Operando tribochemical formation of onion-like-carbon leads to macroscale superlubricity

Robust Low-Friction and Low-Wear TiNbMoTaCr High-Entropy Film Enabled by Periodically Inserting Curved MoS2 Sheets

The well-known integration of physical, chemical, and mechanical properties enables high-entropy alloys (HEAs) to be applied in various fields; however, refractory HEAs are brittle and susceptible to abrasive wear at high coefficients of friction (COF), resulting in insufficient mechanical durability against abrasion. Herein, curved MoS2 nanosheets are periodically introduced into the TiNbMoTaCr film for triggering the self-assembly mixed metal oxides @MoS2 nanoscrolls, which contain hard mixed metal oxides cores and the low-shearing lubricant MoS2 shells, during the friction in the air environment; such mixed metal oxides@MoS2 nanoscrolls in the friction interfaces can contribute to the robust low friction and low wear. Compared to the pure TiNbMoTaCr film (with high COF of ∼0.78, low abrasive durability identified by worn-out event), the periodic incorporation of 10 nm thickness curved MoS2 sheets can successfully achieve a low COF of ∼0.08 and low wear rate of ∼9.561 × 10-8 mm3/ Nm, much lower than the pure MoS2 film (COF = ∼ 0.21, wear rate = ∼ 1.03 × 10-6 mm3/ Nm). Such superior tribological properties originate from the cooperative interaction of TiNbMoTaCr nanolayers and curved MoS2 nanosheets, accompanied by the self-assembly of mixed metal oxides@MoS2 nanoscrolls. In these nanoscrolls, TiNbMoTaCr can act as an 'air-absorbing agent' to form high-loading mixed metal oxide cores and serve as an 'oxygen sacrificer,' preventing the low-shearing lubricant curved MoS2 nanosheets from oxidation. In addition, even with the soft MoS2, the hardness of the TiNbMoTaCr/MoS2 nanomultilayers can still be well maintained and increased above the calculated values by mixing law, further favoring superior mechanical durability. The synergetic effect of TiNbMoTaCr and curved MoS2 nanosheets during the friction in air can provide a route to design HEA films with enhanced tribological properties for better mechanical durability and broader application prospects.

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.

Impact of Static Distortion Waves on Superlubricity

Friction is a major source of energy loss in mechanical devices. This energy loss may be minimized by creating interfaces with extremely reduced friction, i.e., superlubricity. Conventional wisdom holds that incommensurate interface structures facilitate superlubricity. Accurately describing friction necessitates the precise modeling of the interface structure. This, in turn, requires the use of accurate first-principles electronic structure methods, especially when studying organic/metal interfaces, which are highly relevant due to their tunability and propensity to form incommensurate structures. However, the system size required to calculate incommensurate structures renders such calculations intractable. As a result, studies of incommensurate interfaces have been limited to very simple model systems or strongly simplified methodology. We overcome this limitation by developing a machine-learned interatomic potential that is able to determine energies and forces for structures containing thousands to tens of thousands of atoms with an accuracy comparable to conventional first-principles methods but at a fraction of the cost. Using this approach, we quantify the breakdown of superlubricity in incommensurate structures due to the formation of static distortion waves. Moreover, we extract design principles to engineer incommensurate interface systems where the formation of static distortion waves is suppressed, which facilitates low friction coefficients.

Review of two-dimensional nanomaterials in tribology: Recent developments, challenges and prospects

From our ordinary lives to various mechanical systems, friction and wear are often unavoidable phenomena that are heavily responsible for excessive expenditures of nonrenewable energy, the damages and failures of system movement components, as well as immense economic losses. Thus, achieving low friction and high anti-wear performance is critical for minimization of these adverse factors. Two-dimensional (2D) nanomaterials, including transition metal dichalcogenides, single elements, transition metal carbides, nitrides and carbonitrides, hexagonal boron nitride, and metal-organic frameworks have attracted remarkable interests in friction and wear reduction of various applications, owing to their atomic-thin planar morphologies and tribological potential. In this paper, we systematically review the current tribological progress on 2D nanomaterials when used as lubricant additives, reinforcement phases in the coatings and bulk materials, or a major component of superlubricity system. Additionally, the conclusions and prospects on 2D nanomaterials with the existing drawbacks, challenges and future direction in such tribological fields are briefly provided. Finally, we sincerely hope such a review will offer valuable lights for 2D nanomaterial-related researches dedicated on tribology in the future.

