Structural superlubricity in graphite flakes assembled under ambient conditions

The Evolution of Nanoscale Third Body Layer Revealed by Graphite Structural Superlubric Contact

Revealing the evolution of nanoscale third bodies confined between sliding surfaces is essential to understanding the friction and electrical properties for solid contacts. Here, with graphite/graphite contacts in structural superlubricity, a state of no wear and ultralow friction, we in situ reveal the morphological evolution of a third body layer introduced by air through measuring friction and conductance during cyclic hold-slide tests. The directional transport of confined molecules causes apparent elastic deformation of the third body layer, leading to local graphite/graphite direct contact. Together with a proposed quantum tunneling effective thickness (deff) model, a constant volume of third bodies with a sub-nm thickness is observed. Our work provides a feasible approach to investigate the kinetics of substances under nanoscale confinement.

Superlubric Sliding of Graphene Auto-Kirigami with Interfaces Containing Self-Assembled Stripe-Pattern Adsorbates

Van der Waals heterostructures formed by stacked 2D materials show exceptional electronic, mechanical, and optical properties. Superlubricity, a condition where atomically flat, incommensurate planes of atoms result in ultra-low friction, is a prime example enabling, for example, self-assembly of optically visible graphene nanostructures in air via a sliding auto-kirigami process. Here, it is demonstrated that a subtle but ubiquitous adsorbate stripe structure found on graphene and graphitic surfaces in ambient conditions remains stable within the interface between twisted graphene layers as they slide over each other. Despite this contamination, the interface retains an exceptional superlubricious state with an estimated upper bound frictional shear strength of 10 kPa, indicating that direct atomic incommensurate contact is not required to achieve ambient superlubricity for 2D materials. The results suggest that any phenomena depending on 2D heterostructure interfaces such as exotic electronic behavior may need to consider the presence of stripe adsorbate structures that remain intercalated.

Robust structural superlubricity under gigapascal pressures

Structural superlubricity (SSL) is a state of contact with no wear and ultralow friction. SSL has been characterized at contact with van der Waals (vdW) layered materials, while its stability under extreme loading conditions has not been assessed. By designing both self-mated and non-self-mated vdW contacts with materials chosen for their high strengths, we report outstanding robustness of SSL under very high pressures in experiments. The incommensurate self-mated vdW contact between graphite interfaces can maintain the state of SSL under a pressure no lower than 9.45 GPa, and the non-self-mated vdW contact between a tungsten tip and graphite substrate remains stable up to 3.74 GPa. Beyond this critical pressure, wear is activated, signaling the breakdown of vdW contacts and SSL. This unexpectedly strong pressure-resistance and wear-free feature of SSL breaks down the picture of progressive wear. Atomistic simulations show that lattice destruction at the vdW contact by pressure-assisted bonding triggers wear through shear-induced tearing of the single-atomic layers. The correlation between the breakdown pressure and material properties shows that the bulk modulus and the first ionization energy are the most relevant factors, indicating the combined structural and electronic effects. Impressively, the breakdown pressures defined by the SSL interface could even exceed the strength of materials in contact, demonstrating the robustness of SSL. These findings offer a fundamental understanding of wear at the vdW contacts and guide the design of SSL-enabled applications.

Edge-enhanced super microgenerator based on a two-dimensional Schottky junction

Super microgenerator (SMG) refers to a generator that can efficiently convert extremely weak external stimuli into electrical energy and has a small size, high power density and long lifespan, offer ground-breaking solutions for powering wearable devices, wireless distributed sensors and implanted medical equipment. However, the friction and wear between the interfaces of ordinary microgenerator results in an extremely low lifespan. Here, we present a prototype of SMGs based on a 2D-2D (graphite-MoS2) Schottky contact in the state of structural superlubricity (no wear and nearly zero friction between two contacted solid surfaces). What is even more interesting is when the graphite flake is slid from the bulk to the edge of MoS2, the output current will enhance from 31 to 56 A m-2. Through the I-V curve measurement, we found that the conductive channel across the junction can be activated and further enhanced at the edge of MoS2compare to bulk, which provide the explanation for the above-mentioned edge enhancement of power generation. Above results provide the design principles of high-performance SMGs based on 2D-2D Schottky junctions.

