Superlubricity of carbon nanostructures

Journal Pre-proof Superlubricity of carbon nanostructures Xinchun Chen, Jinjin Li PII:

S0008-6223(19)31210-2

DOI:

https://doi.org/10.1016/j.carbon.2019.11.077

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CARBON 14834

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Carbon

Received Date: 15 August 2019 Revised Date:

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Accepted Date: 24 November 2019

Please cite this article as: X. Chen, J. Li, Superlubricity of carbon nanostructures, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.11.077. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Superlubricity of carbon nanostructures

Xinchun Chen, Jinjin Li*

State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China

Corresponding author： *To whom all correspondence should be addressed. Telephone: 8610-62771438 E-mail: [email protected]

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Abstract Superlubricity, a fantastic lubrication sate where friction or resistance to sliding nearly vanishes, has become one of the most important approaches to combat friction-induced energy consumption and devices failures. Emerging carbon nanostructures endow the research community with unprecedented opportunities to realize superlubricity across different length scales. This review provides an overview of the state-of-the-art of nanostructured carbon-based superlubricity, with a specific emphasis on unusual properties and new phenomena in representative carbon materials including layer carbon structure, diamond-like carbon, onion-like/fullerene-like carbons, ultra-nanocrystalline diamond and carbon nanotubes. The scientific fundamentals and technical routes to achieve superlow friction are highlighted for each individual carbon nanostructures. Perspectives on current challenges are put forward, and the possible future directions to guide the development of this field are suggested.

Keywords Superlubricity; carbon nanostructure; friction; sliding interface; thin film

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Table of Contents 1. Introduction ............................................................................................................................ 4 2. Superlubricity of layer structure carbon .............................................................................. 10 3. Superlubricity of diamond-like carbon ................................................................................ 25 4. Superlubricity of onion-like/fullerene-like carbons ............................................................. 43 5. Superlubricity of ultra-nanocrystalline diamond ................................................................. 50 6. Superlubricity of carbon nanotubes ..................................................................................... 52 7. Superlubricity of carbon nanostructures associated with liquid .......................................... 55 8. Discussions and conclusions ................................................................................................ 61

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1. Introduction Carbon is a unique element regarding its diversity in forming short, medium and long ranges bonding configurations, and can crystallize into diamond and graphite. The constant emerging of new forms of carbon structures such as diamond-like amorphous carbon, fullerenes, nanotubes, and graphene has fueled an overwhelming number of studies in the ever-growing fields of nanostructured carbon materials.[1] As one representative application, various carbon-based lubricants have been developed for the use in the field of tribology. Friction is one of the most common but quite complex phenomena causing energy consumption and mechanical devices failure in modern society.[2] From the very early ages to the nowadays, significant strides have been accomplished in not only understanding the origins of friction but also the manipulating technologies from atomic- to macroscales. As a general technique, a small amount of lubricants used on the rubbing surfaces could significantly reduce friction and material losses. This straightforward method opens up numerous possibilities to combat friction and tune the surface and interface behaviors of mechanical objects in relative motions. Obviously, the effective control of friction and wear has profound influences in the energy reservation, lifetime increase of moving mechanical systems, economic efficiency and environmental sustainability.

Conceptually, friction coefficient is a dimensionless value which describes the relationship of friction and normal load between two surfaces in relative motion. It can be sorted into two types, namely static and dynamic friction coefficients, respectively. Superlow friction is generally defined as the dynamic friction coefficient being lower than 0.01 or even towards 4

the vanishing level (0.001 or less, approaching the detection limit for the available tribometers at present).[3] As revealed by the famous Stribeck curve, the friction regimes of lubricated contact surfaces can be broadly divided into (i) boundary lubrication, (ii) mixed lubrication and (iii) elastohydrodynamic lubrication, which is usually determined by the lambda parameter λ.[4] Based on the ability of lubricants to separate the contacting surfaces, a superlow friction coefficient with a thousandth level is more feasible in a hydrodynamic fluid lubrication regime, while it is highly challenging under dry conditions. The present review, however, mainly focuses on the findings enabled by carbon nanostructures to overcome this hardship. It should be pointed out that measuring a superlow friction coefficient is still a great challenge for most commercial or custom-made macroscale tribometers, because the friction force is at least three orders of magnitude smaller than the applied normal load in a superlow friction state, which results in the measurement difficulty or uncertainty in the accuracy of the measured values. For a common pin-on-disc or ball-on-disc tribometer, some uncertainties like load cell calibration, thermal shift and geometrical misalignments can induce measurement inaccuracy during sliding contact.[5] For example, the geometrical misalignment is quite universal for common tribometers, in which a misalignment angle is generally produced owing to the unparallel between the directions of the recorded friction force and real friction force in the horizontal plane. As reported, to achieve a relative error better than 5% when measuring a superlow friction coefficient (0.001-0.01), the misalignment angle must be in the range of 0.003-0.03°.[6] Similarly, for a nanoscale single-asperity contact conducted in atomic force microscopy (AFM) or lateral force microscopy (LFM), the accurate measurement of a superlow friction coefficient is 5

highly dependent on the normal and lateral forces calibration, namely the determination of the normal and torsional spring constants of the AFM cantilevers.[7]

In 1990, a concept of superlubricity predicting the vanishing friction between two sliding surfaces was proposed by Hirano,[8] and soon verified implicitly by the experiments of misfit angle-induced anisotropic friction experiments on cleaved mica surfaces.[9] However, due to the limitations of experimental set-up and the detection resolution of the force microscopy, the as-measured friction forces in Hirano’s work were not small enough to be regarded as reaching the superlubricity level. Even so, the occurrence of this peculiar lubrication state triggers the intensive research in the fundamentals of superlow friction and the relevant lubricious materials with this kind of capacity. As indicated in the pioneering work, the term superlubricity originally refers to the structural lubricity or structural superlubricity,[10] which describes the friction vanishing state between two individual crystalline surfaces in a dry, rigid and clean incommensurate contact.[11] The incommensurate contact phenomenon is generally established through cancellation of one-to-one registry between the surface atoms along the sliding interface. This was first experimentally observed by Martin in 1993 through a macroscale sliding contact of MoS2 coatings in ultrahigh vacuum.[12] The origin of the extremely low friction coefficient (in the range of 10-3 or even less) was due to the frictional anisotropy in the sliding interface, namely the intercrystallite slippage of incommensurate sulfur-rich hexagonal lattices.[13]Until 2004, a more clear evidence of structural superlubricity was achieved by Dienwiebel using a home-made lateral force microscopy to record the friction forces between the graphite 6

nanoflakes-transferred tip and graphite surface as a function of misfit angle.[14] The calculated friction coefficient could be as small as 0.0008. However, before Dienwiebel’s work, superlubricity with the friction coefficient decreasing to the level of 0.001 was first discovered in 1994 on amorphous material surfaces.[15, 16] As a pioneering work, Donnet et al, achieved a superlow friction coefficient of 0.006-0.008 by sliding diamond-like carbon against steel pin in a vacuum below 10-1 Pa, and the critical role of hydrogen in obtaining superlow friction was initially recognized. Later in 2000, the superlubricity of DLC was further verified in a dry inert gaseous atmosphere by tribologists from Argonne National Laboratory.[17] In this finding, Erdemir et al. constructed a self-mated sliding interface of highly-hydrogenated amorphous carbon films and a robust near-frictionless and wearless lubricious state was readily established after a short running-in period. In view of these contributions, Martin proposed that it was better to define superlubricity when the kinetic friction coefficient was in the order of 10-3 or smaller, rather than the material structural characteristics of the sliding interface.[18] Following Erdemir’s work, a noticeable number of anti-friction materials, especially carbon nanostructure-based lubricants, have been confirmed to possess superlubricity properties. As a whole, the unique electronic structures of carbon and the as-formed allotropes provide the unprecedented opportunities for researchers to understand superlubricity from a wide range of scales and environments.

Up to now, the intensive research in 1, 2 and 3D carbon materials has verified the existence of superlubricity in many of them even though the pathways to suppress the friction forces are significantly different from each other.[19-22] For instance, the superlubricity 7

behaviors of 1D carbon nanotubes, 2D graphene and 3D diamond represent three typical carbonaceous sliding systems combating friction and wear. Among these cases, superlow friction was realized either by structural incommensurate contact or by forming distinct physiochemical surface features which can generate a highly passive and chemically inert interface to cancel out the adhesive forces. The possible contact length also covers a wide range from nanoscale to macroscale.[23] Therefore, it is quite necessary to retrospect the state-of the-art of superlubricity in carbon-based materials with the benefits from not only better comprehending but also controlling friction from scientific and technological perspectives.

Figure 1

Carbon nanostructures including graphene, nanodiamond, amorphous carbon,

onion-like carbon and carbon nanotube for superlubricity. Reproduced and modified with permissions from Refs. [24-28].

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As summarized in Figure 1, this review article presents the advances regarding superlubricity in the main types of carbon nanostructured materials found so far [24-28]. Each type of carbon structure represents one specific anti-friction mechanism. The organization frameworks are as follows. The governing laws and fundamentals of friction associated with layered carbon structures such as graphite and graphene are initially highlighted in Section 2 as the interlayer weak interactions are usually the bases for understanding the relevant friction phenomena in most sp2-bonded carbon materials. In the following Section 3, the recent discoveries and developments that enabled superlubricity in diamond-like carbon families of films are discussed with emphasis on the role of film microstructures, counterface materials, contact conditions, environments and tribochemistry on the formation of lubricating tribofilms. Special attention is devoted to the tribo-induced nanostructures along the superlubricious sliding interface that is now being able to be detected with the aid of advanced surface and subsurface analytical methods. Onion-like carbon and fullerene-like carbon with curved graphene structure enabling the occurrence of superlubricity in a unique way are described in Section 4. The advances in lubricity of ultra-nanocrystalline diamond are depicted in Section 5, which demonstrates the capabilities of sp3-bonded carbon materials as solid lubricants to dramatically reduce friction. As another allotrope shown in Section 6, carbon nanotubes provide one exceptional pathway to achieve superlow friction through the relative motion between the inner and outer carbon shells. Section 7 introduces a brief overview of the synergetic collaboration of carbon materials with liquid lubricants and the potential applications of superlubricity in specific conditions. Finally, Section 8 presents a concluding perspective regarding the fundamental restrictions on 9

the realization of robust superlubricity in actual operating environments, the possible approaches to combat these dilemmas and the potential applications in the future.