Template-free scalable growth of vertically-aligned MoS2 nanowire array meta-structural films towards robust superlubricity

Two-dimensional (2D) molybdenum disulfide exhibits a variety of intriguing behaviors depending on its orientation layers. Therefore, developing a template-free atomic layer orientation controllable growth approach is of great importance. Here, we demonstrate scalable, template-free, well-ordered vertically-oriented MoS2 nanowire arrays (VO-MoS2 NWAs) embedded in an Ag-MoS2 matrix, directly grown on various substrates (Si, Al, and stainless steel) via one-step sputtering. In the meta-structured film, vertically-standing few-layered MoS2 NWAs of almost micron length (∼720 nm) throughout the entire film bulk. While near the surface, MoS2 lamellae are oriented in parallel, which are beneficial for caging the bonds dangling from the basal planes. Owing to the unique T-type topological characteristics, chemically inert Ag@MoS2 nano-scrolls (NSCs) and nano-crystalline Ag (nc-Ag) nanoparticles (NPs) are in situ formed under the sliding shear force. Thus, incommensurate contact between (002) basal planes and nc-Ag NPs is observed. As a result, robust superlubricity (friction coefficient μ = 0.0039) under humid ambient conditions is reached. This study offers an unprecedented strategy for controlling the basal plane orientation of 2D transition metal dichalcogenides (TMDCs) via substrate independence, using a one-step solution-free easily scalable process without the need for a template, which promotes the potential applications of 2D TMDCs in solid superlubricity.

Self-Adaptive Macroscale Superlubricity Based on the Tribocatalytic Properties of Partially Oxidized Black Phosphorus

The high-flash heat generated by direct contact at asperity tips under high contact stress and shear significantly promotes the tribocatalytic reaction between a lubricating medium and a friction interface. Macroscale superlubricity can be achieved by using additives with good lubrication properties to promote the decomposition and transformation of a lubricating medium to form an ultralow shear interface during the friction process. This paper proposed a way to achieve self-adaptive oil-based macroscale superlubricity on different tribopairs, including steel-steel and steel-DLC (diamond-like carbon), which is based on the excellent lubricating performance of black phosphorus with active oxidation and the catalytic cleavage behavior of oil molecules on the surface of oBP. This work potentially expands the industrial application of superlubricity.

Compaction of Hydrophobic Molybdenum Disulfide Coatings for Promoting Tribological Behaviors on Engineering Steel

Ambient environment has a crucial effect on the lubrication performance of molybdenum disulfide (MoS₂) coatings. In this work, we fabricated porous MoS₂ coatings via a facile optimized aerosol-assisted chemical vapor deposition (AACVD) method. It is found that the obtained MoS₂ coating demonstrates outstanding antifriction and antiwear lubrication performance with the coefficient of friction (COF) and wear rate as low as 0.035 and 3.4 × 10^-7 mm³/Nm in lower humidity (15 ± 5)%, respectively, which is comparable to the lubrication ability of pure MoS₂ in vacuum. In addition, the hydrophobic property of porous MoS₂ coatings is suitable for infusing lubrication oil to achieve stable solid-liquid lubrication in higher humidity (85 ± 2)%. The composite lubrication system shows excellent tribological behavior in both dry and wet environments, which will alleviate the sensitivity of the MoS₂ coating to the environment and ensure the service life of the engineering steel in complex industrial backgrounds.

Enhanced Tribological Performance of Low-Friction Nanocomposite WSexSy/NP-W Coatings Prepared by Reactive PLD

A novel laser-based method for producing nanocomposite coatings consisting of a tungsten sulfoselenide (WSe_xS_y) matrix and W nanoparticles (NP-W) was developed. Pulsed laser ablation of WSe₂ was carried out in H₂S gas under appropriate laser fluence and reactive gas pressure. It was found that moderate sulfur doping (S/Se ~0.2-0.3) leads to significant improvement in the tribological properties of WSe_xS_y/NP-W coatings at room temperature. Changes in the coatings during tribotesting depended on the load on the counter body. The lowest coefficient of friction (~0.02) with a high wear resistance was observed in a N₂ environment at an increased load (5 N), resulting from certain structural and chemical changes in the coatings. A tribofilm with a layered atomic packing was observed in the surface layer of the coating. The incorporation of nanoparticles into the coating increased its hardness, which may have influenced the formation of the tribofilm. The initial matrix composition, which had a higher content of chalcogen atoms ((Se + S)/W~2.6-3.5), was altered in the tribofilm to a composition close to the stoichiometric one ((Se + S)/W~1.9). W nanoparticles were ground and retained under the tribofilm, which impacted the effective contact area with the counter body. Changes in the tribotesting conditions-lowering the temperature in a N₂ environment-resulted in considerable deterioration of the tribological properties of these coatings. Only coating with a higher S content that was obtained at increased H₂S pressure exhibited remarkable wear resistance and a low coefficient of friction, measuring 0.06, even under complicated conditions.

Hydrogen-Enhanced Catalytic Conversion of Amorphous Carbon to Graphene for Achieving Superlubricity

The solid-state conversion of amorphous carbon into graphene is extremely difficult, but it can be achieved in the friction experiments that induce macroscale superlubricity. However, the underlying conversion mechanisms remain elusive. Here, the friction experiments with Cu nanoparticles and (non-hydrogen (H) or H) a-C in vacuum, show the H-induced conversion of mechanical to chemical wear, resulting in the a-C's tribosoftening and nanofragmentating that produce hydrocarbon nanoclusters or molecules. It is such exactly hydrocarbon species that yield graphene at hydrogen-rich a-C friction interface, through reaction of them with Cu nanoparticles. In comparison, graphene isn't formed at Cu/non-H a-C friction interface. Atomistic simulations reveal the hydrogen-enhanced tribochemical decomposition of a-C and demonstrate the energetically favorable graphitization transformation of hydrocarbons on Cu substrates. The findings are of importance to achieve solid-state transformation between different carbon allotropes and provide a good strategy to synthesize other graphitic encapsulated catalysts with doped elements.

