Superlubricity and Antiwear Properties of In Situ-Formed Ionic Liquids at Ceramic Interfaces Induced by Tribochemical Reactions

Breaking the Tradeoff between Oil Film Thickness and Viscous Friction: n-Alcohol-Containing Lubricants in High-Pressure Contacts

Low-friction lubricant formulations are urgently needed to improve the energy efficiency of machines. Here, we show that blending 1-dodecanol with a hydrocarbon oil improves lubrication in nonconformal sliding/rolling contacts by simultaneously increasing hydrodynamic film thickness and reducing viscous friction. This is due to pressure-induced polymorphic phase transformations in the 1-dodecanol molecules after they flow through the film-thickness-determining inlet and reach the load-supporting zone. At relatively low pressures, 1-dodecanol forms a lamellar hexagonal solid polymorph that gives durable superlubricity and then, at higher pressures, it forms an orthorhombic polymorph. Both polymorphs cause anomalously low friction when blended into various hydrocarbon base oils over a wide range of speed, pressure, and shear rate conditions representative of rolling bearing and gear contacts. By breaking the ubiquitous tradeoff between friction and film thickness and enabling superlubricity, these blends pave the way for considerable energy efficiency improvements in widespread lubricated contacts.

Macroscale Superlubrication Achieved with Shear-Thinning Semisolid Lubricants

Macrosuperlubric materials are pivotal for reducing friction and wear in engineering applications. However, current solid superlubricants require intricate fabrication and specific conditions (e.g., vacuum or inert atmospheres), while liquid superlubricants are prone to creep, leakage, and corrosion. Here, a novel semisolid subnanometer nanowire (SNW) superlubrication material based on the shear-thinning effect is introduced to overcome these challenges. The SNWs achieve an exceptionally low friction coefficient (0.008-0.009) with silicon nitride (Si3N4) and polytetrafluoroethylene (PTFE) tribo-pairs, demonstrating a brief running-in period (≈39 s) and stable superlubrication over extended friction (12 h, >120 000 cycles). The combination of the shear-thinning network structure mechanism, the adsorption membrane mechanism, and hydrodynamic effects provides a synergistic effect, playing a crucial role in achieving superlubricity. Additionally, SNWs can be combined with various base oils to create semisolid gel lubricants with superlubricating properties. This innovative approach addresses the limitations of current superlubrication systems and introduces a new category of semisolid gel lubricants, significantly expanding the applications of superlubrication materials.

Ion-specific ice provides a facile approach for reducing ice friction

HYPOTHESIS

Ice friction plays a crucial role in both basic study and practical use. Various strategies for controlling ice friction have been developed. However, one unsolved puzzle regarding ice friction is the effect of ion-ice interplay on its tribological properties.

EXPERIMENTS AND SIMULATIONS

Here, we conducted ice friction experiments and summarized the specific effects of hydrated ions on ice friction. By selecting cations and anions, the coefficient of ice friction can be reduced by more than 70 percent. Experimental spectra, low-field nuclear magnetic resonance (LF-NMR), density functional theory (DFT) calculations, and Molecular dynamics (MD) simulations demonstrated that the addition of ions could break the H-bonds in water.

FINDINGS

The link between the charge density of ions and the coefficients of ice friction was revealed. A part of the ice structure was changed from an ice-like to a liquid-like interfacial water structure with the addition of ions. Lower charge density ions led to weaker ionic forces with the water molecules in the immobilized water layer, resulting in free water molecules increasing in the lubricating layer. This study provides guidance for preparing ice-making solutions with low friction coefficients and a fuller understanding of the interfacial water structure at low temperatures.

Synergistic Tribological Effects of Ti3C2Tx MXene and ZDDP under Simple Grafting Strategies for Efficient Lubrication

MXenes, a novel class of two-dimensional materials, possess exceptional physical and chemical properties, positioning them as promising candidates for lubricant additives. However, their potential is constrained by challenges in dispersion and stability, coupled with a paucity of research on interactions with additives in full-formula oils. In this study, hexadecylphosphonic acid (HDPA) is grafted onto Ti3C2Tx to formulate a polyalkylene glycol dispersion system. The findings reveal that the HDPA-modified Ti3C2Tx (HDPA-Ti3C2) is successfully synthesized, demonstrating superior dispersion stability and notable friction-reduction and antiwear properties. Notably, when combined with zinc dialkyl dithiophosphate (ZDDP), the HDPA-Ti3C2/ZDDP composite additive outperforms single additives in tribological performance, suggesting synergistic effects between them. This enhanced performance may be attributed to the formation of an amorphous polyphosphate tribofilm offering wear resistance, followed by the generation of a TiO2 tribofilm that further safeguards and repairs the worn surface, thereby enhancing the load-bearing capacity. Concurrently, the interlayer sliding mechanism of nanosheets, which substitutes the relative motion of the friction pair, reduces friction under boundary lubrication, ensuring prolonged effective lubrication. This work broadens the application prospects of Ti3C2Tx MXene for the design and development of commercial lubricating additives.