Filtering Robust Graphite without Incommensurate Interfaces by Electrical Technique

Two-dimensional (2D) van der Waals (vdW) layered materials have attracted considerable attention due to their potential applications in various fields. Among these materials, graphite is widely employed to achieve structural superlubricity (SSL), where the interfacial friction between two solids is almost negligible and the wear is zero. However, the development of integrated SSL systems using graphite flakes still faces a major obstacle stemming from the inherent delamination-induced instability in vdW layered materials. To address this issue, we propose a nondestructive filtering technique that utilizes electrical measurement to identify robust graphite flakes without delamination. Our experimental results confirm that all the filtered graphite flakes exhibit delamination-free behavior after more than 7000 cycles of sliding on a series of 2D and 3D substrates. Besides, we employ three types of characterizing methods to confirm that the filtering process does not impair the graphite flakes. Moreover, with focused ion beam (FIB) assisted slicing characterization and statistical analysis, we have discovered that all of the filtered flakes possess a graphite layer thickness below 100 nm. This is consistent with the thickness of the single crystalline graphite layer of our samples reported in the literature, suggesting the absence of incommensurate interfaces in the filtered graphite flakes. Our work contributes to a deeper understanding of the relationship between graphite conductance and incommensurate interfaces. In addition, we present a possible solution to address the delamination problem in layered materials, and this technique shows the potential to characterize the internal microstructure of grains and the distribution of grain boundaries in vdW materials on a large scale.

van der Waals Epitaxy, Superlubricity, and Polarization of the 2D Ferroelectric SnS

Tin monosulfide (SnS) is a two-dimensional layered semiconductor that exhibits in-plane ferroelectric order at very small thicknesses and is of interest in highly scaled devices. Here we report the epitaxial growth of SnS on hexagonal boron nitride (hBN) using a pulsed metal-organic chemical vapor deposition process. Lattice matching is observed between the SnS(100) and hBN{11̅0} planes, with no evidence of strain. Atomic force microscopy reveals superlubricity along the commensurate direction of the SnS/hBN interface, and first-principles calculations suggest that friction is controlled by the edges of the SnS islands, rather than interface interactions. Differential phase contrast imaging detects remnant polarization in SnS islands with domains that are not dictated by step-edges in the SnS. The growth of ferroelectric SnS on high quality hBN substrates is a promising step toward electrically switchable ferroelectric semiconducting devices.

In Situ Twistronics: A New Platform Based on Superlubricity

Twistronics, an emerging field focused on exploring the unique electrical properties induced by twist interface in graphene multilayers, has garnered significant attention in recent years. The general manipulation of twist angle depends on the assembly of van der Waals (vdW) layered materials, which has led to the discovery of unconventional superconductivity, ferroelectricity, and nonlinear optics, thereby expanding the realm of twistronics. Recently, in situ tuning of interlayer conductivity in vdW layered materials has been achieved based on scanning probe microscope. In this Perspective, the advancements in in situ twistronics are focused on by reviewing the state-of-the-art in situ manipulating technology, discussing the underlying mechanism based on the concept of structural superlubricity, and exploiting the real-time twistronic tests under scanning electron microscope (SEM). It is shown that the real-time manipulation under SEM allows for visualizing and monitoring the interface status during in situ twistronic testing. By harnessing the unique tribological properties of vdW layered materials, this novel platform not only enhances the fabrication of twistronic devices but also facilitates the fundamental understanding of interface phenomena in vdW layered materials. Moreover, this platform holds great promise for the application of twistronic-mechanical systems, providing avenues for the integration of twistronics into various mechanical frameworks.