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.

Tribological Properties of WS2 Thin Films Containing Graphite-like Carbon and Ni Interlayers

The development and production of thin-film coatings having very low friction is an urgent problem of materials science. One of the most promising solutions is the fabrication of special nanocomposites containing transition-metal dichalcogenides and various carbon-based nanophases. This study aims to explore the influence of graphite-like carbon (g-C) and Ni interface layers on the tribological properties of thin WS₂ films. Nanocrystalline WS₂ films were created by reactive pulsed laser deposition (PLD) in H₂S at 500 °C. Between the two WS₂ nanolayers, g-C and Ni nanofilms were fabricated by PLD at 700 and 22 °C, respectively. Tribotesting was carried out in a nitrogen-enriched atmosphere by the reciprocal sliding of a steel counterbody under a relatively low load of 1 N. For single-layer WS₂ films, the friction coefficient was ~0.04. The application of g-C films did not noticeably improve the tribological properties of WS₂-based films. However, the application of thin films of g-C and Ni reduced the friction coefficient to 0.013, thus, approaching superlubricity. The island morphology of the Ni nanofilm ensured WS₂ retention and altered the contact area between the counterbody and the film surface. The catalytic properties of nickel facilitated the introduction of S and H atoms into g-C. The sliding of WS₂ nanoplates against an amorphous g-C(S, H) nanolayer caused a lower coefficient of friction than the relative sliding of WS₂ nanoplates. The detected behavior of the prepared thin films suggests a new strategy of designing antifriction coatings for practical applications and highlights the ample opportunities of laser techniques in the formation of promising thin-film coatings.

Mussel-Inspired Interfacial Modification for Ultra-Stable MoS2 Lubricating Films with Improved Tribological Behavior on Nano-Textured ZnO Surfaces Using the AACVD Method

Friction-associated energy loss and mechanical wear leading to failure is a major problem in industries. To mitigate this, the design and testing of novel lubricants is important. Here, we show the facile one-pot synthesis of ZnO/ZnO nanorods (NRs) and MoS2 films on pre-treated glass substrates via aerosol-assisted chemical vapor deposition. The bearing capacity and wear life of ZnO film/ZnO NRs/MoS2 films were improved due to the lubricant retention capabilities of the NRs. Modification of the ZnO/MoS2 nano-arrays using polydopamine (PDA) allowed the realization of robust and ultra-stable solid lubricants through the triple action of chemical chelation, layered materials, and nanotexture, especially under heavy load conditions. Compared with pristine MoS2, the adhesion and bearing strength of the composite film increased by 11 and 30 times, respectively, while the coefficient of friction and wear rate decreased by 94 and 85%, respectively. This is because the chelation between the transition metal and the groups in the interlayer PDA was fully utilized to improve the interface compatibility, which significantly improves the robustness of ZnO NRs and the adhesion of MoS2. This allowed a stable and firm mechanical lock between the substrate, lubricant films, and the steel ball. It demonstrated a convenient method to achieve the antifriction and anti-wear of solid lubricating materials by PDA interface modification for practical industrial applications.

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.

Molecular-Level Lubrication Effect of 0D Nanodiamonds for Highly Bendable Graphene Liquid Crystalline Fibers

Graphene fiber is emerging as a new class of carbon-based fiber with distinctive material properties particularly useful for electroconductive components for wearable devices. Presently, stretchable and bendable graphene fibers are principally employing soft dielectric additives, such as polymers, which can significantly deteriorate the genuine electrical properties of pristine graphene-based structures. We report molecular-level lubricating nanodiamonds as an effective physical property modifier to improve the mechanical flexibility of graphene fibers by relieving the tight interlayer stacking among graphene sheets. Nanoscale-sized NDs effectively increase the tensile strain and bending strain of graphene/nanodiamond composite fibers while maintaining the genuine electrical conductivity of pristine graphene-based fibers. The molecular-level lubricating mechanism is elucidated by friction force microscopy on the nanoscale as well as by shear stress measurement on the macroscopic scale. The resultant highly bendable graphene/nanodiamond composite fiber is successfully weaved into all graphene fiber-based textiles and wearable Joule heaters, proposing the potential for reliable wearable applications.

