Superlow Friction Research Articles

Smart polymer self-lubricating material: Optimal structure of porous polyimide with base oils for super-low friction and wear

Smart polymer self-lubricating material: Optimal structure of porous polyimide with base oils for super-low friction and wear

The establishment of superlubricity in engineering field for H-DLC composite in multi-environments

The application of hydrogenated diamond-like carbon (H-DLC) with superlubricity can significantly decrease friction and largely save energy dissipation in mechanical systems. Thus, the realization of the near-frictionless state at the engineering scale in both inert and active atmospheres for H-DLC film is of interest. However, the film can achieve the state only under inert and vacuum conditions, while it fails in active atmospheres, such as oxygen and moist air, which seriously hinders wide application of the superlow friction. In previous studies, we have successfully solved this challenge by combining H-DLC film with MoS2 flakes. Nevertheless, the life of the superlubricity achieved by the MoS2/H-DLC composite in an inert gas was extremely short, which was attributed to the failure of MoS2-rich transfer films. To establish the near-frictionless state in various atmospheres, graphene oxide with numerous oxygen-containing groups was used to prepare a GO/MoS2/H-DLC triple composite in this study. The tribological performances were investigated systematacially. The results demonstrated that the triple composite could establish superlubricity in inert gases and ultralow friction in active atmosphere by forming robust transfer films at sliding interfaces. The structures at frictional interfaces including wear tracks and scars were thoroughly investigated. This work provides a valuable approach for the H-DLC composite to achieve an extremely low friction in various atmospheres and can guide the application of superlow friction in engineering fields.

Effects of relative humidities on the superlubricity of GO/MoS2/H-DLC triple composite

The application of superlubricity will largely decrease the dissipation of energy. The MoS2/H-DLC composite can achieve superlubricity in active atmospheres. Unfortunately, in dry air atmosphere, the composite can establish superlow friction with an extremely short life, hindering the application of the MoS2/H-DLC composite with superlow friction in engineering. To establish superlubricity in dry and moist air, we adopted GO with O-containing groups to prepare a GO/MoS2/H-DLC composite to endow the triple composite with superlubricity in dry and moist air. The friction and wear performance of the triple composite were studied comprehensively in this work. The composition and structures of transfer films and wear tracks were also investigated here. This work would provide some helpful advice for application of H-DLC composite with superlubricity in air.

Towards direct superlubricity and superlow wear via amino modification of polyhydroxy alcohol solutions

Friction remains as the primary mode of energy dissipation and components wear, and achieving superlubricity shows high promise in energy conservation and lifetime wear protection. The results in this work demonstrate that direct superlubricity combined with superlow wear can be realized for steel/Si3N4 contacts on engineering scale when polyhydroxy alcohol solution was selectively modified by amino group. Macroscopic direct superlubricity occurs because 3-amino-1,2-propanediol molecules at the friction interface could be induced to rotate and adsorb vertically on the friction surface, forming in-situ thick and dense molecular films to passivate the asperity contacts. Furthermore, amino modification is also conducive to improving the lubrication state from boundary to mixed lubrication regime by strengthening the intermolecular hydrogen bonding interaction, presenting enhanced load-bearing capability and reduced direct solid asperity contacts. Thus, direct superlow average friction of 0.01 combined with superlow wear are achieved simultaneously. The design principle of direct superlubricity and superlow wear in this work indeed offers an effective strategy to fundamentally improve energy efficiency and provide lifetime wear protection for moving mechanical assemblies.

Super-low friction and wear in steel contacts enabled by tribo-induced structural degradation of graphene quantum dots

Reducing friction is a promising strategy to decrease material losses and energy consumption in industrial systems. However, in aqueous-lubricated steel contacts, the contact pressure rarely exceeds 50 MPa during super-low friction due to excessive wear. This work demonstrates that even in steel/steel contacts, by combining graphene quantum dots (GQDs) with aqueous glycerol, it is possible to maintain super-low friction (µ ≈ 0.012) under a contact pressure as high as 316.5 MPa. Moreover, the use of GQDs improved the wear performance by 98 % compared to pure aqueous glycerol due to the formation of a tribochemical film, resulting from the electrostatic adsorption of GQDs on the positively charged sites on the worn surface. In particular, the exfoliation of graphene sheets within GQDs, the shearing of graphene layers inside the GQDs, and the OH–OH repulsion between the asperities shortens the running-in period and consequently reduces the friction and wear. At the same time, the formation of a chemically adsorbed tribofilm containing friction-induced structurally degraded GQDs protects the surface from wear and facilitates the maintenance of super-low friction at high contact pressures by improving the load-carrying capacity. This study suggests that green nano-lubricants based on GQDs have immense potential in sustainable engineering.