Fully automatic transfer and measurement system for structural superlubric materials

Structural superlubricity, a state of nearly zero friction and no wear between two contact surfaces under relative sliding, holds immense potential for research and application prospects in micro-electro-mechanical systems devices, mechanical engineering, and energy resources. A critical step towards the practical application of structural superlubricity is the mass transfer and high throughput performance evaluation. Limited by the yield rate of material preparation, existing automated systems, such as roll printing or massive stamping, are inadequate for this task. In this paper, a machine learning-assisted system is proposed to realize fully automated selective transfer and tribological performance measurement for structural superlubricity materials. Specifically, the system has a judgment accuracy of over 98% for the selection of micro-scale graphite flakes with structural superlubricity properties and complete the 100 graphite flakes assembly array to form various pre-designed patterns within 100 mins, which is 15 times faster than manual operation. Besides, the system is capable of automatically measuring the tribological performance of over 100 selected flakes on Si3N4, delivering statistical results for new interface which is beyond the reach of traditional methods. With its high accuracy, efficiency, and robustness, this machine learning-assisted system promotes the fundamental research and practical application of structural superlubricity.

Design of Superlubricity System Using Si3N4/Polyimide as the Friction Pair and Nematic Liquid Crystals as the Lubricant

Polyimide (PI) is a high-performance engineering plastic used as a bearing material. A superlubricity system using Si3N4/PI as the friction pair and nematic liquid crystals (LCs) as the lubricant was designed. The superlubricity performance was studied by simulating the start-stop condition of the machine, and it was found that the superlubricity system had good reproducibility and stability. In the superlubricity system, friction aligned with the PI molecules, and this alignment was less relevant compared to which substance was rubbing on the PI. Oriented PI molecules induced LC molecule alignment when the pretilt angle was very small, and the LC molecules were almost parallel to the PI molecules due to the one-dimensional ordered arrangement of LC molecules and low viscosity, which is conducive to the occurrence of the superlubricity phenomenon.

Partially Oxidized Violet Phosphorus as an Excellent Lubricant Additive for Tribological Applications

As a novel two-dimensional material, violet phosphorus (VP) has attracted a considerable amount of attention due to its high carrier mobility, anisotropy, wide band gap, stability, and easy stripping properties. In this work, the microtribological properties of partially oxidized VP (oVP) and the mechanism of reducing friction and wear as additives in oleic acid (OA) oil were studied systematically. When adding oVP to OA, the coefficient of friction (COF) decreased from 0.084 to 0.014 with the steel-to-steel pair, and the ultralow shearing strength tribofilm consisting of amorphous carbon and phosphorus oxides that formed resulted in the reductions of COF and wear rate individually by 83.3% and 53.9%, respectively, compared with those of pure OA. The results extended the application scenarios for VP in the design of lubricant additives.

Effect of Amorphous-Crystalline Phase Transition on Superlubric Sliding

Structural superlubricity describes the state of greatly reduced friction between incommensurate atomically flat surfaces. Theory predicts that, in the superlubric state, the remaining friction sensitively depends on the exact structural configuration. In particular the friction of amorphous and crystalline structures for, otherwise, identical interfaces should be markedly different. Here, we measure friction of antimony nanoparticles on graphite as a function of temperature between 300 and 750 K. We observe a characteristic change of friction when passing the amorphous-crystalline phase transition above 420 K, which shows irreversibility upon cooling. The friction data is modeled with a combination of an area scaling law and a Prandtl-Tomlinson type temperature activation. We find that the characteristic scaling factor γ, which is a fingerprint of the structural state of the interface, is reduced by 20% when passing the phase transition. This validates the concept that structural superlubricity is determined by the effectiveness of atomic force canceling processes.