100 km wear-free sliding achieved by microscale superlubric graphite/DLC heterojunctions under ambient conditions

Wear-free sliding between two contacted solid surfaces is the ultimate goal in the effort to extend the lifetime of mechanical devices, especially when it comes to inventing new types of micro-electromechanical systems where wear is often a major obstacle. Here we report experimental observations of wear-free sliding for a micrometer-sized graphite flake on a diamond-like-carbon (DLC) surface under ambient conditions with speeds up to 2.5 m/s, and over a distance of 100 km. The coefficient of friction (COF) between the microscale graphite flake, a van der Waals (vdW) layered material and DLC, a non-vdW-layered material, is measured to be of the order of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${10^{ - 3}}$\end{document}, which belongs to the superlubric regime. Such ultra-low COFs are also demonstrated for a microscale graphite flake sliding on six other kinds of non-vdW-layered materials with sub-nanometer roughness. With a synergistic analysis approach, we reveal the underlying mechanism to be the combination of interfacial vdW interaction, atomic-smooth interfaces and the low normal stiffness of the graphite flake. These features guarantee a persistent full contact of the interface with weak interaction, which contributes to the ultra-low COFs. Together with the extremely high in-plane strength of graphene, wear-free sliding is achieved. Our results broaden the scope of superlubricity and promote its wider application in the future.

Achieving Ultralow Friction and Wear by Tribocatalysis: Enabled by In-Operando Formation of Nanocarbon Films

Under the high-contact-pressure and shear conditions of tribological interfaces lubricated by gaseous, liquid, and solid forms of carbon precursors, a variety of highly favorable tribocatalytic processes may take place and result in the in situ formation of nanocarbon-based tribofilms providing ultralow friction and wear even under extreme test conditions. Structurally, these tribofilms are rather complex and may consist of all known forms of nanocarbon including amorphous or disordered carbon, graphite, graphene, nano-onion, nanotube, etc. Tribologically, they shear readily to provide ultralow friction and protection against wear. In this paper, we review some of the latest developments in catalyst-enabled tribochemical films resulting from gaseous, liquid, and solid sources of carbon. Particular focus is given to the nature and lubrication mechanisms of such in situ derived tribofilms with the hope that future tribological surfaces can be designed in such a way to exploit the beneficial impact of catalysis in friction and wear control.

Structural superlubricity in 2D van der Waals heterojunctions

Structural superlubricity is a fundamentally important research topic in the area of tribology. Van der Waals heterojunctions of 2D materials are an ideal system for achieving structural superlubricity and possessing potentially a wide range of applications in the future due to their ultra-flat and incommensurate crystal interfaces. Here we briefly introduce the origin and mechanism of structural superlubricity and summarize the representative experimental results, in which the coefficient of friction has achieved the order of 10^-5. Furthermore, we analyze the factors affecting structural superlubricity of 2D materials, including dynamic reconstruction of interfaces, edge effects, interfacial adsorption, etc, and give a perspective on how to realize the macroscopic expansion and where it can be applied in practice.

Angstrom-Scale Transparent Overcoats: Interfacial Nitrogen-Driven Atomic Intermingling Promotes Lubricity and Surface Protection of Ultrathin Carbon

Lubricity, a phenomenon which enables the ease of motion of objects, and wear resistance, which minimizes material damage or degradation, are important fundamental characteristics for sustainable technology developments. Ultrathin coatings that promote lubricity and wear resistance are of huge importance for a number of applications, including magnetic storage and micro-/nanoelectromechanical systems. Conventional ultrathin coatings have, however, reached their limit. Graphene-based materials that have shown promise to reduce friction and wear have many intrinsic limitations such as high temperature and substrate-specific growth. To address these concerns, a great deal of research is currently ongoing to optimize graphene-based materials. Here we discover that angstrom-thick carbon (8 Å) significantly reduces interfacial friction and wear. This lubricant shows ultrahigh optical transparency and can be directly deposited on a wide range of surfaces at room temperature. Experiments combined with molecular dynamics simulations reveal that the lubricating efficacy of 8 Å carbon is further improved via interfacial nitrogen.

Tribochemistry of superlubricating amorphous carbon films

Tribochemistry refers to a series of physical and chemical reactions that occur at a sliding interface under friction action, and the tribological properties of materials also change significantly. Understanding the effect of tribochemical reactions on the tribological properties of materials is important for controlling the structure and composition of materials by chemical means and promoting the engineering application of materials. This study primarily introduces the tribochemical reactions of diamond-like carbon (DLC) films during the friction process in different environments and the relationship between tribochemistry and the tribological properties of DLC films. From this, the study proposes strategies to achieve the superlubricity of DLC films through tribochemistry. Finally, challenges and countermeasures in the engineering application of DLC films are discussed.

Robust Superlubricity and Moiré Lattice's Size Dependence on Friction between Graphdiyne Layers

Structural superlubricity is a fascinating physical phenomenon that plays a significant role in many scientific and technological fields. Here, we report the robust superlubricating state achieved on the interface of relatively rotated graphdiyne (GDY) bilayers; such an interface with ultralow friction is formed at nearly arbitrary rotation angles and sustained at temperatures up to 300 K. We also identified the reverse correlation between the friction coefficient and size of the Moiré lattice formed on the surface of the incommensurate stacked GDY bilayers, particularly in a small size range. Our investigations show that the ultralow friction and the reduction of the friction coefficient with the increase in size of the Moiré lattice are closely related to the interfacial energetics and charge density as well as the atomic arrangement. Our findings enable the development of a new solid lubricant with novel superlubricating properties, which facilitate precise modulation of the friction at the interface between two incommensurate contacting crystalline surfaces.