2. Superlubricity of layer structure carbon Graphite, consisting of multiple layers of graphene sheets with a spacing of 0.3 nm, has extremely high strength, stiffness, and thermal conductivity along the basal plane.[29] Its layered structure, bonded by very weak van der Waals (vdW) forces, allows the easy sliding of interlayer planes,[30] which has been widely used as friction pairs or lubrication additives.[31-36] In 1987, Mate et al. measured the friction force using the friction force microscope (FFM) when a tungsten tip slid on a freshly cleaved graphite surface at the atomic scale, finding that the atomic structure of graphite manifested itself directly in the dynamical frictional properties.[37] They observed a clear “stick-slip” movement, leading to the friction force varying periodically with the crystal lattice of graphite, and the superlow friction coefficient of 0.005 was obtained between graphite and tungsten tip. After that, lots of studies on superlubricity of graphite have been conducted theoretically and experimentally at the nanoscale and microscale. Dienwiebel et al. fabricated a silicon tribolever, which allowed the measurement of the forces on the scanning tip in three independent directions. The similar “stick-slip” movement was observed in some sliding orientations, and the lattice periodicity of the graphite surface can be recognized vaguely in the force variations (Figure 2a).[14] They found that the friction force varied periodically as a function of the rotational angle between graphite and tip. There were two narrow peaks with very high friction at the rotational angles of 0° and 61°, separated by a wide angular interval with near-zero friction 10

(Figure 2b). The rotational angle difference between the two high friction peaks was 61°, which was consistent well with the 60° symmetry of individual atomic layer in the graphite lattice. This indicates that the superlow friction of graphite appears in the incommensurate contact between the rotated graphite layers. Further experimental evidence shows that there were graphite nanosheets transferred onto the AFM tip,[38] and thus it formed the incommensurate contact between two rotated graphite layers, which is the origin of superlubricity.[39] This result gives the direct evidence of structural superlubricity achieved by two sliding graphite layers at the atomic scale. Hod et al. also demonstrated the presence of a direct relation between the interlayer commensurability and superlubricity between two graphite layers by using the registry-index concept that can quantify the registry mismatch and reproduce their interlayer sliding energy landscape.[40]

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Figure 2

(a) Typical friction loops and friction force images measured at three different

tip-surface orientation angles of 60°, 72°, and 38°. (b) Friction force as a function of rotation angle of the graphite substrate. There were two narrow peaks with high friction appeared at 0° and 61°, which corresponds to the commensurate configurations. The solid curve was the simulated results by the model calculation with a 96-atom graphite nanoflake. Reprinted from Ref. [41] with permission from the American Physical Society (Copyright 2004).

It was supposed that the structural superlubricity of graphite may break down when the contact scale increases from atomic scale to micro- or macro scale, because the two graphite lattices are not perfectly rigid, and the misfit dislocations can cause much more energy dissipation.[42, 43] However, Zhen’s group recently fabricated a microscopic graphite mesas, and sheared it by a micro-manipulator, and thereafter, they observed that the sheared graphite flake self-retracted to its original positions on the mesas rapidly due to the tendency to minimize the surface free energy (Figure 3).[44] It exhibited the self-retraction reproducibly with the small flakes of 1 µm, but the self-retraction probability reduced to 12% for 10 µm mesas, and zero for 20 µm mesas, respectively, confirming that the large contact size limits the superlubricity. It was observed that the lock-in states (no self-retraction) appeared at some rotation angles, with a clear 60° symmetry, which shows the direct evidence that the self-retraction arises from the structural superlubricity, where the superlow friction appears when the two graphite surfaces are located in the incommensurate contact. The shear stress was measured to be 0.02-0.04 MPa in the incommensurate sliding, which was three orders of magnitude lower than that between commensurate graphitic layers. This result provides a 12

novel way to probing superlubricity, and extends the contact area of superlubricity to ~10 µm scales. Further study shows that the self-retraction of the graphite flake could occur with a high speed of up to 25 m/s at a temperature over 200

, which was comparable to the

theoretical speed limit for structural superlubricity.[45] There was a strong temperature dependence of the retraction speed, which was attributed to the thermally activated motion that involves pinning and depinning events at interfacial defects. When the sliding occurs between graphite surfaces under ambient conditions, the friction coefficients were close to zero and had no correlation with the external applied load up to the maximal pressure of 1.67 MPa.[46] Based on the fabrication of graphite mesas structures, the basal plane cleavage energy of graphite was measured as 0.37±0.01 Jm−2 for the incommensurate contact of two graphite layers, and it was independent on the temperature and twist angle.[47] Meanwhile, Elad et al. measured the adhesion energy between graphite layers of 0.227 joules per square meter, and found that the friction was fundamentally stochastic in the incommensurate lattices contact,[48] which was attributed to the disappearance of the lateral force acting on the interfacial atoms.

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Figure 3 (a) Schematic illustrations of a graphite mesa being partially sheared by a micro-manipulator, to form a self-retracting graphite flake. After releasing the graphite flake, it spontaneously returned to its original position on the graphite mesa. (b) SEM image of the self-retraction process in vacuum conditions. (c) Optical microscopy image of the self-retraction process under ambient conditions. Reprinted from Ref. [44] with permission from the American Physical Society (Copyright 2012).

It is known that the friction forces increased with the reduction of graphite layers due to the formation of a pucker, which requires additional energy to move the puckered region forward.[49] Therefore, the achievement of superlubricity on the thin-layer graphene is more difficult than that on graphite. Feng et al. used the scanning tunneling microscopy to study the friction behavior of free graphene nanoflakes sliding on a graphene surface, which exhibited a facile translational and rotational motions between commensurate initial and final states at a low temperature of 5 K.[50] The graphene nanoflakes were stable only in the commensurate contacts with the graphene layer. As long as the contact switched to the incommensurate state by the tip interaction, the graphene flakes diffused over hundred nanometers until another commensurate state appeared (Figure 4). The sliding distance became smaller at a higher temperature, indicating that thermal fluctuations triggered the transitions from superlubricity back to high friction states. Recently, Sun et al. used the first-principle calculation to simulate the friction between graphene and graphene surfaces, and observed that the friction was decreased with the increase of contact pressures, and finally became nearly zero after exceeding a certain critical load.[51] The superlubricity 14

under this critical load originated from the pressure-induced transition of the potential energy surface from the corrugated states to the flattened states and final to the counter-corrugated states.

Figure 4

(a) Schematic illustration of the graphene nanoflake rotating on the graphene

surface by the tip interaction, reaching the incommensurate state (the superlubricity state), and after that the nanoflake slid easily until another commensurate state is reached with either the same orientation or 60° rotation. (b) Interaction energy between the graphene nanoflake and graphene surface as a function of the rotation angle. The four states of the nanoflakes in (a) are marked in this energy curve, respectively. (c) Sliding distances of the nanoflakes at two cryogenic temperatures of 5 and 77 K. Reprinted from Ref. [50] with permission from the American Chemical Society (Copyright 2013).

One of the key problems that limits the application of structural superlubricity is the high anisotropy of friction due to the inevitable formation of commensurate contact, which 15

can lead to the failure of superlubricity under specific sliding orientations. One of the strategies to achieve a robust superlubricity state (independent of sliding orientations) is to avoid the formation of commensurate between graphite or graphene interfaces under all sliding orientations. Miura et al. found that the superlow friction coefficient of less than 0.001 could be obtained in all sliding directions after C60 was embedded in the graphite layers,[52] because C60 acts as a rolling bearing sliding on the graphite surface smoothly, which plays the key role on the robustness of superlubricity.[53] Recently, our group found another simple method to avoid the formation of commensurate contact is to form multiple contact points in the contact zone. Liu et al. fabricated a graphene-coated AFM probe by the metal-catalyst-free chemical vapor deposition, and formed the multiple contacts with graphite or boron nitride.[54] The superlow friction coefficient of 0.003 was achieved at arbitrary sliding orientations (no observed friction peaks), which is because of the fact that the multiple asperities contact covered with randomly oriented graphene nanograins leads to the formation of overall incommensurate geometry. Li et al. found that the graphene nanoflakes could be transferred onto the silica probe by the tribointeractions, and the superlubricity between graphite and multiple transfer graphene nanoflakes was achieved under ambient conditions.[55] The friction coefficient was reduced to approximately 0.0003 with excellent robustness, which arose from the extremely weak interaction between the graphite surface and multiple transferred graphene nanoflakes in the incommensurate contact (Figure 5). Moreover, it was observed that the superlubricity was independent of the sliding velocities, surface roughness, and rotation angles because of the formation of multiple contacts. After the graphene nanoflakes being transferred onto a 16

sharp silicon tip by tribointeractions, the robust superlubricity of graphite sliding against graphene could be achieved until the contact pressures exceeded 2.52 GPa.[56] In addition, Wang et al. proposed a different method to eliminate the anisotropy of friction by applying a strain on the graphene surface. They used molecular dynamics simulation to show that the robust superlubricity state was realized via both biaxial and uniaxial stretching, and the friction force was no longer dependent on the sliding orientations, which was due to the formation of the complete lattice mismatch.[57]

Figure 5

(a) Schematic illustration of shear plane between the graphite surface and silica

probe before tribointeractions. (b) Schematic illustration of transferred graphene nanoflakes on the silica probe and the formation of new shear plane after tribointeractions. (c) Schematic illustration of common contact between graphite and silica probe before tribointeractions. (d) Schematic illustration of incommensurate contact between graphite and multiple transferred graphene nanoflakes after tribointeractions. (e) Friction forces versus normal loads for different probes with random roughness sliding against graphite after tribointeractions. Inset is the measured roughness of eight different silica probes. Reprinted from Ref. [55] with permission from Wiley (Copyright 2018).