Unravelling High-Load Superlubricity of Ionic Liquid Analogues by In Situ Raman: Incomplete Hydration Induced by Competitive Exchange of External Water with Crystalline Water

A high load-carrying capacity is the key to the practicality of liquid superlubricity, but it is difficult to achieve high load and low friction simultaneously by relying solely on a liquid film. Herein, a choline chloride-based ionic liquid analogue (ILA) macroscale superlubricant is first reported by tuning down strong hydrogen bonding in the ILA via introducing 2-10 wt % water, with a high load of 160 MPa and a low coefficient of friction of 0.006-0.008. In situ Raman reveals that competitive exchange between external water and crystalline water induces weak H-bond-dominated incomplete hydration, conferring a low-shear interface and considerable load-carrying capacity inside the lubricant. It is a hydrodynamic lubrication film rather than a tribochemical/physical adsorption film, allowing it to be applied to friction pairs of various materials. This study unveils the principle of water mediation of high-viscosity ILAs and also provides new insights into the design of practical ILA-based superlubrication materials with high load-carrying capacity.

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.

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.

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.

Microscale Schottky superlubric generator with high direct-current density and ultralong life

Miniaturized or microscale generators that can effectively convert weak and random mechanical energy into electricity have significant potential to provide solutions for the power supply problem of distributed devices. However, owing to the common occurrence of friction and wear, all such generators developed so far have failed to simultaneously achieve sufficiently high current density and sufficiently long lifetime, which are crucial for real-world applications. To address this issue, we invent a microscale Schottky superlubric generator (S-SLG), such that the sliding contact between microsized graphite flakes and n-type silicon is in a structural superlubric state (an ultra-low friction and wearless state). The S-SLG not only generates high current (~210 Am-2) and power (~7 Wm-2) densities, but also achieves a long lifetime of at least 5,000 cycles, while maintaining stable high electrical current density (~119 Am-2). No current decay and wear are observed during the experiment, indicating that the actual persistence of the S-SLG is enduring or virtually unlimited. By excluding the mechanism of friction-induced excitation in the S-SLG, we further demonstrate an electronic drift process during relative sliding using a quasi-static semiconductor finite element simulation. Our work may guide and accelerate the future use of S-SLGs in real-world applications.

Infinite Approaching Superlubricity by Three-Dimensional Printed Structures

The rapid development of three-dimensional (3D) printing technology opens great opportunities for the design of various multiscale lubrication structures. 3D printing allows high customization of arbitrary complex structures and rapid prototyping of objects, which provides an avenue to achieve effective lubrication. Current experimental observations on superlubricity are limited to atomically smooth clean surfaces, extreme operating conditions, and nano- or microscales. With the in-depth exploration of 3D printed lubrication, construction of multifunctional 3D structures with refined dimensions spanning from micronanoscale to macroscale is increasingly regarded as an important means to approach superlubricity and has aroused great scientific interest. To document recent advances in 3D printing for structural lubrication, a detailed literature review is provided. Emphasis is given on the design and lubrication performance of geometric and bioinspired lubrication structures with characteristic dimensions. The material requirements, merits, drawbacks, and representative applications of various 3D printing techniques are summarized. Potential future research trends aiming at the design strategy and manufacturing process of 3D printed lubrication structures are also highlighted.

Sliding Friction of Amorphous Asperities on Crystalline Substrates: Scaling with Contact Radius and Substrate Thickness

Disorder in the contact between an amorphous slider and a crystalline substrate leads to a cancellation of lateral forces. Atomically flat, rigid surfaces exhibit structural superlubricity, with the frictional stress in circular contacts of radius a vanishing as 1/a. The inclusion of elasticity allows relative motion of domains on the surface in response to the random interfacial forces. The competition between disorder and elastic deformation is predicted to limit structural superlubricity and produce a constant frictional stress for a larger than a characteristic domain size λ that depends on the ratio of the shear modulus G to the magnitude of interfacial shear stresses τ₀. Extensive simulations of a flat, amorphous punch sliding on a crystalline substrate with different system sizes and G/τ₀ are used to test scaling predictions and determine unknown prefactors that are needed for quantitative analysis. For bulk systems, we find an exponential decrease of the large a frictional stress and 1/λ with increasing G/τ₀. For thin free-standing films, the stress and 1/λ are inversely proportional to G/τ₀. These results may help explain the size-dependent friction of nanoparticles and plate-like materials used as solid lubricants.