Highly Concentrated Electrolyte Superlubricants Enhanced by Interfacial Water Competition Around Chemically Active MgO Additives

The low concentration of water-based lubricants and the high chemical inertness of the additives involved are often regarded as basic norms in the design of liquid lubricants. Herein, a novel liquid superlubricant of an aqueous solution containing a relatively high concentration of salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), is reported for the first time, and the superlubricity stability and load-bearing capacity of the optimized system (MgO0.10/LiTFSI10) are effectively strengthened by the addition of only trace (0.10 wt %) water-chemically active MgO additives. It demonstrates higher applicable loads, lower COF (∼0.004), and stability relative to the base solution. Only a trace amount of MgO additive is needed for the superlubricity, which makes up for the cost and environmental deficiencies of LiTFSI10. The weak interaction region between free water and the outer-layer water of Li+ hydration shells becomes a possible ultralow shear resistance sliding interface; the Mg(OH)2 layer, generated by the reaction of MgO with water, further creates additional weakly interacting interfaces, leading to the formation of an asymmetric contact between the clusters/particles within the hydrodynamic film by moderating the competition between interfacial water and free water, thus achieving high load-bearing macroscopic superlubricity. This study deepens the contribution of electrolyte concentration to ionic hydration and superlubricity due to the low shear slip layer formed by interfacial water competition with water-activated solid additives, providing new insights into the next generation of high load-bearing water-based liquid superlubricity systems.

Study on the Superlubricity Behavior of Ions under External Electric Fields at Steel Interfaces

Realizing macroscopic superlubricity in the presence of external electric fields (EEFs) at the steel interfaces is still challenging. In this work, macroscopic superlubricity with a coefficient of friction value of approximately 0.008 was realized under EEFs with the lubrication of LiPF6-based ionic liquids at steel interfaces. The roles of cations and anions in the superlubricity realization under EEFs were studied. Based on the experimental results, the macroscopic superlubricity behavior of Li(PEG)PF6 under EEFs at steel interfaces is attributed to the strong hydration effect of Li+ cations and the complete reactions of anions that contributed to the formation of a boundary film on the appropriate surface. Moreover, the reduction in the number of iron oxides in the boundary film on the disc was beneficial for friction reduction. We also provide a calculation model to describe the relationship between the hydration effect and the optimal voltage position, at which the lowest friction might occur. Ultimately, this work proves that macroscopic superlubricity can be realized under EEFs at steel interfaces and provides a foundation for engineering applications of superlubricity in an electrical environment.

Macroscale Superlubricity of Hydrated Anions in the Boundary Lubrication Regime

Cations can achieve excellent hydration lubrication at smooth interfaces under both microscale and macroscale conditions due to the boundary layer composed of hydration shells surrounding charges, but what about anions? Commonly used friction pairs are negatively charged at the solid/solution interface. Achieving anionic adsorption through constructing positively charged surfaces is a prerequisite for studying the hydration lubrication of anions. Here we report the hydration layer composed of anions adsorbed on the positively charged polymer/sapphire interface at acidic electrolyte solutions with pH below the isoelectric point, which contributes to the hydration lubrication of anions. Strongly hydrated anions (for the case of SO42-) exhibit stable superlubricity comparable to cations, with strikingly low boundary friction coefficient of 0.003-0.007 under contact pressures above 15 MPa without a running-in period. The hydration lubrication performance of anions is determined by both the ionic hydration strength and ion adsorption density based on the surface potential and tribological experiments. The results shed light on the role of anions in superlubricity and hydration lubrication, which may be relevant for understanding the lubrication mechanism and improving lubrication performance in acidic environments, for example, in acid pumps, sealing rings of compressors for handling acidic media, and processing devices of nuclear waste.