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In addition to the above superlubricity achieved between graphite and graphene interfaces, the superlubricity state of graphite or graphene can also be realized with the sliding against other layered materials (forming the heterojunction structures), such as hexagonal boron nitride (hBN), molybdenum disulfide, and other transition metal dichalcogenides.[58-62] Leven et al. used the density functional theory (DFT) calculations to simulate the sliding of graphene nanoflake on the hBN layer, finding that when the size of graphene flakes became large enough, the overall sliding friction was vanishingly small regardless of the variation of the relative orientation when the sliding occurred between graphene and hBN.[58] This was completely different from the cases of the homogeneous graphene or graphite interfaces where the nanoflake reorientations are able to eliminate the superlubricity state. It was further found that the superlubricity of graphene/hBN heterojunction can bear higher loads than the homogeneous graphitic interface, which was attributed to the reduction of the effects of edge atom pinning under high normal loads caused by the intrinsic incommensurability.[61] Recently, Song et al. reported the experimental realization of the robust superlubricity between graphite flakes and hBN at the microscale. The friction force exhibited a six-fold symmetry, showing the clear hexagonal features of the underlying lattices, but the frictional anisotropy was much smaller than that measured for the homogeneous graphite interfaces (Figure 6).[60] The atomistic molecular dynamics (MD) simulations demonstrated that the friction energy was dissipated through the internal degrees of freedom of the contacting layers in the graphite/hBN heterojunction, which caused the friction was independent of the misalignment angle, while for the homogeneous graphitic contact, it was dissipated through the centre-of-mass motion. This result for graphite/hBN heterojunction clearly exhibited that 18

the structural superlubricity could persist even when there were external loads applied for the incommensurate contact under ambient conditions.

Figure 6

Relationship between the frictional stress and rotation angle between graphite and

hBN under ambient conditions. The frictional stress in linear (a) and polar (b) both showed clearly the anisotropy with six-fold symmetry. Reprinted from Ref. [60] with permission from Nature Publishing Group (Copyright 2018).

Another case for the superlubricity of non-identical lattices was the formation of the metal/graphite or graphene interfaces, such as the antimony, gold and platinum nanoparticles sliding on the crystalline graphite, and the graphene nanoribbons sliding on the gold surface.[63-67] These dissimilar interfaces were fundamentally different from that formed by identical surfaces because the commensurate orientations are hard to form due to the mismatch between the lattices. Kawai et al. measured the friction behavior of graphene nanoribbons sliding on the crystalline gold surface (111) under ultra high vacuum (UHV) conditions at cryogenic temperatures.[65] They observed that the static friction force per unit length reduced with increasing the length of graphene nanoribbon, demonstrating that the structural superlubricity took place. By attaching the nanoribbons to the AFM tips, and then dragging them laterally on the gold surface, they found that there was a stick-slip motion 19

during the graphene nanoribbon sliding process, and the shift of resonance frequency in the noncontact AFM was periodically modulated in steps of 0.28 nm, which arose from the particular configuration between the nanoribbon and gold substrate. In recent years, the nanomanipulation based AFM have been established as powerful techniques in analyzing superlubricity mechanism of non-identical lattices.[63, 68, 69] Dietzel et al. using the nanomanipulation based AFM demonstrated that both the amorphous antimony and crystalline gold islands can slide on crystalline graphite surface under UHV with the structural superlubricity model.[63] For the amorphous antimony, the friction forces exhibited a sub-linear relationship with the contact area for the disordered surfaces. However, for crystalline gold particles, the results exhibited a more complex scaling power behavior due to the different shapes of gold particles, where the perfectly straight edge crystals, such as triangles and trapezoids, exhibited a scaling power of γ = 0 in an incommensurate registry and γ = 0.5 in a pseudo-commensurate registry, and the perfectly round crystals exhibited a scaling power of γ = 0.25 both at commensurate and incommensurate orientations. Although the shape of gold particles varied greatly, all the scaling power was in the range of 0 - 0.5, demonstrating that the superlow friction regime was imposed by the structural superlubricity. Recently, Cihan et al. found that the structural superlubricity at the gold-graphite interface was not limited to UHV conditions but it could also be achieved under ambient conditions (Figure 7), with a mean scaling power of γ = 0.16.[64] The Ab initio calculations showed that the gold–graphite interface remained largely free from the contaminant molecules under ambient conditions, leading to the robust structural superlubricity. Because the gold crystal cannot be oxidized under 20

ambient condition, the atomically flat and tight interface between graphite and gold can keep intact over a long period, which was the key factor influencing superlubricity, and also makes them become the promising friction pairs to exhibit robust superlubricity.

Figure 7 (a) Friction force versus contact area measured during the nano-manipulation of gold island on graphite (37 manipulation events). The friction was always less than 1 nN. (b) Normalized friction forces versus number of sliding atoms at the gold/graphite interface.

All the scaling powers were less than 0.3, which were in the regime defined for structural lubricity (0 < γ < 0.5), with an average scaling power of γ = 0.16. The scatter originated from the variability for the circumferential shape of the gold islands. Reprinted from Ref. [64] with permission from Nature Publishing Group (Copyright 2016).

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Although the graphite or graphene is easy to achieve superlubricity by forming the incommensurate contact, it is limited to the nano- or microscale because it requires the atomically smooth contact in the absence of any defects for the structural superlubricity. Therefore, the friction coefficient of graphite or graphene-based materials was usually above 0.1 at the macroscale,[31, 33, 70, 71] which was much larger than that measured at the nanoscale or microscale. Recently, Berman et al. observed that the superlubricity of graphene can be achieved at the macroscale when the graphene patches was used in combination with the nanodiamond particles and DLC.[72] The friction coefficient could be reduced down to 0.004 between the graphene-coated surface and DLC-coated ball after a running-in period under dry nitrogen environments (Figure 8), but in the humid environment, the friction coefficient would become above 0.2, implying the failure of superlubricity. They used the molecular dynamics simulations to explore the superlubricity mechanism, and observed that the graphene patches at the sliding interface wrapped around the nanodiamonds, leading to the formation of graphene nanoscrolls in the contact zone. Such a structure could reduce the contact area, and could act as rolling ball bearing between the sliding surfaces, and meanwhile it could realize an incommensurate contact when slid against the DLC surface, all of which in turn substantially made the friction coefficient reduce down to superlubricity state.[72] If in the humid air, the water molecules could enter the sliding interface, which would prevent the formation of the graphene nanoscrolls and thus cause a high friction. Recently, Li et al. observed the instantaneous superlubricity of graphite at the macroscale after many multilayer graphene nanoflakes were transferred onto the surface of steel ball by tribointeractions.[73] The minimum friction coefficient was reduced to the level of 0.001, 22

and randomly appeared with a very short duration as the test progressed. The friction at the macroscale mainly originated from the statistical friction forces of many graphene nanoflakes sliding on the graphite surface in the contact zone. Because there are many atomic steps on the graphite surface, it would greatly reduce the appearance probability of superlubricity when some graphene nanoflake sliding across these atomic steps with a high friction. This result provides a possible method to achieve superlubricity of graphite or graphene at the macroscale by the discretization of a large contact zone into multiple contacts.

Figure 8

(a) Schematic illustration of the superlubricity test between a DLC-coated ball and

a silica substrate deposited with many graphene patches and nanodiamonds. (b) Coefficient of friction versus number of cycles in sliding test performed in a dry nitrogen environment 23

for three different friction systems, where the silica substrate is covered with graphene-plus-nanodiamonds (blue), graphene patches only (green), and nanodiamonds only (brown), respectively. Inset shows the detail of coefficient of friction reducing to less than 0.01 after 2000 cycles for graphene-plus-nanodiamonds, demonstrating the superlubricity state. Reprinted from Ref. [72] with permission from AAAS (copyright 2015).

To make the superlubricity of graphite or graphene be available in the engineering application, the extremely low friction should be robust and stable. However, many studies found that the superlubricity is very sensitive to friction conditions and test environments. Wijk et al. observed that the friction had a sudden increase with load when the graphene flakes slid on graphite, which was attributed to the vertical distortions of the carbon atoms at the edge of the graphene nanoflake.[74] Kim et al. showed that the superlubricity between incommensurate surface was dependent on the tip compliance and normal load, because the adjustment of the tip to substrate with higher load or weaker harmonic interaction could lead to the local commensurability and consequently to the failure of superlubricity.[75] Li et al. found that the friction coefficient between graphene nanoflake and graphite increased approximately by 10 times (superlubricity failure) when the contact pressure exceeded 2.52 GPa, which originates from the additional exfoliation energies due to the lamination of the topmost graphene layers on the graphite substrate under ultrahigh pressures.[56] Song et al. observed a logarithmic increase of frictional stress with increasing sliding velocity when the superlubricity was achieved in the graphite/hBN interface, which was attributed to the thermally activated friction process.[60] Liu et al. demonstrated that the occurrence of 24

superlubricity between graphite interfaces was dependent on the contact size; when the contact area exceeded 400 µm2, the superlubricity become impossible due to the presence of inevitable defect on the graphite surfaces. Dietzel et al. studied the limitations of structural superlubricity through the nanomanipulation experiments, revealing that the high chemical interaction energies could lead to the loss of incommensurability-driven superlubricity.[66] In addition to these, the surface chemical reaction, roughness, contamination, defects, and deformations, all could lead to the failure of superlubricity of graphene or graphite surface.[23, 42, 76]

3. Superlubricity of diamond-like carbon Diamond-like carbon (DLC), also called amorphous carbon, is a metastable material with carbon atoms mainly hybridized in sp2 and sp3 bonds [77]. Since its discovery in 1971, DLC films have been widely studied because of their exceptional properties and have widespread applications in many fields. Thanks to the bonding diversity of carbon and the significant strides in CVD and PVD deposition techniques, DLC has a great variety of structures. Among them, amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) are the two most basic structures in DLC family. As early in 1994, the superlubricity capacity of DLC film had been discovered by Donnet et al. Later in 2000, a more detailed breakthrough of superlubricity is accomplished by Erdemir in highly-hydrogenated amorphous carbon films.[17] A stable and extremely low friction coefficient down to 0.001 was established in self-mated a-C:H (40 at.% H) tribo-couple, as shown in Figure 9a. The most striking finding is that almost no wear could be detected on the sliding surfaces even 25

after a 32 days’ long-duration test in dry N2 environments. This pioneering work triggers the extensive research in the superlubricity phenomena and the relevant mechanisms in DLC films. To date, the major studies in superlubricity regarding DLC are concentrated on a-C:Hs with the emphasis on the pivotal impact of hydrogen in achieving superlow friction. Meanwhile, new types of DLC films with novel nanostructures or alloyed with foreign elements are also found to possess capacities of superlubricity. This section discussed the state-of-the-art of DLC superlubricity from the perspectives of structural dependence, environmental effects and the crucial role of tribofilm formed on the contact surfaces.