Interplay of mechanics and chemistry governs wear of diamond-like carbon coatings interacting with ZDDP-additivated lubricants

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.

Wear reduction in diamond-like carbon interacting with ZDDP-additivated oils is essential for current automotive applications. Here, the authors present an atomic-scale study revealing that this can be achieved by tailoring diamond-like carbon’s stiffness, surface nano-topography, and hydrogen content.

The pivotal role of oxygen in establishing superlow friction by inducing the in situ formation of a robust MoS2 transfer film

The achievement of superlow friction is vital for the engineering application of hydrogenated diamond-like carbon (H-DLC), but it always fails in an oxygen atmosphere. In this paper, robust superlow friction was achieved by MoS₂ flakes and H-DLC composite films in a large range of atmospheres, especially in oxygen. The results showed that the composite structure could only retain the superlow friction for an short time in pure argon, nitrogen and carbon dioxide; surprisingly, oxygen was capable of remaining in the near frictionless state with a friction coefficient as low as 0.002, and the duration was prolonged significantly by the introduction of oxygen in those other gases. The stability of the transfer film that induced the near frictionless state was also studied comprehensively. The experimental results and first-principle calculations demonstrated that oxygen could bond with the molybdenum, sulfur and aluminum atoms to form bridge bonds that fixed the MoS₂ transfer film on the counterface; this led to the formation of incommensurate contact between the MoS₂ tribo-layer and H-DLC film, which enabled robust superlow friction. This finding supports a simple strategy to resolve the challenge of superlubric failure and opens a path for the actual application of H-DLC in oxygen-rich environments.

Macroscale Superlubricity on Engineering Steel in the Presence of Black Phosphorus

Friction and wear are the main reasons for decreasing the lifetime of moving mechanical components and causing energy loss. It is desirable to achieve macroscale superlubricity on industrial materials for minimizing friction. Herein, the two-dimensional material black phosphorus (BP) is prepared as an oil-based nanoadditive in oleic acid (OA) and shown to produce macroscale superlubricity at the steel/steel contact under high pressure. Experiments and molecular dynamics simulation reveal that BP quickly captures the carboxylic group and, as a result of the high contact pressure and heat, OA decomposes to release passivating species and recombines to form amorphous carbon giving rise to a composite solid tribofilm with BP. The OA and passivating groups adsorb onto the solid tribofilm to produce the passivating layer, thus resulting in macroscale superlubricity. The findings provide fundamental insight into the nature of tribochemical mechanisms and suggest a new approach to achieve macroscale superlubricity of industrial materials.

Role of OH Termination in Mitigating Friction of Diamond-like Carbon under High Load: A Joint Simulation and Experimental Study

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

Macroscale Superlubricity Accomplished by Sb2O3-MSH/C Under High Temperature

Here, we report the high-temperature superlubricity phenomenon accomplished in coatings produced by burnishing powders of antimony trioxide (Sb₂O₃) and magnesium silicate hydroxide coated with carbon (MSH/C) onto the nickel superalloy substrate. The tribological analysis performed in an open-air experimental setup revealed that with the increase of testing temperature, the coefficient of friction (COF) of the coating gradually decreases, finally reaching the superlubricity regime (the COF of 0.008) at 300°C. The analysis of worn surfaces using in-situ Raman spectroscopy suggested the synergistic effect of the inner Sb₂O₃ adhesion layer and the top MSH/C layer, which do not only isolate the substrate from the direct exposure to sliding but also protect it from oxidation. The cross-sectional transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results indicated the tribochemically-activated formation of an amorphous carbon layer on the surface of the coating during sliding. Formation of the film enables the high-temperature macroscale superlubricity behavior of the material system.

Multilayer Tribofilm: An Unique Structure to Strengthen Interface Tribological Behaviors

Tribofilm configuration under the boundary lubrication region is an essential concern to gain insight into the tribological enhancement of the two-dimensional (2D) nano materials toward base oils. In this work, several alkyl carboxylic acids modified 2D triazine-based covalent organic frameworks (ATC) nano platelets were fabricated and served as lubrication additives. When carbon atoms add up to 16, the best lubricating performance is exhibited at an additive concentration of 0.05 wt % and the friction coefficient and wear volume are, respectively, reduced by 56.0% and 89.6% as compared to those of pure PAO 10 base oil. The analysis of the focused ion beam-transmission electron microscope (FIB-TEM) on the worn surface reveals that an alternately multilayer tribofilm consisting of 2D platelet additives and oxides and/or metallic soap salts is formed on the frictional interface of steel substrate, with a well-ordered arrangement along the sliding direction, which dominates the contributions of the excellent lubrication.