Thermal Stability and High-Temperature Super Low Friction of γ-Fe2O3@SiO2 Nanocomposite Coatings on Steel

The thermal stability of the γ-Fe2O3@SiO2 nanocomposites and super low friction of the γ-Fe2O3@SiO2 nanocomposite coatings in ambient air at high temperature are investigated in this paper. X-ray diffraction, scanning electron microcopy, transmission scanning electron microcopy, high-temperature tribometer, thermogravimetric analysis and differential scanning calorimetry were used to investigate the microstructure, surface morphology and high-temperature tribological properties of the γ-Fe2O3@SiO2 nanocomposite coatings, respectively. The results show that the γ-Fe2O3@SiO2 nanocomposite with the core–shell structure has excellent thermal stability because the SiO2 shell inhibits the phase transition of the γ-Fe2O3 phase to the α-Fe2O3 phase in the nanocomposites. The temperature of the phase transition in γ-Fe2O3 can be increased from 460 to 829 °C. The γ-Fe2O3@SiO2 nanocomposite coatings exhibit super low friction (0.05) at 500 °C. A high-temperature super low friction mechanism is attributed to γ-Fe2O3 and the tribochemical reactions during sliding.

The development of B/Cr co-doped DLC coating by FCVA deposition system and its tribological properties at 300 °C

The diamond-like carbon (DLC) coating prepared using filtered cathodic arc deposition (FCVA) lost its promising wear resistance under high temperatures. Element-doped DLC coatings have been investigated to suppress carbon oxidation reaction and subsequently enhance the wear resistance in high-temperature environments. Previous research clarified the excellent tribological properties of boron-doped DLC (B-DLC) but identified challenge such as arc discharge extinguishment and coating declamation (under high-temperature friction test). In this study, the introduction of Argon gas (10 sccm) during the deposition process stabilized stable arc discharge, resulting in high hardness and deposition rate. Moreover, a chromium interlayer and varying concentration of Cr dopant (0.5 %, 1.0 %, 3.0 % at.) were added to B-DLC to develop a stable, co-doped DLC with low friction and high wear resistance. Raman spectroscopy and nano-indentation revealed an increase in the disordered carbon structure and reduction in hardness and Young's Modulus with Cr doping. Macroscopic ball-on-disk friction test, showed 1 % Cr-doped B-DLC (a-C:B:Cr1) coating exhibited super low friction (avg. coefficient 0.02) and a low specific wear rate (<5.0 × 10−6 mm3/Nm). Raman spectroscopy indicated that the transferred graphite-like tribo-film onto the surface of Si3N4 counterparts contributed to stable low friction. Additionally, XPS analysis on the DLC coatings' wear track suggested the rich BB bond might contribute to its high wear resistance. This successful improvement on element-doped DLC prepared by FCVA provided valuable insights for further exploration of DLC coating applied to various working situations.

Bioinspired Slippery Surfaces for Liquid Manipulation from Tiny Droplet to Bulk Fluid.

Slippery surfaces, which originate in nature with special wettability, have attracted considerable attention in both fundamental research and practical applications in a variety of fields due to their unique characteristics of superlow liquid friction and adhesion. Although research on bioinspired slippery surfaces is still in its infancy, it is a rapidly growing and enormously promising field. Herein, a systematic review of recent progress in bioinspired slippery surfaces, beginning with a brief introduction of several typical creatures with slippery property in nature, is presented. Subsequently,this review gives a detailed discussion on the basic concepts of the wetting, friction, and drag from micro- and macro-aspects and focuses on the underlying slippery mechanism. Next, the state-of-the-art developments in three categories of slippery surfaces of air-trapped, liquid-infused, and liquid-like slippery surfaces, including materials, design principles, and preparation methods, are summarized and the emerging applications are highlighted. Finally, the current challenges and future prospects of various slippery surfaces are addressed.

