication and wear resistance limit their further application. Here, silicon-doped DLC (Si-DLC) films with different Si contents were deposited using a superimposed HiPIMS-DCMS deposition system to improve their high-temperature lubrication and wear resistance performance. The influence of Si content on the microstructure, mechanical and high-temperature friction and wear behavior of Si-DLC films was investigated. The results showed that Si-DLC films had a dense structure and nanoscale smooth surface. The Si doping enhanced the adhesion strength and relieved the residual stress of Si-DLC films, but decreased the hardness and Young's modulus. Meanwhile, Raman and XPS spectroscopy revealed significant local structural changes in Si-doped DLC films. Furthermore, the results showed that DLC films doped with low Si content displayed excellent tribological performance at elevated temperatures due to the graphite lubricating layer on the surface of wear marks. Specifically, Si-DLC film with 3.31 at% Si had an ultra-low average friction coefficient of 0.04 at 450 °C and still provided effective lubrication at temperatures up to 500 °C. In contrast, pure DLC film failed to lubricate at 450 °C due to the severe graphitization transition and Si-DLC film with 17.47 at% Si quickly failed to lubricate at 200 °C due to the SiC and SiO2. This work sheds light for the high-temperature application of DLC films doped with low Si content by superimposed HiPIMS-DCMS deposition system.

In-situ vibrational spectroscopic observation for thermally activated structural changes of 100% cellulose nanofiber molding with ultralow friction

This paper reports the thermally activated ultralow friction of 100% cellulose nanofiber (CNF) molding. The mechanism of friction reduction was investigated using a laboratory-built in-situ Raman tribometer. Our experimental results showed that a CNF molding exhibited an ultralow friction coefficient of below 0.04 in a CNF ring and steel disk tribopair under high-temperature conditions (T &amp;gt; 100°C). The results of the temperature-rise friction test showed that the friction coefficient of the CNF molding strongly depended on the temperature and decreased linearly with increasing temperature. The in situ tribo-Raman monitoring results, during friction, indicated a change in the structure of the CNF molding. Therefore, the crystallinity indices and lengths of the CNF fibers gradually changed as the temperature increased. Moreover, transfer tribofilms were observed on the counter-steel surface against the CNF rings. When the CNF molding exhibited thermally activated ultralow friction, the tribofilm was mainly composed of cellulose and graphitic carbon. Our results suggest that the thermal and friction-activated structural transformations of CNF molding and CNF-derived transfer film formation are pivotal factors contributing to the ultralow friction phenomenon observed in CNF molding at high temperatures.

Mechanochemical exfoliation of sugar-assisted boron nitride nanosheets for achieving superlubricity and wear distinction

Boron nitride nanosheets (BNNS) possess extensive potential applications across various fields. However, their limited dispersibility in liquids constrains their effectiveness as lubricating additives. In this study, a streamlined sugar-assisted mechanochemical exfoliation technique was employed to concurrently exfoliate and functionalize BNNS. This approach enabled the covalent grafting of sucrose molecules onto BNNS, enhancing their dispersion in water. The application of water containing sucrose-modified BNNS (S-BNNS) facilitated an ultralow coefficient of friction (COF = 0.001) at the Si₃N₄-Si₃N₄ interface. Additionally, using S-BNNS nanosheets as lubricating additives in water achieved superlubricity with a significant surface roughness (Ra ≤120 nm). We explored the correlation between surface roughness and superlubricity, revealing that tribological performance was enhanced by the formation of a tribofilm composed of S-BNNS and a silica layer at the interface through tribochemical reactions. The exceptional anti-wear properties and significantly reduced shear strength of S-BNNS within the tribofilm substantially contributed to achieving the ultralow COF, thus enhancing lubrication performance.

Facilitating macroscopic superlubricity through graphene oxide nanosheet additives in phosphoric acid

The field of superlubricity is garnering significant global interest amid the ongoing energy crisis. Various liquids can achieve superlubricity under ambient conditions; however, this limits their applications, such as in acidic environments. Consequently, enhancing anti-wear properties and reducing the coefficient of friction (COF) have become pressing challenges. Graphene-based materials are being extensively studied for tribological applications, attributed to their unique molecular structures and lubricating properties, often serving as lubricating additives to significantly reduce COF. In this study, graphene oxide (GO) nanosheets were utilized as lubricating additives in phosphoric acid (H3PO4; pH = 1.5) to explore lubrication enhancement in acidic environments. An ultralow COF of 0.001 was achieved, accompanied by reduced surface roughness and increased contact pressure (by 96.42 %), following the lubrication with GO-H3PO4. The reduction in COF post-lubrication with GO-H3PO4 is ascribed to three primary factors: the formation of a tribofilm via chemical reactions (comprising silica and phosphorus oxide layers), the hydrogen bond effect leading to a hydrated water layer with low shear strength, and the adsorption of GO nanosheets on the friction surface, facilitating friction transfer from Si3N4/Si3N4 to GO/GO.