Specific Features of Reactive Pulsed Laser Deposition of Solid Lubricating Nanocomposite Mo-S-C-H Thin-Film Coatings

This work investigates the structure and chemical states of thin-film coatings obtained by pulsed laser codeposition of Mo and C in a reactive gas (H₂S). The coatings were analysed for their prospective use as solid lubricating coatings for friction units operating in extreme conditions. Pulsed laser ablation of molybdenum and graphite targets was accompanied by the effective interaction of the deposited Mo and C layers with the reactive gas and the chemical states of Mo- and C-containing nanophases were interdependent. This had a negative effect on the tribological properties of Mo–S–C–H nanocomposite coatings obtained at H₂S pressures of 9 and 18 Pa, which were optimal for obtaining MoS₂ and MoS₃ coatings, respectively. The best tribological properties were found for the Mo–S–C–H_5.5 coating formed at an H₂S pressure of 5.5 Pa. At this pressure, the x = S/Mo ratio in the MoS_x nanophase was slightly less than 2, and the a-C(S,H) nanophase contained ~8 at.% S and ~16 at.% H. The a-C(S,H) nanophase with this composition provided a low coefficient of friction (~0.03) at low ambient humidity and 22 °C. The nanophase composition in Mo–S–C–H_5.5 coating demonstrated fairly good antifriction properties and increased wear resistance even at −100 °C. For wet friction conditions, Mo–S–C–H nanocomposite coatings did not have significant advantages in reducing friction compared to the MoS₂ and MoS₃ coatings formed by reactive pulsed laser deposition.

Structural lubricity in soft and hard matter systems

Over the recent decades there has been tremendous progress in understanding and controlling friction between surfaces in relative motion. However the complex nature of the involved processes has forced most of this work to be of rather empirical nature. Two very distinctive physical systems, hard two-dimensional layered materials and soft microscopic systems, such as optically or topographically trapped colloids, have recently opened novel rationally designed lines of research in the field of tribology, leading to a number of new discoveries. Here, we provide an overview of these emerging directions of research, and discuss how the interplay between hard and soft matter promotes our understanding of frictional phenomena.

Structural lubricity is one of the most interesting concepts in modern tribology, which promises to achieve ultra-low friction over a wide range of length-scales. Here the authors highlight novel research lines in this area achievable by combining theoretical and experimental efforts on hard two-dimensional materials and soft colloidal and cold ion systems.

Tribochemistry as an Alternative Synthesis Pathway

While reactions driven by mechanical force or stress can be labeled mechanochemical, those specifically occurring at a sliding interface inherit the name tribochemical, which stems from the study of friction and wear: tribology. Increased perception of tribochemical reactions has been gained through technological advancement, and the development of new applications remains on-going. This surprising physico-kinetic process offers great potential in novel reaction pathways for synthesis techniques and nanoparticle interactions, and it could prove to be a powerful cross-disciplinary research area among chemists, engineers, and physicists. In this review article, a survey of the history and recent usage of tribochemical reaction pathways is presented, with a focus on forging new compounds and materials with this sustainable synthesis methodology. In addition, an overview of tribochemistry’s current utility as a synthesis pathway is given and compared to that of traditional mechanochemistry.

Robust Superhydrophobic Membrane for Solving Water-Accelerated Fatigue of ZDDP-Containing Lubricating Oils

Superwetting materials show distinct advantages in interfacial applications such as oil-water separation. However, it remains a challenge to solve water-accelerated fatigue of lubricating oils owing to the poor mechanical durability of superhydrophobic surfaces and the intractable emulsions stabilized by additives. In this work, a robust superhydrophobic membrane for solving water-accelerated fatigue of lubricating oils containing zinc dialkyldithiophosphate (ZDDP) as a typical antiwear additive is presented. An all-inorganic coating is constructed by SiO₂ nanoparticles and aluminum phosphate using a simple spray-coating method. After silanization, the prepared membrane can extremely repel water and effectively separate ZDDP-stabilized water-in-lubricating oil emulsions (the purities of the collected lubricating oils are over 99.995%), even after sand impingement for 100 cycles. Ball-on-disk tribological tests at severe contact pressures reveal that the reclaimed lubricating oils recover the protective ability, and the catalytic dehydrogenation of lubricating oil is dramatically suppressed to avoid producing a mass of unwanted carbon-based wear debris. This work advances the development of superwetting materials in the lubricating oil industry.

Water deteriorates lubricating oils: removal of water in lubricating oils using a robust superhydrophobic membrane

Water is recognized as a contaminant in lubricating oils. Recently, interfacial materials with special wettability have been broadly developed for oil-water separation. However, solving lubricating oil failure caused by water remains a challenge. Here, a robust superhydrophobic membrane is presented to effectively remove water in lubricating oils to recover their lubricating capability. Compared to pure lubricating oils without or with an additive, lubricating oils collected from their emulsions using the superhydrophobic membrane have an equivalent friction coefficient and wear volume, which are much lower than that of lubricating oils contaminated by water. Water in lubricating oils accelerates the oxidation of metallic substrates and wear corrosion. Moreover, the metallic ions dissolved in water-containing lubricating oils induce the catalytic dehydrogenation of lubricating oils, leading to the deposition of a good deal of carbon-based wear debris. Importantly, the prepared membrane shows steady performance in regard to extreme water repellency, high-efficiency purification of lubricating oils, and low wear volume even after harsh mechanical damage. Robust interfacial materials have potential advantages in practically solving lubricating oil failure caused by water.