Durability of Super-Low Friction of Hydrogenated Carbon Nitride Coatings in High-Vacuum Environment

Durability of Super-Low Friction of Hydrogenated Carbon Nitride Coatings in High-Vacuum Environment

Layered Structure and Wear Mechanism of Concentrated Polymer Brushes.

Concentrated polymer brushes (CPBs), which are significantly denser and thicker than conventional semidilute polymer brushes, have received increasing attention in the field of tribology because of their superlow friction properties. However, despite numerous studies aimed at enhancing CPBs for mechanical applications, the relationship between the specific layered structure and lubrication mechanisms of CPBs is still not completely understood. In this study, to reveal the relationship, simultaneous time-resolved measurements of the interfacial gap, static mechanical response, and dynamic mechanical response of the CPB at the contact interface were conducted using optical interference and precise force measuring methods. Two types of tests (i.e., the "indentation" and "sliding" tests) were alternately performed on a glass substrate coated with the CPB against a steel ball immersed in an ionic liquid. The indentation tests measuring the time-resolved interfacial gap and changes in static and dynamic mechanical responses quantitatively confirmed the presence of dilute, middle, and concentrated layers in the CPB. In the sliding tests, the wear of the CPB was detected by observing a decrease in the interfacial gap at the contact interface. Moreover, the thickness of the dilute layer remained constant with sliding, whereas the thicknesses of the other layers decreased, indicating that the dilute layer was continuously formed due to sliding. Therefore, CPB wear occurs randomly at the friction interface alongside the formation of a dilute layer with low density and stiffness on the surface.

High temperature superlubricity behaviors achieved by AlSiN coatings against WS2 coatings at 600 °C

In this paper, AlSi coatings and AlSiN coatings on 22MB5 high strength steel and MoS2 coatings on the die steel were prepared by magnetron sputtering respectively. The tribological properties of the coatings were studied from room temperature to 800 °C. The hardness and the bond strength between the coatings and the substrate were also investigated. The experimental results show that AlSi coatings have poor anti-friction behaviors comparing with the steel, and coefficient of friction (CoF) is as 0.2 at room temperature, but 0.8 at 600 °C. The N element was doped into AlSi coatings with FeAl transition layers to improve the tribological performances of the coatings at high temperature. CoF of AlSiN coatings and H13 steel is high. However, AlSiN coatings have excellent wear-resistance. WS2 coatings were prepared on the surface of the die steel to improve high temperature anti-friction behaviors of AlSiN coatings and H13 steel pin. The experimental results show that WS2 coatings exhibit good high temperature anti-friction behaviors. Super low friction about 0.003 is achieved at 600 °C. High temperature superlubricity mechanisms of AlSiN coatings and WS2 coatings are discussed systematically.

Strategies to lower the interface sliding friction force: A first-principles study case for the WSe2/graphene system

Lowering the sliding friction force is a major way to improve the energy loss, mechanical failures, and inefficiencies of a moving mechanical system. We investigate the possibility of lowering the friction force for the WSe2/graphene system by importing rotation angle and surface functionalization through a DFT study. The introduction of a surface rotation angle of 19.11° can reduce the friction force by nearly one order of magnitude to 6.37 × 10−5∼7.92 × 10−5 nN/atom, and the surface F-functionalized graphene can further reduce the friction force by more than half to 3.10 × 10−5∼3.56 × 10−5 nN/atom. The low interface interaction, smooth interface, and irregular atom arrangement are responsible for the superlow friction force. Our study suggests possible ways to lower the interface sliding friction force.

Super-Low Friction Electrification Achieved on Polytetrafluoroethylene Films-Based Triboelectric Nanogenerators Lubricated by Graphene-Doped Silicone Oil.

The novel proposal of Wang's triboelectric nanogenerator (TENG) has inspired extensive efforts to explore energy harvesting devices from the living environment for the upcoming low-carbon society. The inevitable friction and wear problems of the tribolayer materials become one of the biggest obstacles for attaining high-performance TENGs. To achieve super-low friction electrification of the TENGs, the tribological and electrical behaviors of the sliding-mode TENGs based on polytetrafluoroethylene (PTFE) films and metallic balls under both dry friction and liquid lubrication conditions were investigated by using a customized testing platform with a ball-on-flat configuration. Most interestingly, a super-low friction coefficient of 0.008 was achieved under graphene-doped silicone oil lubrication. The corresponding wear rate of the PTFE film was drastically decreased to 8.19 × 10-5 mm3/Nm. Simultaneously, the output short-circuit current and open-circuit voltage were enhanced by 6.8 times and 3.0 times, respectively, compared to the dry friction condition. The outstanding triboelectrical performances of the PTFE film when sliding against a steel ball are attributed to the synergistic lubricating effects of the silicone oil and the graphene nanosheets. The current research provides valuable insights into achieving the macro-scale superlubricity of the TENGs in practical industrial applications.