Figure 9 (a) Long-lasting superlow friction in self-mated highly-hydrogenated a-C:H films and (b) the proposed non-adhesive interface model based on surface hydrogen passivation. Reprinted from Ref. [19] with permission from Springer (Copyright 2014).

As mentioned above, superlow friction was obtained by introducing significant hydrogen into the a-C matrix in Erdemir’s work [78, 79], in which most carbon dangling bonds were speculated to be passivated by hydrogen terminations and a non-reactive sliding interface was created.[19, 80] From the atomic-scale viewpoint (Figure 9b), the authors 26

proposed that the re-distribution of electrical charge density and the repulsive forces generated between two positively charged hydrogen protons along the sliding interface are the origins of suppression of adhesive interactions and hence friction [81]. This mechanism was theoretically supported by MD simulations and DFT calculations on highly-terminated diamond and DLC surfaces [82-84]. It seems that a well-established hydrogen passivation interface is an ideal pathway to cancel out the friction forces and avoid material wear, especially in self-mated H-terminated surfaces as there is no necessary to lose carbon material to form lubricative tribofilm (as will be discussed below). Clearly, the vanishing state of friction highly depends on the hydrogen concentration in the film, namely the hydrogen coverage degree of carbon surfaces [83]. This kind of hydrogen-rich DLC film with hydrogen content exceeding 40 at.% is usually designated as polymer-like carbon (PLC).[85] The hydrogen incorporation into amorphous carbon matrix is expected to induce a softening structural change including an increase in the voids density and the corresponding reduction in the cross-linking of carbon network and therefore the release of its intrinsic residual stress. Generally, varying the source gas chemistry [78] during deposition (i.e., the H/C chemical ratio in the feeding gas) or the ion energy [86] (i.e., the accelerating bias voltage applied to the substrate) is a common method to control the hydrogen incorporation in the films. Most of hydrogen atoms exist in the carbon matrix by forming C-H covalent bonds, while some of them remain as unbonded interstitials.[81] To some extent, these unbonded hydrogen molecules are speculated to replenish the desorbed surface hydrogen during sliding contact. Alternatively, from the extrinsic perspective, testing the hydrogen-deficient or hydrogen-free carbon films in hydrogen atmosphere provides another feasible way to reduce the friction 27

coefficient towards an ultralow level. A minimum equilibrium partial pressure of hydrogen was theoretically predicted to achieve a fully atomic hydrogen passivated carbon surface and hence superlow friction at a specific temperature.[87] The supply of hydrogen atoms from the film itself will reduce the hydrogen pressure required to establish a hydrogen-passivated interface.

The doping of foreign elements or introduction of nanostructures in the hydrocarbon matrix brings a new pathway to realize superlubricity. Fluorine, another monovalent atom, can also endow the fluorinated DLC film (a-C:H:F) with superlow friction in UHV.[88] It is well known that polytetrafluoroethylene (PTFE) is a fluoropolymer lubricant consisting of long parallel macro-molecules, and each molecule is composed of a carbon skeleton chain surrounded by fluorine atoms. The fluorine atoms fully terminate the carbon chain and repeal other PTFE molecules on account of repulsive forces caused by the strong electronegative properties of fluorine. Due to the very weak interactions between neighboring molecules, an excellent lubricating performance is provided by PTFE at low shear stress. Therefore, the role of fluorine is speculated to be similar as hydrogen in view of the surface passivation nature with C-F terminations. As a common dopant, silicon incorporation into a-C:H is usually beneficial for reducing the density and size of sp2 graphitic defects, stabilizing the tetrahedral sp3-bonded network (thermal stability) and promoting the development of polymer-like structure.[86] The occurrence of a superlow friction coefficient of 0.001 for self-mated a-C:H:Si films (31.9 at.% H, 9.3 at.% Si) in dry N2 is found to originate from the fact that silicon is involved in the tribo-induced activities to assist the phase transformation and the 28

corresponding tribo-induced softening of the sliding contact interface, which is accompanied by the in situ formation of a polymeric tribofilm.[89, 90] Interestingly, a superlow friction (µ = 0.007) obtained for a-C:H:Si film rubbing against steel ball in vacuum was also highly correlated with the transfer of polymer-like hydrocarbon to the bearing steel surface and the orientation along the sliding direction.[91] The most unique characteristic inherent to Si-containing DLC films is the structural suppression of moisture and the resulted humidity insensitivity in ambient environments.[92] To realize superlubricity in ambient and humid atmosphere is still a great challenge to date. It is noticed that the formation of a nanoscale silica-like tribofilm is the key factor to restrain the influence of water molecules for Si-doped DLC.[93] By tailoring the surface density of silicon hydroxyl group (controlled by the Si content in the film), superlow friction was also feasible at a specific humidity level. A similar humidity-insensitive anti-friction performance (µ = 0.004 ± 0.002) was also observed for sulfur-doped a-C:H film (5 at.% S) in a wide relative humidity range of 0-50%.[94] The authors argued that the generation of thiol-like (-C-S-H) chemical groups on the amorphous carbon film surfaces is the structural origin of moisture suppression, in which the smaller dipole moment of S-H in contrast to C-H could reduce the dipole–dipole binding forces between adsorbed water molecules and the carbon surface and hence the established equilibrium water coverage. Nitrogen-doped DLC, also called carbon nitride (CNx), represents another special type of superlubricious carbon film since the realization of superlow friction is not dependent on the hydrogen passivation effect.[95] A superlow friction coefficient of 0.005-0.01 could be realized in dry N2 atmosphere rather than other gaseous environments. The running-in stage is quite crucial in establishing the highly 29

chemically inert and non-adhesive sliding interface. A thin amorphous tribolayer with a cross-sectional thickness of 10 nm formed on the bearing ball surface is responsible for the superlubricity behaviors.[96] As compared to the above non-metal elements, the incorporation of simple metal elements such as tungsten, chromium and titanium into hydrocarbon matrix (metal-doped DLC, a-C:H:Me) hardly lowers the friction coefficient below 0.01. The superior self-lubricating performances were reported for a-C:H:Ti films with a friction coefficient down to 0.05 in ambient humid air and 0.02 in dry air.[97] Generally, the presence of metal carbide-forming elements in amorphous carbon matrix promotes the formation of metal carbide nanocrystallites, which is beneficial for improving the hardness and toughness.[98] However, these hard nanoparticles may behave as abrasive third-bodies during the friction contact. This is probably the major hinder to realize superlow friction and the underlying origin of noticeable material wear. Alternatively, some researchers adopted the co-doping of metal and non-metal elements to overcome this drawback. For instance, as shown in Figure 10, an extremely elastic (Si, Al)/a-C:H dual-elements-doped film was designed to realize a superlow friction coefficient of 0.001 in high vacuum.[99] The existence of self-assembled dual nanostructures of cross-linking networks and fullerene-like structures exceptionally contributes to the macroscale friction-vanishing behaviors. Another co-doped film (Si, Ti)/a-C:H with a friction coefficient of 0.007 realized in ambient air was also reported.[100] Note that the content of metal incorporated should be controlled in a relatively low level (usually below 10 at.%) before the emergence of numerous metal carbide nanocrystallites in the film.

30

Figure 10

(a) Self-assembled dual nanostructures of (Si, Al)/a-C:H film and (b) the

corresponding superlow friction and wear behaviors under vacuum environment. Reprinted from Ref. [99] with permission from Wiley (Copyright 2012).

The operating environments are the extrinsic factors dramatically influencing the tribological properties of DLC films. The usually encountered environments are dry N2, Ar, CO2, H2, H2-He mixture, UHV, O2, H2O, and humid air. According to the structural characteristic of molecules, the surrounding gaseous species can be sorted into two types: chemically inert one and reactive one. Dry N2 atmosphere is the most common inert gaseous background for evaluating the superlubricity behaviors of DLC films, as seen in Erdemir’s and other researchers’ studies.[17, 89, 95, 96] For instance, the lowest friction coefficient of DLC film is often obtained in dry N2 atmosphere. The above-mentioned carbon nitride films also exhibit the most sustainable superlow friction state in N2.[95] Meanwhile, it has been frequently found that the frictional responses of the same DLC films are deeply changed even when tested in different dry inert gases. Interestingly, the friction properties of a-C:H films in dry N2 and CO2 gases are better than that in dry Ar.[101] The origin of this diversity is 31

speculated to depend on the molecular characteristic and the gas-surface interactions. After chemisorption on the carbon film surface, the presence of lone pair electrons at both orbital sides of the N2 and CO2 molecules is expected to cause charge transfer and hence the electron density increase between the molecules and surface carbon atoms.[101, 102] This oriented gaseous adsorption does not only passivate the surface dangling bonds but also prevents π–π∗ interactions between the counterfaces, finally resulting in superlubricity. In comparison, the Ar molecules are incapable of establishing this specific gas-surface interfacial interaction network owing to the lack of lone pair electrons. It should also be pointed out that the gas-surface interaction efficiency is dependent on the exposure time of carbon surface to the gaseous environment. The gas-related adsorption kinetics modeled by the Elovich equation emphasized that the superlubricity of DLC films could be maintained, suppressed or even recovered by controlling the exposure time during sliding contact.[103] Specifically, the superlow friction coefficients (0.003-0.008) could be sustained for Erdemir’s DLC coatings when the exposure time was smaller than 5 s per sliding cycle. Therefore, the sliding duty cycle should be taken into account when exploiting the lubricity of carbon-based films.