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.

Tribochemistry: A Review of Reactive Molecular Dynamics Simulations

Tribochemistry, the study of chemical reactions in tribological interfaces, plays a critical role in determining friction and wear behavior. One method researchers have used to explore tribochemistry is “reactive” molecular dynamics simulation based on empirical models that capture the formation and breaking of chemical bonds. This review summarizes studies that have been performed using reactive molecular dynamics simulations of chemical reactions in sliding contacts. Topics include shear-driven reactions between and within solid surfaces, between solid surfaces and lubricating fluids, and within lubricating fluids. The review concludes with a perspective on the contributions of reactive molecular dynamics simulations to the current understanding of tribochemistry, as well as opportunities for this approach going forward.

Slippery and Wear-Resistant Surfaces Enabled by Interface Engineered Graphene

Friction and wear remain the primary cause of mechanical energy dissipation and system failure. Recent studies reveal graphene as a powerful solid lubricant to combat friction and wear. Most of these studies have focused on nanoscale tribology and have been limited to a few specific surfaces. Here, we uncover many unknown aspects of graphene's contact-sliding at micro- and macroscopic tribo-scales over a broader range of surfaces. We discover that graphene's performance reduces for surfaces with increasing roughness. To overcome this, we introduce a new type of graphene/silicon nitride (SiN_x, 3 nm) bilayer overcoats that exhibit superior performance compared to native graphene sheets (mono and bilayer), that is, display the lowest microscale friction and wear on a range of tribologically poor flat surfaces. More importantly, two-layer graphene/SiN_x bilayer lubricant (<4 nm in total thickness) shows the highest macroscale wear durability on tape-head (topologically variant surface) that exceeds most previous thicker (∼7-100 nm) overcoats. Detailed nanoscale characterization and atomistic simulations explain the origin of the reduced friction and wear arising from these nanoscale coatings. Overall, this study demonstrates that engineered graphene-based coatings can outperform conventional coatings in a number of technologies.

Fluorescence and Physico-Chemical Properties of Hydrogenated Detonation Nanodiamonds

Hydrogenated detonation nanodiamonds are of great interest for emerging applications in areas from biology and medicine to lubrication. Here, we compare the two main hydrogenation techniques—annealing in hydrogen and plasma-assisted hydrogenation—for the creation of detonation nanodiamonds with a hydrogen terminated surface from the same starting material. Synchrotron-based soft X-ray spectroscopy, infrared absorption spectroscopy, and electron energy loss spectroscopy were employed to quantify diamond and non-diamond carbon contents and determine the surface chemistries of all samples. Dynamic light scattering was used to study the particles’ colloidal properties in water. For the first time, steady-state and time-resolved fluorescence spectroscopy analysis at temperatures from room temperature down to 10 K was performed to investigate the particles’ fluorescence properties. Our results show that both hydrogenation techniques produce hydrogenated detonation nanodiamonds with overall similar physico-chemical and fluorescence properties.

Triboemission of hydrocarbon molecules from diamond-like carbon friction interface induces atomic-scale wear

Understanding atomic-scale wear is crucial to avoid device failure. Atomic-scale wear differs from macroscale wear because chemical reactions and interactions at the friction interface are dominant in atomic-scale tribological behaviors, instead of macroscale properties, such as material strength and hardness. It is particularly challenging to reveal interfacial reactions and atomic-scale wear mechanisms. Here, our operando friction experiments with hydrogenated diamond-like carbon (DLC) in vacuum demonstrate the triboemission of various hydrocarbon molecules from the DLC friction interface, indicating its atomic-scale chemical wear. Furthermore, our reactive molecular dynamics simulations reveal that this triboemission of hydrocarbon molecules induces the atomic-scale mechanical wear of DLC. As the hydrogen concentration in hydrogenated DLC increases, the chemical wear increases while mechanical wear decreases, indicating an opposite effect of hydrogen concentration on chemical and mechanical wear. Consequently, the total wear shows a concave hydrogen concentration dependence, with an optimal hydrogen concentration for wear reduction of around 20%.