Boron-graphene composite for efficient electromagnetic interference shielding with strong strength and near-zero thermal expansion

The quest for the materials that boast efficient electromagnetic interference (EMI) shielding, strong strength and superb thermal dimensional stability is a burgeoning research area, particularly due to their critical applications in safeguarding sensitive circuits against microwave radiation, especially in the space environments. Graphene-based composites, leveraging the remarkable attributes of individual graphene nanosheets, emerge as prime contenders for fulfilling these sophisticated application requirements. In this study, we prepared boron-graphene composites via spark sintering, combining graphene sheets and boron nanoparticles. This method not only ensures high-performance outcomes but also remains cost-effective and suitable for large-scale production. Boron serves as a binder, facilitating the connection between adjacent graphene sheets and enhancing the graphitization process. The resulting composites demonstrated exceptional electrical conductivity (4.53 × 105 S m−1) and superior EMI shielding effectiveness (average SET 83 dB, with the thickness of 0.25 mm), markedly surpassing previous graphene-based materials in terms of compressive strength (171.3 MPa), and exhibiting low thermal expansion and an ultra-low friction coefficient (0.04). Additionally, to unravel the evolution of boron in the graphene composite and the impact of boron on electrical conductivity, first principles calculations and density functional theory (DFT) were utilized. This investigation underscores the significant promise of boron-graphene composites as high-performance, multifunctional materials across various domains.

Earthworm-Inspired Ultra-Durable Sliding Triboelectric Nanogenerator with Bionic Self-Replenishing Lubricating Property for Wind Energy Harvesting and Self-Powered Intelligent Sports Monitoring.

Triboelectric nanogenerators (TENGs), a promising strategy for harvesting distributed low-quality power sources, face inevitable bottlenecks regarding long-term abrasion and poor durability. Herein, both issues are addressed by selecting an earthworm-inspired self-replenishing bionic film (ERB) as the tribo-material of sliding-freestanding TENGs (SF-TENGs), it consists of an interconnected 3D porous network structure capable of storing and releasing lubricant under cyclic mechanical stimuli. Thanks to the superiority of self-replenishing property, there is no need for periodic replenishment and accurate content control of lubricant over the interfacial-lubricating SF-TENGs based on dense tribo-layers. Additionally, an SF-TENG based on ERB film (ERB-TENG) demonstrates remarkable output stability with only a slight attenuation of 1% after continuous operation for 100000 cycles. Moreover, the ERB-TENG displays a distinguished anti-wear property, exhibiting no distinct abrasion with an ultra-low coefficient of friction (0.077) and maintaining output stability over a prolonged period of 35 days. Furthermore, integration with an energy management circuit enables the ERB-TENG to achieve a 39-fold boost in charging speed. This work proposes a creative approach to enhance the durability and extend the lifespan of TENG devices, which is also successfully applied to wind energy harvesting and intelligent sports monitoring.

Ultra-low friction system using special wetting interfaces: Bridging across various wetting regimes

Friction was the main way of energy loss in society, and reducing friction has become a focal point in current research. Inspired by special wetting phenomena, an asymmetric multiphase composite (AMPC) friction system was constructed. Tribological behavior of the friction system was evaluated using hydrogel on anodized aluminum as friction pair and superhydrophobic alumina with micro/nano-composite as substrate. Ultra-low friction coefficients (∼10−3) were measured under both Cassie and Wenzel regimes. Especially in Wenzel state, increased contact stress or extended shear rate promoted the formation of transfer lubricant film, thus enabling the ultra-low friction coefficient. Moreover, the system showed mechanical stability during prolonged water lubrication. This novel lubrication approach may provide valuable scientific solutions for specific tribology applications.

Effect of vacuum atomic oxygen irradiation on tribological properties of MoS2/WC multilayer films

The adaptability of molybdenum disulfide (MoS2) films in space environments is crucial for aerospace applications. However, atomic oxygen (AO) in low Earth orbit can damage the friction and wear performance of MoS2 films, leading to a reduction in the service life of the film. To enhance the resistance of the film to atomic oxygen irradiation, we constructed a nano-multilayer structure composed of MoS2-WC-MoS2. Studies have shown that MoS2/WC multilayer films exhibit an ultra-low friction coefficient of 0.010 due to their (002) preferred orientation. Even after high-dose AO irradiatione at 1.84 × 1020 atoms·cm−2, the film still maintains a friction coefficient of 0.009. This is attributed to the presence of MeOX nanoparticles in the MoS2 transfer film at the friction interface after AO irradiation, and the existence of MeOX nanoparticles plays a key role in maintaining ultra-low friction of the film. These findings are of great significance for extending the service life of molybdenum disulfide films in space environments and enhancing their resistance to atomic oxygen irradiation.

Comparative study of dry high-temperature tribological performance of hydrogen-free and hydrogenated DLC films deposited by HiPIMS in DOMS mode

Solid lubricants are crucial for industries operating at temperatures beyond 300 ºC, where liquid lubricants encounter limitations. Diamond-like carbon (DLC) films, known for exceptional solid lubrication and mechanical properties, need higher thermal stability for effective use in high-temperature applications. This study focuses on developing DLC films with the required thermal stability and solid lubricating properties. Hydrogen-free and hydrogenated DLC films were deposited utilizing deep oscillation magnetron sputtering (DOMS). Thermal characterizations revealed both films surpassed 500 ºC in thermal stability, rendering them suitable for high-temperature tribological applications. However, the hydrogenated DLC film exhibited superior solid lubricating properties, achieving an ultra-low friction coefficient below 0.05 at elevated temperatures, along with enhanced wear resistance, while effectively protecting its counterpart up to 500 ºC.