It is known that the hydrogen or dilute hydrogen gas is a source of atomic hydrogen to tribochemically passivate the sliding interface, owing to which the hydrogen-deficient or even hydrogen-free DLC film can also exhibit superlow friction and wear.[104] Under the shear stress and flash temperature exerted at the rubbed asperities, hydrogen molecules are expected to be decomposed into atomic hydrogen and take part in the tribochemical reactions with the carbon film surfaces. This process favors the production of numerous hydrocarbon 32

termination groups on the contact surface as confirmed by the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis,[105] and promotes the hydrogenation of the sliding interface. As revealed in Figure 11, a very interesting friction fading-out phenomenon with the friction coefficient vanishing down to 0.0001 was discovered for polymer-like carbon film when rubbed against ZrO2 ball under a heavy load (producing a maximum Hertzian contact pressure of 2.6 GPa) in H2 gas atmosphere [106, 107]. The authors argued that the ZrO2 ball behaves as a catalytic to promote the dissociative reactions of H2 gas when the H2 molecules are coming near to the Zr and O atoms of ZrO2. The dissociative H atoms adsorb on the catalytic agent and tend to be involved in hydrogenation reactions with carbon atoms possessing double or triple bonds, i.e., transforming C2H2 into C2H4. A blister-rich tribofilm was formed on the ZrO2 contact surface [107]. In addition to the repulsive forces derived from paired interfacial H-atoms, the lubricating effect from some produced tribochemical substances and the possibility of elasto-hydrostatic gas bearing lubrication are also proposed by the authors to explain this range of superlow friction.[108] UHV is one of the challenging environments for DLC film to maintain durable lubricity. Clearly, the non-hydrogenated DLC films are always subjected to high friction coefficients and high wear rates. This is usually attributed to the strong tribochemical interactions between the carbon dangling bonds and the counterface surfaces.[109] For hydrogen-rich DLC, without the surrounding protection from the gaseous molecules, the permanent loss of hydrogen from the carbon film surface can also cause the deterioration of the passivated sliding interface and the following high friction and severe wear (shortened operating life of the film).

33

Figure 11 (a) An extremely low friction coefficient of 0.0001 achieved under heavy contact pressure of 2.6 GPa in aqueous-ethanol-vapored H2 atmosphere and (b) the as-formed blister-rich tribofilm on the contact surface. Reprinted from Ref. [107] with permission from Japanese Society of Tribologists (Copyright 2017).

As typical representatives of reactive gaseous molecules, the presence of O2 and H2O will significantly affect the frictional behaviors of DLC films. For a-C:H films, the friction coefficient generally increases with the elevated pressure of O2 or H2O in a vacuum background

environment.[110]

One

possible

explanation

is

the

physicochemical

adsorption-induced surface modification of the carbon films. The hydrocarbon surface readily undergoes oxidization when a small number of the adsorbed gaseous molecules are dissociated and forming chemical bonds. The DFT calculations reveal that the chemisorption of O2 molecules on C-C bonds of surface hydrocarbon chains can induce the breaking of C-C bonds and finally cause the cleavage of the hydrocarbon chains.[111, 112] A CO-terminated sub-chain is then generated on hydrocarbon surfaces. A very thin native oxide layer is therefore present on the DLC film surfaces, which is believed to be closely related to the abnormal high friction in the running-in stage during the friction test. However, it should be emphasized here that recent studies have revealed that the extrinsic O2 molecules can only 34

erode the surface of carbon films with a maximum depth of about a few nanometers, leaving the bulk of the film intact.[113] Therefore, the oxygen-induced erosion of carbon film surfaces is a gradually-evolved process. As compared to the oxygen molecules, the adsorbed water layers are expected to behave as a stronger physical barrier owing to its greater dipole-dipole polar interactions and the resulted larger cohesive energy. Moreover, the presence of thicker water layers under high relative humidity can induce viscous drag and capillary forces between the sliding surfaces.[114] The surface chemistry and characteristic are to a large extent affected by the surface coverage of water molecules (determined by the water vapor pressure). Another significant factor is the tribochemical reactions occurred along the sliding interface induced by these oxygen-containing gaseous species. Strong chemical interactions take place not only between the a-C:H film surface and the counterfacing materials but also between the contact surfaces and the gas species. For instance, the rubbing process occurred between the a-C:H film and bearing steel ball is usually characteristic of the formation of a complex tribofilm, which is composed of various iron carbides, oxides and covalent compounds, and responsible for the as-observed high friction in humid air.[90] However, it should be pointed out that in some cases the water adsorption and the formation of water layers can be used to suppress the moisture influence. As one example mentioned above, the water dissociation-induced growth of a hydrophilic silica-like tribofilm on the a-C:H:Si film and counterface surfaces and the following oriented nanostructures of adsorbed surface water molecules are found to be related to the humidity insensitivity (Figure 12).[93] One critical influencing factor is the degree of hydroxylation of carbon surface deriving from silicon atoms oxidation (tailored by the doping content of 35

silicon) and water molecules adsorption together with water molecules dissociation (determined by the environmental relative humidity level). The formation of a hydroxyl-rich surface can trigger a highly directed growth of the adsorbed water layers by hydrogen bonds with surface OH sites and with other adsorbed water molecules. The MD simulations results also reveal the microscopic activities that the adsorbed water molecules confined in nanoscale contact space can still form a layered structure under high normal pressure and shear stress, which is speculated to behave as a boundary lubrication layer to lower the interfacial friction.[115]

Figure 12 Lubricating mechanisms of a-C:H:Si film showing (a) the evolution of friction coefficient with respect to the relative humidity, and interfacial boundary water lubrication behaviors on Si-OH surface: (b) layer-like water network formed under an intermediate relative humidity, and (c) liquid-like water network formed under a high relative humidity. Reprinted from Ref. [93] with permission from IOP Publishing (Copyright 2013). 36

In most cases of DLC films, the in situ formed tribofilms on the rubbing areas are the accompanying tribo-products during sliding contact. However, the frictional behaviors of carbon-based interfaces are highly affected by them. The occurrence of a superlubricity state is generally correlated with the nanostructures formed in the tribofilm. Throughout the sliding process, initial running-in is a crucial stage for the tribofilm build-up. During this period, a number of tribochemical activities occur between the contact surfaces, resulting in the reconstruction of the sliding interface. It involves numerous bond-breaking and bond-forming events occurred within the film itself (intrafilm) or between the paired films (interfilm) or between the film and the paired counterface (heterogeneous interface).[83] These events may include structural phase transformation of sp3-to-sp2 in the carbon matrix, hydrocarbon gas emission from the film bulk, exposing and re-saturation of surface carbon dangling bonds, orientation of local clusters with the bonding structures parallel to the sliding direction, in situ growth of nanostructured tribofilm and so on.[116] When the number of events regarding the bonds breaking and reforming reaches a steady-state value, the interfacial friction will usually come to a lower state as compared to that of the initial contact.[117] Obviously, it is an intractable issue to in situ track and record the interfacial friction activities as they are occurring in the buried contact area. Moreover, it is nearly impossible to obtain direct imaging of the area of interests for most modern characterization methods such as Raman and X-ray photoelectron spectroscopy (XPS), which hinders the visual inspection of the sliding contact interface. However, with the development of focus ion beam (FIB) microscopy in recent years, the preparation of high-quality nanometer-thick lamella in a highly-localized, 37

site-specific manner enables the cross-sectional observation and analysis of the contact area by transmission electron microscopy (TEM).[118] Moreover, a newly-emerging microscope called scanning TEM (STEM) based on Z-contrast imaging principles provides the sub-angstrom imaging resolution. Combined with electron energy-loss spectroscopy (EELS), the compositional and bonding features of the tribofilm can be distinguished at an extremely high spatial resolution.[90] Among the numerous factors, the counterfacing materials, contact pressure (applied normal load), sling velocity and environmental medium (as discussed above) are the most remarkable parameters affecting the structural evolution of tribofilms.

For self-mated hydrogen-rich a-C:H tribocouple, it seems that the realization of superlubricity in dry N2 owing to the hydrogen passivation effect (repulsive force model) dose not necessarily rely on the formation of tribofilms as the contact film surface is very clean and no wear loss can be detected after the friction test. However, above a threshold contact pressure, partial hydrogen atoms are expected to be detached from the hydrocarbon asperities and nanoscale wear occurs. Accompanying this is the occurrence of a shear band with clustering and ordering of sp2-hybridized sites in the top-most few-nm region of the sliding interface.[90] With sufficient hydrogen remaining, this shear layer possesses a bonding structure more like a highly-hydrogenated graphite-like carbon.[85] A significant reduction of the shear stress in this oriented shear band (i.e., 0.6 MPa) is the structural origin of the extremely low friction (µ=0.001) observed in dry N2 in spite of sacrificing some film material loss.[90] For self-mated a-C:H:Si tribocouple, there is another recently-discovered pathway to significantly reduce the frictional resistance in dry contact. Upon contact, the film 38

material coated on the ball surface was completely transferred onto the film surface on Si wafer and the simultaneous in situ growth of nanostructured tribofilms (Figure 13a) on both surfaces resulted in the reconstruction of the sliding contact interface. In contrast to the normal wear debris as found on both sides of the wear track, the as-formed tribofilm was covering the whole wear track and expected to provide the lubricity all through the sliding contact. The tribofilm is rich in sp2-hybridized sites and hydrogen-related bonds, and possesses a very low hardness (~0.25 GPa). Therefore, the bonding nature of this tribofilm is more like a polymeric compound. A silicon-nucleation sublayer (Figure 13b) found along the initial sliding interface is speculated to behave as a starting point for promoting the growth of such an anti-friction polymeric tribofilm. This tribo-induced polymerization of the sliding contact interface is a new finding to the superlubricity mechanisms in DLC films.[90]

39

Figure 13

Tribo-induced polymerization as a new superlubricity mechanism in self-mated

a-C:H:Si film: (a) a nanostructured tribofilm in situ formed on the steel ball scar surface through material transfer and reconstruction and (b) a silicon-nucleation sublayer formed along the initial sliding interface as a starting point for promoting the growth of the anti-friction polymeric tribofilm. Reprinted from Ref. [90] with permission from Nature Publishing Group (Copyright 2017).