Tribochemical Behaviors of Onion-like Carbon Films as High-Performance Solid Lubricants with Variable Interfacial Nanostructures

Onion-like carbon (OLC), spherical nanoparticles consisting of carbon shells, is capable of providing exceptional lubrication effects. Nevertheless, the underlying mechanism, especially the tribo-induced evolution of interfacial nanostructures and their correlation with the friction states, is not clear. In this work, OLC films with a thickness of ∼1 μm were synthesized by electrophoretic deposition on the mirror-polished stainless steel. The lubricity was evaluated by tailoring the sliding aspects including applied normal load, contact time, and counterface materials. It is found that the friction reduction level is highly dependent on the material transfer and transformation of the OLC surface and the physicochemical nature of the as-formed tribolayer in the contact areas. The subsurface of the OLC film always undergoes a deep amorphization transformation upon sliding. It is interesting to note that the tribolayer formed on the bare steel ball is mainly composed of highly ordered graphene-like nanoflakes derived from the sliding-induced degradation of OLC nanospheres. In comparison, the nanospherical carbon structure can be retained in the topmost subsurface of the tribolayer formed on the ceramic Si₃N₄ ball. Such a nanosphere-/amorphization-coupled interface is capable of providing a robust lubrication state under high contact stresses. The findings identify a new lubrication mechanism for the spherical carbon nanostructure, rendering them effective solid lubricants.

Neutron visualization of inhomogeneous buried interfaces in thin films

When designing some functions in thin film systems, one of the key concepts is the structure of the constituent layers and interfaces. In an actual system, the layers and interfaces are often inhomogeneous in different scales, from hundreds of microns to several nanometers, causing differences in properties, despite very similar average structures. In this case, the choice of the observation point is critical to clarify the problem. Another critical aspect is the identification of these points by surveying the entire inhomogeneous thin film system. This article presents a description of a novel promising solution that is suitable for nondestructive visualization of inhomogeneous buried layers and interfaces in thin films. Such observations have been impossible until now. In this investigation, a unique extension of neutron reflectometry is proposed. While conventional neutron reflectivity just gives average depth-profiling of the scattering length density of layered thin films, the present method provides full picture of the inhomogeneity. In general, achieving a high spatial-resolving power for neutron scattering is not straightforward because the neutron counts become fairly limited at the sample or the detector position when the beam size is reduced. As a result, XY scanning of a sample with a small neutron beam is fairly difficult because of the required long measurement time. To address these issues, new concepts have been introduced for neutron reflectivity. The proposed method uses a wide beam instead of reducing the beam size. In addition, it measures the projection reflection profile instead of the total integrated intensity. These profiles are collected at a set of different in-plane angles. Similar to computed tomography, it is possible to obtain the specimen's two-dimensional (2D) neutron reflectivity distribution as one image. Because the spatial resolution is limited by the detection method, a Hadamard coded mask is employed to measure the reflection projection with only 50% loss of the primary neutron intensity. When the time-of-flight (ToF) mode is used for the neutron experiment, one can obtain many images as a function of ToF, i.e., the wavevector transfer. Such series of images can be displayed as a video. This indicates that the neutron reflectivity profiles of local points can be retrieved from the above video images. This paper presents the first report on the development of neutron reflectivity with imaging capability, and the analysis of local points in inhomogeneous layered thin-films without utilizing a small neutron beam. In the present work, the feasibility of the proposed method with approximately 1 mm spatial resolution was examined. In addition, further improvements of the approach are discussed. It is anticipated that this technique will facilitate new opportunities in the study of buried function interfaces.

In situ formation of spherical MoS2 nanoparticles for ultra-low friction

The motion resistance and energy dissipation of rolling friction are much lower than those of sliding friction at the macroscale. But at the microscale, the impact of rolling on friction remains an open question. Here, we show that spherical MoS2 nanoparticles can be formed in situ at a friction interface by scrolling and wrapping MoS2 nanosheets under the induction of a reciprocating shear stress, when an MoS2 coating constructed from loosely stacked nanosheets is tested in a vacuum of 3.5 × 10-3 Pa. An ultra-low friction state can be readily realized with friction coefficients of 0.004-0.006, which are one order of magnitude lower than that of a pulse laser deposited MoS2 coating without nanoparticles formed in a friction process. Accordingly, the spherical nanoparticles are highlighted as the key factor in the ultra-low friction. Classical molecular dynamics simulations further reveal that the motion mode of the MoS2 nanoparticle is stress-dependent. This finding confirms access to ultra-low friction by introducing rolling friction based on the microstructural evolution of the coating.

Renewable Cr2O3 Nanolayer on Cr(W)N Surface for Seizure Prevention at Elevated Temperatures

Chromium nitride coating is now the norm for improving the wear resistance of high-performance mechanical components. Even so, to prevent the seizure issue between the contacting interfaces, the prerequisites are oil or solid lubricants which would however lose the lubricating functionality at elevated temperatures due to breakdown or degradation. In this research, we utilize a Cr₂O₃ nanolayer formed on modified Cr(W)N coating to prevent the adhesive seizure for steel-based components. X-ray photoelectron spectroscopy (XPS) analyses show that the chromium oxide can be generated at 200-400 °C. At 400 °C, the Cr₂O₃ nanolayer is in situ formed and maintains a consistent thickness of 2.2 nm due to the oxide renewal during the heating-sliding operation. The in situ, renewable oxide nanolayer provides a novel approach to the technically unsolved seizure problem occurring in high-performance machines operated at elevated temperatures.