Lithium Citrate Triggered Macroscopic Superlubricity with Near-Zero Wear on an Amorphous Carbon Film

An amorphous carbon (a-C) film shows substantial potential for friction and wear reduction. In this work, the robust superlubricity state with a coefficient of friction of 0.002 at the maximal pressure of 1.15 GPa was realized when lithium citrate (LC) was applied as the lubricating additive in ethylene glycol (EG) to lubricate the Si₃N₄/a-C friction pair based on the ball-on-plate friction test. The wear rate of the a-C film was 4.5 × 10^-10 mm³/N·m, which was reduced by 98.3% compared to that of the film lubricated with EG. Friction promoted the chemisorption of the LC molecules via the tribochemical reaction between the carboxylate radicals and the a-C film. The exposed lithium ions could adsorb water molecules to form a hydration layer, providing extremely low shear strength. Furthermore, the colloidal silica layer formed on the Si₃N₄ ball via the tribochemical reaction could reduce friction. It was difficult to destroy the formed tribochemical films under high contact pressure because they were robust, preventing the direct contact of the friction pair and resulting in the near-zero wear of the a-C film.

Macroscale Superlubricity with Ultralow Wear and Ultrashort Running-In Period (∼1 s) through Phytic Acid-Based Complex Green Liquid Lubricants

Liquid superlubricity has attracted much attention, due to its ability to significantly reduce friction on the macroscale. However, the severe wear caused by the long running-in period is still one of the bottlenecks restricting the practical application of liquid superlubricating materials. In this work, the obtained polyethylene glycol-phytic acid (PEG-PA) composite liquid lubricants showed outstanding superlubricating properties (μ ≈ 0.006) for Si3N4/glass friction pairs with an ultrashort running-in period (∼1 s) under high Hertzian contact pressure of ∼758 MPa. More importantly, even after up to 12 h (∼700 m of travel), only about 100 nm deep wear scars were found on the surface of the glass sheet (wear rate = 2.51× 10-9 mm3 N-1 m-1). From the molecular point of view, the water molecules anchored between the two friction pairs have extremely low shear force during the friction process, and the strong hydrogen bond interaction between PEG and PA greatly improves the bearing capacity of the lubricant. This work addresses the challenge of liquid superlubricant simultaneously exhibiting low shear force and high load-carrying capacity and makes it possible to obtain liquid superlubrication performance with an extremely short running-in time.

Macroscale Superlubricity of Black Phosphorus Quantum Dots

In the present work, Black Phosphorus Quantum Dots (BPQDs) were synthesized via sonication-assisted liquid-phase exfoliation. The average size of the BPQDs was 3.3 ± 0.85 nm. The BPQDs exhibited excellent dispersion stability in ultrapure water. Macroscale superlubricity was realized with the unmodified BPQDs on rough Si3N4/SiO2 interfaces. A minimum coefficient of friction (COF) of 0.0022 was achieved at the concentration of 0.015 wt%. In addition, the glycerol was introduced to promote the stability of the superlubricity state. The COF of the BPQDs-Glycerol aqueous solution (BGaq) was 83.75% lower than that of the Glycerol aqueous solution (Gaq). Based on the above analysis, the lubrication model was presented. The hydrogen-bonded network and silica gel layer were formed on the friction interface, which played a major role in the realization of macroscale superlubricity. In addition, the adsorption water layer could also prevent the worn surfaces from making contact with each other. Moreover, the synergistic effect between BPQDs and glycerol could significantly decrease the COF and maintain the superlubricity state. The findings theoretically support the realization of macroscale superlubricity with unmodified BPQDs as a water-based lubrication additive.

Liquid Superlubricity Enabled by the Synergy Effect of Graphene Oxide and Lithium Salts

In this study, graphene oxide (GO) nanoflakes and lithium salt (LiPF₆) were utilized as lubrication additives in ether bond−containing dihydric alcohol aqueous solutions (DA(aq)) to improve lubrication performances. The apparent friction reduction and superlubricity were realized at the Si₃N₄/sapphire interface. The conditions and laws for superlubricity realization have been concluded. The underlying mechanism was the synergy effect of GO and LiPF₆. It was proven that a GO adsorption layer was formed at the interface, which caused the shearing interface to transfer from solid asperities to GO interlayers (weak interlayer interactions), resulting in friction reduction and superlubricity realization. In addition to the GO adsorption layer, a boundary layer containing phosphates and fluorides was formed by tribochemical reactions of LiPF₆ and was conducive to low friction. Additionally, a fluid layer contributed to friction reduction as well. This work proved that GO−family materials are promising for friction reduction, and provided new insights into realizing liquid superlubricity at macroscale by combining GO with other materials.