For a heterogeneous sliding interface, the presence of a metal or non-metal counterfacing surface induces the formation of tribofilms with more specific features. It is capable for a-C:Hs to realize superlubricity in this situation by forming a nanoscale tribolayer in the contact area. For instance, a carbonaceous tribolayer with a thickness of ~27 nm was confirmed on the bearing steel ball surface after the superlubricity test in dry N2. As compared to the self-mated case, it was found that a noticeable running-in period is necessary to build up this lubricating tribofilm. Due to the high chemical reactivity of iron and the presence of pristine oxide layers, the as-formed tribofilm possessed a subtle multilayered structure, namely a highly-crystallized C-Fe-O bonding sublayer near the steel ball surface, a carbon-rich low-density sublayer formed in the middle and a nanoparticle-passivated sublayer near the sliding interface. Hydrogen was still plentiful in the outer-most region of the tribofilm, further passivating the sliding interface and lowering the friction resistance. In comparison to steel ball surface, the relatively inert ceramic counterface surface such as silicon nitride ball only needs a very short running-in period to reach the superlow friction state. Meanwhile, the as-formed carbonaceous tribofilm was very thin and the thickness was about 5 nm. The bonding structure was mainly amorphous. A common point as like the case 40

of bearing steel ball is that the major composition of this tribofilm was carbon with sp2-C bonding fraction up to 65-80% and the outer-most region was also rich in hydrogen.[90] As mentioned above, the contact pressure is a decisive factor to induce structural change of the film upon sliding. In general, a higher contact pressure can intensify the possibility of hydrogen detachment from the hydrocarbon asperities and promote the degree of graphitization in the as-formed tribofilm. Beyond a threshold value, the occurrence of a more specific shear localization mechanism owing to high-degree phase transformation and gradual bonding ordering may govern the lubricity of carbon films. It is demonstrated that amorphous carbon matrix does not exhibit homogeneous Newtonian-fluid-like shear deformation in the film bulk, but rather behave as a localized deformation in a very thin shear band. This localization process significantly promotes the clustering and ordering of sp2-hybridized sites, resulting in the occurrence of graphene-like nanoflakes or shear band along the sliding interface.[119] As already mentioned above, the environmental medium involved in the rubbing process also highly affects the compositional characteristics and interfacial mechanical properties of the as-formed tribofilms. For instance, it was found that the shear stress of the tribofilm was lower in dry air condition than in ambient air, mainly due to the tribochemical reactions occurred between the gaseous molecules and the carbon film surface.[120] Meanwhile, moisture can promote the agglomeration of wear particles to form third-bodies, in which water molecules behave as ‘bonding glue’ to link the debris together. Furthermore, the adhesion of the tribofilm to the non-coated counterface surface is also reinforced owing to the moisture effect. Another important issue is the stability of the as-formed interfacial tribofilm during the sliding contact. The friction and wear states highly 41

depend on whether and when a stationary and stable tribofilm formed on the contact surface in the running-in stage. Both the environmental effect and the tribocouple characteristics determine the interfacial evolution of the tribofilm. For instance, the delayed adherence of the transferred materials to Al2O3 ball surface from a-C:H:W film tested in dry air is the non-reactive characteristic of W with sapphire whether or not oxygen is present. These speculations comply with bonding-related reactions predicted by thermochemical equilibrium calculations. In response to the buildup of tribofilm, different velocity accommodation modes (VAM) are expected to account for the interfacial friction behaviors. For high friction states with no or non-stationary tribofilm, the VAM mainly occurs in the form of shearing movement and extrusion of third bodies. In contrast, the VAM for steady-state low friction is dominated by interfacial sliding between the tribofilm and the pristine film surface for the case of a stationary tribofilm formed in the contact area.[120]

All these findings emphasize the fact that the formation of a carbonaceous (mainly sp2-C rich in the composition) tribofilm is a quite universal and fundamental mechanism governing the tribological behaviors of amorphous carbon films in spite of the contact parameters and environmental conditions. In most superlubricity states, a well-constructed tribofilm can maintain a robust lubricity for a long time even though its thickness is only in the range of a few or dozens of nanometers. The most crucial factor is the optimized evolution of the bonding structure and the interfacial stability of the in situ formed tribofilm. With the broadening applications of DLC films in more specialized fields, the realization of

42

long-lasting superlubricity under extreme operating conditions is usually relying on the design and establishment of robust tribofilms.

4. Superlubricity of onion-like/fullerene-like carbons As a unique spherical carbon material, onion-like carbon (OLC) is entirely composed of sp2-hybridized sites and has excellent self-lubricating properties.[121] Through systematic studies in the solid lubricating mechanism of OLC, it can give a detailed explanation on how the carbon atoms are arranged on the friction contact interface, how the sp2- and sp3-bonded structures transform to each other, and how the potential rolling effect works in the lubrication. Inspired by these merits, many researchers investigated the intrinsic spherical structure in detail, such as the existence state of interfacial carbon atoms, the microstructure of the as-formed tribofilm and the underlying lubricating mechanisms. Ugarte et al. showed that OLC, a spherically-layered graphene structure, epitaxially evolves from the outer layer to inner layer.[122] From the viewpoint of energy, the structure of a limited icosahedron with a nearly spherical shape possesses the smallest energy. This structure distributes the stress into individual atoms, forcing the structure more stable. The chemical bonds between carbon atoms in the graphene-like layer are stronger than those in the diamond structures, thereby forming a tight microstructure, and the high elasticity (the elastic recovery is ~92 %) can reduce the surface damage in the friction process.[26] In addition, the spherical OLC units peeled from the substrate could probably act as bearings, effectively reducing the friction coefficient (less than 0.01) and wear rate (~6.41 × 10-9 mm3/Nm). Yao et al. employed highly-graphitized OLC with a diameter of ~25 nm as lubricating additives in base oil to 43

study the friction properties.[123] It is believed that the key aspect of OLC lubricating is the excellent dispersibility, high chemical inertness and excellent structural strength of the spherical structure. The ribbon-like carbon materials, such as carbon nanotubes which are also layered structures, easily crimp with each other during the friction process, so the lubricating effect becomes deteriorated. Matsumoto et al. used OLC as basic additives in PAO (poly-alpha-olefin) to conduct the friction tests under the contact pressure of ~1 GPa, finding that the OLC structure was intact and a tribofilm of ~100 nm thick was formed on the interface.[124] Therefore, it is considered that the lubricating mechanism of OLC film mainly depends on its inherent structural property. That is, the OLC invariably maintains a concentric structure during the friction process, which is significantly different from the lubricating mechanism of layered structures such as MoS2 or WS2. Song et al. studied the friction properties of a-C:H films in vacuum. Initially, the passivation effect of hydrogen in the film resulted in a super-low friction coefficient.[125] After continuous friction testing, the hydrogen was lost gradually. However, the formation of a spherical OLC on the interface at the same time maintained the super-low friction state.

Fullerene-like carbon (FLC) possesses unique structures with curved and cross-linked graphene nanosheets covalently connected by tetrahedral sp3 carbon bonds,[126] which is comparatively similar with OLC. As mentioned above, the bonding state of PLC film is mainly amorphous. The occurrence of fullerene-like nanostructures in a-C:Hs endows the carbon films with unprecedented mechanical and tribological properties.[127] Wang et al. studied the FLC structure in detail and considered that the excellent lubrication depended on 44

the unique OLC structure formed on the contact interfaces.[128] Through analyzing the annealed hydrogenated FLC film from 200 to 500 °C in nitrogen, they found that the film exhibited maximal hardness and compaction in 300 °C, owing to which the FLC film had the optimal anti-wear and friction-reducing performances. In addition, annealing in different temperatures can simulate the actual application environments like in engines and piston rings as the properties of films are usually affected by high temperature and pressure. As one example in Figure 14, after friction test, the interlayer spacing of FLC decreased and induced strain between carbon layers, so the FLC transformed to OLC in which the reorientation and rehybridization of carbon layers occurred outside firstly.[129] However, the inner part was still amorphous as the initial outer carbon restricted the phase transformation.[130] A super-low friction coefficient of 0.005 was realized by low-shear stress owing to the formation of the OLC-like particles even if the inner was amorphous carbon. Doping of FLC provides another method to achieve a lower friction coefficient. A nitrogen-doped FLC:N film was prepared by combining plasma enhanced chemical vapor deposition with plasma nitriding so as to enhance the adhesion with the steel substrate.[131] Meanwhile, the iron nitride particles were introduced into the films through sputtering method. A super-low friction coefficient of ≤0.01 was then attained due to the introduction of Fe and N elements. Another reason about long wear life of FLC film is the nanocrystalline graphite formed on the friction interface.[132] In the vacuum friction test, the fullerene-like clusters fractured and then nucleated during the reconstruction process of tribofilm, in which the odd rings vanished and six-membered graphene sheet stacks were formed. Therefore, the friction coefficient of solid film was lowered and the service life was prolonged. 45

Figure 14 (a) Frictional behaviors of FLC and GLC films under different applied normal loads and (b) the formation of spherical onion-like nanoparticles with outer graphite shells. Reprinted from Ref. [129] with permission from Elsevier (Copyright 2018).

Even though the transition metal sulfides are not carbon-based lubricants, their interesting correlations with the fullerene-like structures are still emphasized here. For instance, MoS2 and WS2 are typical two-dimensional materials, and can be converted to fullerene-like nanoparticles in some conditions. Rosentsveig et al. obtained high quality fullerene-like MoS2 nanoparticles from the molybdenum oxide powder, and these nanoparticles as additives dispersed in PAO had an excellent lubricating performance than the pure PAO.[133] It is believed that the fullerene-like MoS2 nanoparticles possesses smaller size, so the agglomeration phenomenon can be effectively suppressed. And then the nanoparticles can enter in the gap between the tribocouples, resulting in the friction reduction. Joly-Pottuz et al. used inorganic fullerene-like WS2 nanoparticles as basic additives dispersed in oil, leading to the friction reducing and wear-resistance enhancing performances.[134] Nevertheless, the lubrication mechanism is different from MoS2 nanoparticles. It is probably 46

due to the fact that larger WS2 particles may behave as additional bearing structures around tribocouples even though not directly entering the contact area, which caused the friction coefficient to being 20% lower than MoS2.