Achieving superlubricity and high adhesion strength of hydrogenated amorphous carbon film with Al/Cr/Si-doping

Excellent tribological properties of hydrogenated amorphous carbon films make it an important candidate for friction reduction in industry. However, the great challenge of achieving both high adhesion strength and super-low friction limits its wide applications. In this work, a novel Al, Cr and Si co-doped hydrogenated amorphous carbon (AlCrSi/a-C:H) film was prepared by high-power impulse magnetron sputtering (HiPIMS) method. The AlCrSi/a-C:H film showed favorable mechanical performance and high adhesion (∼80 N), which was attributed to the controlled elements doping and special transition layers produced by HiPIMS method. Meanwhile, superlubricity was obtained with coefficient of friction 0.0014. Detailed characterizations suggested that doping elements played significant roles in tribo-chemical reactions during friction. On one hand, doping elements can promote shear-induced graphitization and formation of graphite-like layers at the friction interface. On the other hand, the preferential oxidation of doping elements can protect the formed graphite-like layers from oxidation and the formed oxides nanoparticles can also be wrapped by carbon layers to prevent abrasive wear. It was the synergistic effects of graphitization and oxidation actions that led to superlubricity state of the film. The proposed AlCrSi/a-C:H film architecture with strong adhesion and superlubricity properties provides design criteria of superlubricious films for industrial applications.

Operando tribo-Raman spectroscopic observation for wear processes of superlow frictional concentrated polymer brushes at frictional interface

Concentrated polymer brushes (CPBs) synthesized by advanced surface-initiated controlled radical polymerization have received increasing attention in the tribological field because CPBs in good solvents exhibit superlow friction phenomena under macroscopic severe sliding conditions. To realize tribological applications of CPBs, an understanding of the wear mechanisms of CPBs is required to develop a method to enhance their durability. In this study, an operando tribo-Raman spectroscopic experiment, which is an effective approach for revealing the wear mechanisms of tribo-materials, was conducted on the frictional interface between a polymethyl methacrylate (PMMA)-CPB and a bearing steel ball immersed in an ionic liquid (IL), which is a good solvent for PMMA-CPBs. From the operando tribo-Raman results, the concentrations and molecular structures of the PMMA-CPBs and IL at the frictional interface were found to be constantly changing with the sliding cycles during operation. Moreover, their time-dependent changes strongly affected the wear process and superlow friction phenomena of the PMMA-CPB lubricated with the IL. Consequently, our results suggest that controlling the degree of swelling of CPBs at frictional interface is an effective approach for realizing a durable-superlow frictional tribo-system of CPBs.

Superlow friction and wear enabled by nanodiamond and hexagonal boron nitride on a-C:H films surfaces in dry nitrogen

Hydrogen-deficient amorphous carbon (a-C:H) is subjected to abnormally high friction in an inert environment, and the possibility of maintaining ultra-low friction is still a great challenge. Here, nanodiamond and hexagonal boron nitride are used to improve the tribological properties of a-C:H films. Particularly, an excellent synergistic lubrication effect was obtained after surface treatment using a solution droplet of nanodiamond + h-BN mixture with a mass ratio of 1:1 and a concentration of 0.1 mg/mL, and concurrently vacuum heated at 80 °C. The COF also decreased sharply to be below 0.001, about two orders of magnitude lower than the case without surface treatment. Correspondingly, the wear rate of steel ball was also decreased by ∼97%. The dominant factor to obtain ultra-low friction is the incommensurate contact between the tribolayer formed by nanodiamond + hexagonal boron nitride and the hydrogen passivation layer. Meanwhile, the incommensurate contact of mismatched h-BN nanoflakes promotes the formation of ultra-low friction. Besides, low shear slip at the interface formed by the in-situ growth of h-BN layered shear band and ND + h-BN nano-composited shear band, respectively, rubbing against the upper ND + h-BN mixed tribolayer also makes a contribution. The ultra-low friction is achieved under the joint action of these factors. These findings can enrich the understanding of the surface modification of a-C:H films by low-dimensional nano-lubricants. It is helpful to develop solid carbon films with more adaptability and robustness under extreme working conditions and provide important scientific and technical guidance.