Macroscale Robust Superlubricity on Metallic NbB2

Robust superlubricity (RSL), defined by concurrent superlow friction and wear, holds great promise for reducing material and energy loss in vast industrial and technological operations. Despite recent advances, challenges remain in finding materials that exhibit RSL on macrolength and time scales and possess vigorous electrical conduction ability. Here, the discovery of RSL is reported on hydrated NbB₂ films that exhibit vanishingly small coefficient of friction (0.001-0.006) and superlow wear rate (≈10^-17 m³ N^-1 m^-1 ) on large length scales reaching millimeter range and prolonged time scales lasting through extensive loading durations. Moreover, the measured low resistivity (≈10^-6 Ω m) of the synthesized NbB₂ film indicates ample capability for electrical conduction, extending macroscale RSL to hitherto largely untapped metallic materials. Pertinent microscopic mechanisms are elucidated by deciphering the intricate load-driven chemical reactions that generate and sustain the observed superlubricating state and assessing the strong stress responses under diverse strains that produce the superior durability.

Role of Interfacial Bonding in Tribochemical Wear

Tribochemical wear of contact materials is an important issue in science and engineering. Understanding the mechanisms of tribochemical wear at an atomic scale is favorable to avoid device failure, improve the durability of materials, and even achieve ultra-precision manufacturing. Hence, this article reviews some of the latest developments of tribochemical wear of typical materials at micro/nano-scale that are commonly used as solid lubricants, tribo-elements, or structural materials of the micro-electromechanical devices, focusing on their universal mechanisms based on the studies from experiments and numerical simulations. Particular focus is given to the fact that the friction-induced formation of interfacial bonding plays a critical role in the wear of frictional systems at the atomic scale.

Hysteresis in the MD Simulations of Differential Capacitance at the Ionic Liquid-Au Interface

In this Letter, we report the first observation of the capacitance-potential hysteresis at the ionic liquid | electrode interface in atomistic molecular dynamics simulations. While modeling the differential capacitance dependence on the potential scan direction, we detected two long-living types of interfacial structure for the BMImPF₆ ionic liquid at specific charge densities of the gold Au(111) surface. These structures differ in how counterions overscreen the surface charge. The high barrier for the transition from one structure to another slows down the interfacial restructuring process and leads to the marked capacitance-potential hysteresis.

Superlubrication obtained with mixtures of hydrated ions and polyethylene glycol solutions in the mixed and hydrodynamic lubrication regimes

HYPOTHESIS

Superlubricity is known to dramatically reduce frictional energy consumption and to improve service life of mechanical devices and biological systems. However, reduction of wear during the running-in period of friction pairs, especially under high contact pressures, still remains an unresolved issue affecting all machines.

EXPERIMENTS

Here the lubrication, adsorption, and conformational properties of hydrated ions and polyethylene glycol (PEG) mixtures were evaluated at different mass fractions and concentrations of PEG and salts by ball-on-disc tribometer, ζ-potential, quartz crystal microbalance with dissipation (QCM-D), and dynamic light scatting (DLS) analyses.

FINDINGS

These mixtures exhibited superlubricity between Si₃N₄ and sapphire surfaces in a wide range of concentrations and ions valency. Interestingly, a running-in phase shorter than 1 min and low wear rate of 1.85 μm³/(N·m) were observed at contact pressures up to 555 MPa, significantly higher to earlier findings. PEG chains retain random coils filling the bulk of the interfacial film without strongly adsorbing on the interfaces but significantly increasing the viscosity of lubricating film, thereby favoring hydrodynamic lubrication. Hydrated ions are strongly adsorbed on the negatively charged ceramic surfaces, ensuring a sustained hydration effect maintaining superlubricity. The outstanding lubrication characteristics of the PEG/ions mixtures were attributed to the synergistic action of hydration and hydrodynamic lubrication, which appears as a promising avenue for developing new green lubricants and has implications for industrial and biological applications.