Superlubricity between Graphite Layers in Ultrahigh Vacuum

Graphite has been conventionally believed to exhibit an inferior lubricating performance with significantly larger friction coefficient and wear rate in a vacuum environment than in ambient air. Dangling bonds at the edge planes of graphite, accounting for the high friction in inert atmosphere are saturated by chemisorbed vapor molecules in air, which contributes to low surface adhesion and low friction. However, there is still a lack of direct experimental evidence whether basal planes of graphite excluding the negative effects of edges or dangling bonds shows intrinsic lubricity when sliding under ultrahigh vacuum (UHV) conditions. By the interlayer friction measurement enabled by graphite flake-wrapped atomic force microscope tips in UHV, we show a record-low friction coefficient of 4 × 10^-5 (slope of friction vs normal force curve) when sliding between graphite layers, which is much lower than that in ambient air. This discrepancy manifests the intrinsic sliding frictional behavior between the graphite basal planes when the tribo-materials and experimental conditions are well-designed and strictly controlled. In addition, the temperature dependence of the kinetic friction between the graphite layers has been investigated under UHV conditions over the temperature range of 125-448 K, which is consistent with the thermally activated process.

Structural Superlubricity Based on Crystalline Materials

Herein, structural superlubricity, a fascinating phenomenon where the friction is ultralow due to the lateral interaction cancellation resulted from incommensurate contact crystalline surfaces, is reviewed. Various kinds of nano- and microscale materials such as 2D materials, metals, and compounds are used for the fabrication. For homogeneous frictional pairs, superlow friction forces exist in most relative orientations with incommensurate configuration. Heterojunctions bear no resemblance to homogeneous contact, since the lattice constants are naturally mismatched which leads to a robust structural superlubricity with any orientation of the two different surfaces. A discussion on the perspectives of this field is also provided to meet the existing challenges and chart the future.

Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite

Structural lubricity is an intriguing tribological concept, where extremely low friction is anticipated, if two surfaces in relative motion do not share the same lattice structure and consequently instabilities originating from interlocking surface potentials are strongly reduced. Currently, the challenges related to the phenomenon of structural lubricity are considered to be twofold. On one hand, experimental systems suitable for showing structural lubricity must be identified, while at the same time, it is also crucial to understand the intricate details of interface interaction. Here, we introduce a new material combination, namely NaCl-particles on highly oriented pyrolithic graphite (HOPG), where the nanoparticles coalesce under the influence of ambient humidity. Our experiments reveal that the interfacial friction can be described by the concept of structural lubricity despite the seemingly unavoidable contamination of the interface. By systematically analyzing the friction versus area scaling, this unlikely candidate for structural lubricity then shows two separate friction branches, with distinct differences of the friction versus area scaling. The exact tribological behavior of the nanoparticles can ultimately be understood by a model that considers the influence of nanoparticle preparation on the interface conditions. By taking into account an inevitable water layer at the interface between particle and substrate that can exist in different crystalline configurations all friction phenomena observed in the experiments can be understood.

A Hybrid Two-Axis Force Sensor for the Mesoscopic Structural Superlubricity Studies

Structural superlubricity (SSL) is a state of nearly zero friction and zero wear between two directly contacted solid surfaces. Recently, SSL was achieved in mesoscale and thus opened the SSL technology which promises great applications in Micro-electromechanical Systems (MEMS), sensors, storage technologies, etc. However, load issues in current mesoscale SSL studies are still not clear. The great challenge is to simultaneously measure both the ultralow shear forces and the much larger normal forces, although the widely used frictional force microscopes (FFM) and micro tribometers can satisfy the shear forces and normal forces requirements, respectively. Here we propose a hybrid two-axis force sensor that can well fill the blank between the capabilities of FFM and micro tribometers for the mesoscopic SSL studies. The proposed sensor can afford 1mN normal load with 10 nN lateral resolution. Moreover, the probe of the sensor is designed at the edge of the structure for the convenience of real-time optical observation. Calibrations and preliminary experiments are conducted to validate the performance of the design.