The above studies show that the intrinsic spherical structures of OLC/FLC/fullerene-like nanoparticles are the key factors in obtaining superlow friction. Compared with the intrinsic structure of OLC, some spherical structures are transformed from other materials under certain conditions, and for instance, the formation of a scrolled structure of sp2-hybridized carbon layers also has a good lubricating effect. Weingarth et al. [135] used the nanodiamond particles (rich in sp3-C constituent) to prepare the OLC (made up of sp2-hybridized sites) at the annealed temperatures of 1300, 1500 and 1750 °C, respectively, in vacuum, indicating that the structure of carbon material can convert to others and thus form solid lubricants. As shown in Figure 15, Berman et al. [72] wrapped nanodiamond particles with graphene sheets to form an OLC structure, and obtained a macroscopic superlubricity in dry nitrogen atmosphere. Without adding graphene, the periodic shear stress and frictional heat generate the sp2-bonded graphene sheets on the interface. The highly active and unpaired σ bonds were easily adsorbed by nanodiamond cores existing at the edge, where a core-shell structure formed, and then the system energy reduced observably [72, 136]. However, only sp2-bonded graphene sheets cannot form a wrapping structure without adding nanodiamond cores, and the friction coefficient is greater than 0.2, failing to achieve superlubricity. This is because the sp2-bonded graphene wraps nanodiamond particles and then form a spherical OLC structure, which effectively lowers the contact area and the as-established incommensurate contact 47

further reduces the friction and wear.

Figure 15

Macroscale superlubricity endowed by graphene nanoscrolls formation in dry N2

atmosphere: (a, b) HRTEM images showing the as-produced wear debris after DLC sliding against graphene-plus-nanodiamonds and the as-formed core-shell nanostructures. Insets are the EELS spectra for both diamond and graphene found in the wear debris. Reprinted from Ref. [72] with permission from AAAS (Copyright 2015).

In addition to the above cases, many researchers have synthesized a series of spherical or scrolled OLC films with rich sp2-hybridized sites, and obtained a number of excellent tribological properties. Cao et al. proved that when the contact pressure was higher than 2 GPa, the tribolayer formed on the surface of OLC film produced an incommensurate contact, then obtaining industrial-level superlubricity.[137] The introduction of abundant Ar+ ions can increase the graphene nanostructure formation during the deposition process, so the prepared carbon film possesses more excellent recovery ability, superlow friction and wear resistance because of the more sp2-hybridized sites. Li et al. synthesized a high quality 48

nano-scroll composed of continuous monolayer graphene with a yield over 95 %.[138] While measured by AFM, the graphene nano-scroll has stronger adhesion than monolayer graphene, thus producing a spherical-like lubricating effect under the physical adhesion condition.

Besides the as-achieved experimental results, Shi et al. used DFT calculations to achieve the hybridization and interfacial binding energies of the sp2-bonded carbon layers,[139] and theoretically verified the orbital interaction on contact interface, thus favoring the design of superlow friction systems. Bejagam et al. employed reactive molecular dynamics (RMD) to simulate graphene wrapping over different nanoparticles.[140] The results demonstrated that the energy criterion promoted the formation of sp2-bonded graphene nano-scrolls, and provided the essential reference for preparing metal and graphene hybrid composites. Jariwala et al. used MD simulations to prove that the abundant hydrogen introduced in carbon materials

can

convert

the

sp2-C

to

sp3-C.[141]

Therefore,

tribotesting

in

a

hydrogen-containing atmosphere can form a passivation structure and effectively reduce friction, but it also inevitably destroys sp2-bonded structure. The above simulation calculations provided the theoretical bases about the sp2-bonded phase graphene of scrolling or wrapping particles to form lubricating structures, and the key factor of super-low friction is the unique rolling effect on the contact interface.

49

5. Superlubricity of ultra-nanocrystalline diamond Ultra-nanocrystalline diamond (UNCD), one of the smoothest diamond films, is a carbon material mainly composed of σ-bonds of sp3-C, which exhibits superior tribological properties in specific operating environments.[142, 143] Intrinsically, the friction performances of UNCD films are affected by grain size and orientation, amorphous matrix, sp2-C/sp3-C bonds fraction, chemical impurities in the grain boundary and surface roughness.[144] In general, this kind of sp3-rich carbon surface is subjected to poor lubricity in dry inert gaseous atmosphere or UHV, where the friction increases to a high level (i.e., µ=0.5-0.7) after a few cycles of low friction owing to the initial presence of adsorbed species on the surface.[145] Similar to hydrogen-free DLC films, the high friction phenomenon is generally attributed to some kind of “welding” phenomenon because of strong covalent interactions between surface carbon dangling bonds. In contrast, the most prominent characteristic of UNCD films is the ultralow or even superlow friction under specific tribotesting conditions in humid air. For traditional polycrystalline diamond films, the low friction and wear in ambient humid environments is speculated to be resulted from the phase transformation during sliding or the surface passivation of dangling bonds. However, for UNCD films, the experimental and theoretical studies by Konicek et al. have confirmed that surface passivation rather than graphitization/rehybridization is the dominating mechanism for the observed ultralow friction (µ = 0.008) and wear (on the level of 10-9 mm3/Nm) as negligible rehybridization was detected even in a high load and low humidity condition (Figure 16a).[146, 147] At a sufficiently high relative humidity level, the UNCD interface readily achieves a quite low steady-state friction coefficient. The pathway is the surface 50

passivation of carbon dangling sites by OH and H terminations from the tribo-induced dissociation of water molecules.[148] More specifically, as revealed by near-edge x-ray absorption fine structure (NEXAFS) shown in Figure 16b,[146, 147] and XPS,[144] the formation of hydroxyl and carboxylic functional groups such as C=O, O–H, C–COO, CH3COH and CH2–O on the carbon surface accounts for the ultralow friction resistance of UNCD films in the presence of moisture.

Figure 16 (a) Friction behaviors of UNCD film under different loads and relative humidities, and (b) NEXAFS spectra of C 1s recorded from the pristine and worn region of the film. Reprinted from Ref. [146] with permission from the American Physical Society. (Copyright 2018).

Interestingly, besides water-containing atmosphere, UNCD films can achieve ultralow friction in O2 and H2 environments,[145, 149] which is also attributed to the surface passivation effect from the oxygen and hydrogen fragments. The MD simulations and DFT

51

calculations provide strong theoretical support to the atmospheric effects and the surface passivation mechanism.[150, 151] However, the results indicate that both H2 and H2O + O2 environments can easily achieve low friction and adhesion interface by dissociative passivation of the surface with H and OH chemical groups at very low partial pressures. Nevertheless, only H2O vapor itself needs abnormally high partial pressure to effectively passivate the diamond surface. However, a trace amount of O2 can dramatically reduce the partial pressure of water for efficient passivation. In addition, by considering all the possible tribochemical reactions occurred in the modeling situations, the authors put forward the argument that the partial pressure as-required for realizing low friction and wear resistance increases rapidly with the environmental equilibrium temperature.[151] Meanwhile, this temperature-dependent vapor passivation is highly related with the intrinsic surface chemistry and crystalline orientations of the diamond film materials.

6. Superlubricity of carbon nanotubes Carbon nanotubes are composed of coaxial cylindrical graphene layers with a high aspect ratio, which are ideal friction pairs for the achievement of superlubricity, because its surface is atomically smooth, and can form the highly incommensurate lattice.[152] The theoretical calculations predict that the perfect multiwalled carbon nanotubes (MWCNTs) in the absence of any defect could become the ‘‘smoothest bearings’’, with nearly vanished friction because of the suppression of stick-slip motion when the carbon nanotubes are in the incommensurate contact.[153] However, the friction for nanotubes was difficult to measure at the nanoscale due to their nano-size diameters, and only a few experimental studies reported the sliding or 52

rotational behavior of MWCNT shells. Yu et al. used a mechanical-loading stage to realize the sliding between nested shells of MWCNTs in a SEM chamber. They observed a stick-slip motion and a smooth pullout motion for the two separate MWCNTs, which was attributed to the shear interaction, the “capillary” effect, and the edge effect in nanotubes.[154] Kis et al. performed

the nanotube sliding measurements

in

a TEM

chamber using the

nanomanipulation stage with a force sensor. The force acting between the core of MWNTs and the outer casing generally exhibited superlow friction, which was less than the measurement limit of 1.4 ×10-6 nN/atom and led to the total energy dissipation being lower than 0.4 meV/atom per cycle. Although some defects in the form of dangling bonds can lead to the temporary mechanical dissipation, the innate ability of nanotubes to rapid self-healing restores the smooth motion with ultralow friction.[155]

However, the above measured friction for the carbon nanotubes was usually much higher than the theoretical calculation, because it is difficult to produce the carbon nanotubes with ideal, defect-free structure. Many defects such as vacancies, adatoms, and Stone-Wales defects were inevitably introduced during carbon nanotubes growth, which can lead to much more frictional energy dissipation during sliding.[156] Recently, the superlubricity sliding has been demonstrated for the centimeter-long double-walled carbon nanotubes (DWCNTs) under ambient conditions after the success in producing the ultra-long DWCNTs with perfect atomic structures.[157] Zhang et al. focused on the fabrication of ultra-long DWCNTs to observe the structural superlubricity at macroscale contacts. The inner shell of DWCNTs was pulled against the outer shell in an incommensurate state (Figure 17), and the intershell 53

friction was measured during the pulling process. They observed that the friction had no correlation with the pull-out length and nanotube length, and the average intershell friction could be reduced to 2 nN, giving the shear stress of 2.6 Pa for 9-mm-long DWCNTs with outer diameter of 2.73 nm. The shear stress was at least four orders of magnitude lower than the latest reported value (0.04 MPa) for MWCNTs, which indicates the achievement of superlubricity state.[157] The superlubricity in DWCNTs was attributed to the incommensurate contact between inner and outer shells of DWCNTs and the absence of the energetically preferred position with respect to one another. On the other hand, the friction mainly arises from the very weak vdW forces between the inner and outer shells. In the overlapped section (Figure 17b), the shear stress vanished due to the repetitive breaking and reforming of vdW forces between adjacent shells.[158] Thus, only the edge section of carbon nanotube is responsible for the friction between inner and outer shells of DWCNTs during the pulling process, which is the reason why the superlubricity was independent of the length of carbon nanotubes. Therefore, the perfect structure of the DWCNTs plays the dominate role in the achievement of macroscale superlubricity of carbon nanotubes.