Macroscale Superlubricity Achieved on the Hydrophobic Graphene Coating with Glycerol

Introduction of graphene-family nanoflakes in liquid results in a reduction in friction and enhanced wear resistance. However, the high demand for dispersity and stability of the nanoflakes in liquid largely restricted the choice of graphene-family nanoflakes thus far. This study proposed a new strategy to overcome this limitation, involving the formation of a graphene coating with deposited graphene-family nanoflakes, followed by the lubrication of the coating with glycerol solution. Pristine graphene (PG), fluorinated graphene (FG), and graphene oxide (GO) nanoflakes were chosen to be deposited on the respective SiO₂ substrates to form graphene coatings, and then an aqueous solution of glycerol was used as lubricant. The coefficient of friction (COF) and wear rate were reduced for all deposited coatings. However, the PG coating exhibited better lubrication and antiwear performance than FG and GO coatings. A robust superlubricity with COF of approximately 0.004 can be achieved by combining glycerol with the PG coating. The superlubricity mechanism was attributed to the formation of a tribofilm, mainly composed of graphene nanoflakes in the contact zone. The extremely low friction achieved on the hydrophobic graphene coating with liquid can aid in the development of a high-performing new lubrication system for industrial applications.

Ultrastable Lubricating Properties of Robust Self-Repairing Tribofilms Enabled by in Situ-Assembled Polydopamine Nanoparticles

Aqueous lubrication in nature is attracting increasing attention in the tribological fields for reducing friction energy consumption and improving anti-wear durability. Generally, adding nanolubricant additives is one of the most important strategies to effectively enhance the interface performance under boundary lubrication via the formation of a protective tribofilm on rubbing surfaces. However, the adsorbed tribofilms are unstable and are prone to failure during friction, and the interaction mechanism between the tribofilms and frictional interfaces is partly disclosed. In this study, inspired by mussels, an in situ-assembled polydopamine (PDA) tribofilm is achieved with PDA nanoparticles as aqueous lubricant additives, which shows excellent lubrication properties. The coefficient of friction is interface-independent and is reduced by as much as 83%. The results show that the PDA tribofilm can not only form chemical bonding with metal interfaces but also present a synergistic lubrication effect with the upper ceramic surface. Especially, a self-repairing effect of the PAD tribofilm is observed, by which the ultrastable lubricating properties can be achieved during friction, and thus, the friction and wear can be effectively controlled. This work provides an effective method for improving the interface stability of friction pairs under aqueous lubrication and also shows great meaning for industrial applications.

Origins of Superlubricity Promoted by Hydrated Multivalent Ions

Strong hydration repulsion exists between two negatively charged surfaces in the alkali metal salt solutions, together with the fluid response to the shear of hydration layers, leading to superlubricity. However, whether the multivalent ions can obtain superlubricity has not been revealed yet. Here, we evaluate the lubrication and adsorption properties of multivalent ions at different concentrations between Si₃N₄ and sapphire surfaces. The divalent and trivalent ions exhibit extremely low friction coefficients of 0.005-0.006 and 0.002-0.004, respectively, under contact pressures above 0.25 GPa, and three trivalent ions can achieve superlubricity at quite low sliding speeds (3.1 mm/s), which is a significant breakthrough for superlubricity under boundary lubrication. Moreover, compared with monovalent ions, divalent ions can reduce surface potential and lower surface charge density even further, and trivalent ions can neutralize the negatively charged ceramic surfaces and even lead to charge inversion due to excess adsorption of the cations, which ensures strong adsorption of hydrated multivalent ions on friction surfaces.

On Lubrication States after a Running-In Process in Aqueous Lubrication

Recently, many studies have reported the ultralow friction coefficient of sliding friction between rigid solid surfaces in aqueous lubrication. A running-in process that goes through high-friction and friction-decreasing regions to a stable ultralow friction region is often required. However, the role of the friction-decreasing region is often ascribed to tribofilm formation in which complexity hindered the quantitative description of the running-in process and the prediction of its subsequent lubrication state. In this work, the frictional energy (E_f) dissipated in the running-in process of a poly(oligo(ethylene glycol) methyl ether acrylate) aqueous lubrication was related to the wear of solid surfaces under different conditions and lubrication states. Experimental results indicated that the high-friction region was in a boundary lubrication state, contributed to most of the wear, and significantly reduced the contact pressure, whereas the friction-decreasing region was in a mixed lubrication state, contributed only to the slight and slow removal of materials, and slightly reduced the contact pressure. Therefore, by establishing relationships among the wear scar diameter, E_f, and the Stribeck curve of the tribological system, the subsequent lubrication state after a running-in process under various working loads and sliding speeds could be quantitatively predicted. The running-in experiments with different aqueous lubrication systems showed good agreement with the prediction of this method. This investigation provides an effective method for the wear and lubrication state prediction after a running-in process, further proving the importance of the Stribeck curve for a lubrication system. This study may also have important implications for the strategy design of the running-in process in various industrial applications.