Preparation of MXene/EP coating for promising anticorrosion and superlow friction properties

MXene, a two-dimensional (2D) material consisting of Ti3C2Tx, has demonstrated significant potential for numerous applications, particularly in the areas of corrosion and wear resistance to protect metals. In this study, we reported the facile preparation of MXene/epoxy resin (EP) coatings through blending, which was successfully confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). With the lubrication of oleic acid (OA), an EP coating containing 1 wt% MXene exhibited superlubricity with a remarkably low coefficient of friction (COF) of 0.008. Furthermore, compared to the pristine substrate, the coating achieved an impressive reduction in the wear rate of 99.73 %. Under simulated seawater conditions, the coating demonstrated an exceptional corrosion protection efficiency (PE) of 99.38 %, which was attributed to the corrosion inhibition effect of MXene. The observed superlow friction was primarily attributed to the synergistic effects of a tribo-film of OA, the presence of the MXene film, and the improved mechanical properties of the EP coating due to the incorporation of MXene. Our study highlighted the potential of MXene as a protective material in long-term marine applications.

Novel Environmentally Superior Tribomaterial with Superlow Friction: 100% Cellulose Nanofiber Molding

In this paper, we report on a novel, environmentally superior tribomaterial with superlow friction of 100% cellulose nanofiber (CNF) molding. Based on our experimental results, the CNF molding exhibited a superlow friction coefficient of approximately 0.01 under lubrication with a fatty acid: glycerin monooleate (GMO) diluted with poly-alfa-olefine. Attenuated total reflection Fourier-transform infrared spectroscopy and high-resolution frequency-modulation atomic force microscopy analyses demonstrated that superlow friction of the CNF molding was realized by GMO-assisted functionalization of the CNF surface, which effectively promoted the formation of a soft absorption film or soft swollen CNF layer. Our findings indicate that the in-situ functionalization of OH-terminated CNF surfaces during the friction process plays a crucial role in achieving superlow friction.Graphical

High-Temperature Superlubricity Performance of h-BN Coating on the Textured Inconel X750 Alloy

The high-temperature superlubricity performances of h-BN coatings on the nontextured and textured surface of an Inconel X750 alloy is reported in the present paper. The hardness and bond strength of the h-BN coating and alloy were investigated. The tribological properties of the X750 alloy and coatings on the X750 alloy substrate were investigated at different temperatures. The surface texture was manufactured on the surface of the X750 alloy, and then coatings were deposited on the textured surface to reduce the cracking of the coating and enhance the stability of the antifriction behaviors of the h-BN coatings. The tribotest results showed that the texture is helpful to enhance the interface thermal compatibility of the coating and substrate and store the wear debris generated during sliding. Therefore, a stable superlubricity was achieved at high temperatures, and a super low friction mechanism is also discussed.

Understanding and Preventing Lubrication Failure at the Carbon Atomic Steps.

Two-dimensional (2D) lamellar materials are normally capable of rendering super-low friction, wear protection, and adhesion reduction in nanoscale due to their ultralow shear strength between two basal plane surfaces. However, high friction at step edges prevents the 2D materialsfrom achieving super-low friction in macroscale applications and eventually leads to failure of lubrication performance. Here, taking graphene as an example, the authors report that not all step edges are detrimental. The armchair (AC) step edges are found to have only a minor topographic effect on friction, while the zigzag (ZZ) edges cause friction two orders of magnitude larger than the basal plane. The AC step edge is less reactive and thus more durable. However, the ZZ structure prevails when step edges are produced mechanically, for example, through mechanical exfoliation or grinding of graphite. The authors found a wayto make the high-friction ZZ edge superlubricious by reconstructing the (6,6) hexagon structure to the (5,7) azulene-like structure through thermal annealing in an inert gas environment. This will facilitate the realization of graphene-based superlubricity over a wide range of industrial applications in which avoiding the involvement of step edges is difficult.