54

Figure 17 (a) Schematic illustration of an inner shell being pulled out of DWCNTs. (b–e) TEM images of different nanotube parts in the DWCNTs in (a) during pulling-out process. The scale bars are 5 nm. (f) vdW forces between inner and outer shells of DWCNTs during the pulling-out process. The friction force mainly comes from the vdW forces in the edge section of carbon nanotube. (g) Measured friction force for the five independent ultra-long DWCMTs during pulling-out process. Reprinted from Ref.[157] with permission from Nature Publishing Group (Copyright 2013).

7. Superlubricity of carbon nanostructures associated with liquid In addition to the above superlubricity achieved in the ambient condition or nitrogen atmosphere or UHV, the carbon nanostructures combined with aqueous solution can also 55

significantly reduce friction either by forming boundary molecular layers or fluid lubricating films, resulting in the superlubricity state.[159-161] It is completely different from the structural superlubricity, because there are no incommensurate contact, and instead the water molecules are essential in the achievement of superlubricity. Our group recently found that the friction coefficient between graphite and silica modified with amino groups was reduced to approximately 0.005 in sodium dodecyl sulfate (SDS) solution.[162] We observed that the SDS molecules could adsorbed on the modified silica and graphite surface by self-assembly, and therefore, the shear plane would transfer from silica/graphite interface to SDS molecules/graphite interface in the aqueous medium. The origin of the ultralow friction was attributed to the extremely weak interaction between SDS molecules and graphite surface, which provides an extremely low shear stress as well as extremely low adhesion.[162]

In additional to the SDS molecules, the phospholipid molecules were also attached on the silica probe to form the bilayer structure, and then slid the bilayer probe against the graphene layers (Figure 18a).[163] The friction coefficient was observed to fall to the level of 0.001, entering the superlubricity regime. MD simulation shows that there was a subnanometer hydration layer with a thickness of about 2 nm confined between the bilayer probe and graphene (Figure 18b), which remains as a liquid phase even under a normal pressure of 2.3 MPa. It is because the water molecules can be firmly attached onto the zwitterions in the lipid bilayer through hydration interactions to form the hydration shells, and meanwhile, the water molecules can also be adsorbed on the graphene surface to form the water cluster through weakly vdW interactions.[164, 165] Thus, when the lipid bilayer slid against the graphene 56

surface, the shear occurred at water/graphene interface due to the extremely low shear stress between water molecules and graphene surface, which is the origin of the superlubricity.[163] The friction force was observed to have a logarithmic increase with increasing the sliding speed, which was associated with the thermal activation process for sliding occurred at the liquid-solid interface.[166] This result demonstrates that the formation of hydration layer on the graphene surface is possible to achieve a superlubricity state.

Figure 18 (a) Friction forces versus normal loads between the lipid bilayer probe and graphene layers in water under the sliding velocity of 2000 nm/s. Inset is illustration of lipid bilayer probe sliding on the graphene surface in water. (b) MD simulation model of lipid bilayer sliding on the graphene surface in water. The grey atoms are silica, and the green atoms are the graphene layers. Blue and yellow atoms are the lipid bilayers. Reprinted from Ref. [163] with permission from the Royal Society of Chemistry (Copyright 2018). (c) Friction forces versus normal loads between the self-assembled hydrophobic fluoroalkyl monolayers probe and graphene layers in water under the sliding velocity of 4000 nm/s. Inset 57

is the illustration of the self-assembled hydrophobic fluoroalkyl monolayers sliding against graphene layers in water. (d) Shear stress as a function of sliding velocity in the experimental (1 – 50 µm/s) and simulation (104 – 106 µm/s) regions. The fitting line was from the equation of τ s = b + aln ( vs ) , where a = 7.9 kPa·s/m, and b =3.1 kPa. Reprinted from Ref. [167] with permission from the American Chemical Society (Copyright 2019).

Recently, it was found that the superlubricity of graphene was also achieved with a highly hydrophobic surface in water (Figure 18c). A self-assembled hydrophobic fluoroalkyl monolayers (SAFMs) was first formed on the silica probe, and then the probe slid on the graphene surface in water. The superlubricity is achieved with an extremely low friction coefficient of 0.0003 at contact pressures of up to 14.5 MPa, which indicates that the shear stress could become extremely low at graphene/SAFMs interfaces in water. MD simulation confirms that there was a thin water layer confined between graphene and SAFMs during sliding, which arose from the packing effect of water molecules on the atomically smooth graphene surface. Because the interactions between water molecules and graphene is extremely weak, the energy barrier for water molecules sliding on the graphene would become very small, which leads to the achievement of superlubricity. Moreover, the experimental results in the low speed range of 1 – 50 µm/s show a logarithmic increase of shear stress with increasing the sliding velocity (Figure 18d). The shear stress in the low speed range are consistent with the simulation results in the high sliding speed, confirming that the shear occurs at the water/graphene interface, which leads to an extremely low friction. These results provided the direct evidence that minimizing the friction of graphene 58

or graphite by introducing a self-assembly molecular layer was an effective approach in aqueous environments.

Moreover, the combination of carbon nanostructures and liquid was also conducive to achieve superlubricity at the macroscale. Matta et al. found that the friction coefficient of below 0.01 was achieved on DLC coated surfaces with the lubrication of pure glycerol at 80 °C at the macroscale.[168] The mechanism underlying this superlubricity is the formation of easy sliding interface on the tribo-formed OH-terminated surfaces. Besides, some evidences from both experimental and MD simulations showed that the extremely low friction could also be attributed to tribo-induced degradation of glycerol, generating a nanometer-thick film of organic acids and water at a lower temperature. During the recent five years, the carbon nanostructures were widely used in combination with oil or water based lubricant to reduce friction because of their excellent anti-wear and easy shear properties.[35, 169, 170] Ge et al. observed the robust superlubricity at the macroscale which was attained by taking advantage of the synergistic effect of graphene oxide (GO) nanoflakes and ethylene glycol (EG) at the ceramic/silica interface.[171] The friction coefficient decreased to µ < 0.01 after a running-in period of 10 min (Figure 19), and thereafter, the friction coefficient reduced further to approximately 0.004 and remained very stable. The wear results showed that the wear rate under lubrication with GO—EG solution was only 5% of that under lubrication with only EG solution. GO nanoflakes were directly observed to adsorb on the worn regions of friction pairs, thereby protecting the contact surfaces from severe wear and generating super-low wear volumes. Besides, the adsorption of GO nanoflakes contributes toward the 59

transformation of the shear plane from solid asperities to GO interlayers owing to their easy-shear property. Thus, the extremely low friction at the macroscale could be attributed to the extremely low shear stress of GO interlayers. Meanwhile, the generation of partial-slip boundary condition at the GO/EG interface together with the formation of hydrated GO−EG networks contributes toward the generation of superlubricity after the combination of GO nanoflakes and EG solution.[171] Moreover, they used GO nanoflakes in combination with ionic liquid as lubricant at ceramic/sapphire interface, and observed a robust superlubricity state (µ ≈ 0.005) at the macroscale under an extreme pressure of 600 MPa, which greatly improved the upper limitation of contact pressure of liquid superlubricity.[172] These work offer a novel method—combining carbon nanostructures with specific liquids—to achieve superlubricity at the macroscale.

Figure 19 Proposed mechanism underlying the achievement of liquid superlubricity by GO−EG. (a) Overall view of contact region; (b) Evolution of friction coefficient at the ceramic/silica interface versus test time with the lubrication of GO−EG, exhibiting the extremely low friction coefficient of 0.0037 – 0.0052 after a running-in process (c) Diagram of the asperities contact with GO nanoflakes in the contact region; (d) Diagram of confined 60

liquid film in the contact region with partial-slip boundary condition. Inset describes the schematic structure of hydrated GO−EG networks. Reprinted from Ref. [171] with permission from the American Chemical Society (Copyright 2018).

8. Discussions and conclusions Since the superlubricity concept was proposed, it has attracted more and more attentions from researchers in various fields of tribology, materials, physics, and chemistry. Many solid and liquid lubricant with excellent superlubricity properties have been found and designed. Among them, the carbon nanostructures and materials are most widely used as the friction pairs for superlubricity system because of their chemical inertness and easiness to fabricate and prepare, which has great potential in the engineering lubrication. Considering the different carbon nanostructures to achieve superlubricity at different scales discussed in this review, the superlubricity mechanisms of them are also different. With the development of AFM technology, MD simulation, and DFT calculations, the mechanism of superlubricity and frictional energy dissipation of these carbon nanostructures has been studied in detail from atomic to microscale, which helps us establish the conditions for robust superlubricity and eliminate the unfavorable factors causing the failure of superlubricity.

As for graphene, graphite, and carbon nanotube, the superlubricity can be mainly attributed to the formation of incommensurate contact, which directly depends on the lattice misfit between two contact rigid surfaces. The essence of the structural superlubricity mainly arises from the significant reduction of the energy barrier against atom sliding in the 61

incommensurate contact. Once the incommensurate contact is formed, the lateral force experienced by every atom in the interface during sliding can be eliminated by a corresponding atom, leading to the complete disappearance of friction forces. Therefore, the structural superlubricity of these carbon nanostructures is strongly dependent on the surface qualities, and often exhibits the sub-linear dependence of friction force on the contact area. Many experiment results showed that the small change of the qualities of graphene or graphite surfaces or test environments would cause the friction significantly increased, leading to the failure of superlubricity. For example, the surface defects, deformation, contamination, temperature, velocity, and humidity, could all lead to the increase of friction in the superlubricity state, which limits their actual applications because of the very high sensitiveness to these factors.

To better understand the superlubricity mechanism of them, MD simulations and DFT calculations were widely used in carbon layered structures to reveal the friction energy dissipation and evolution of contact structure during sliding process. Based on the simulations, several extrinsic factors were observed to suppress the superlubricity of graphene or graphite. The first one is the introduction of contamination within the friction interface, which could cause the pinning of incommensurate interface, and lead to the appearance of static friction and increase of kinetic friction.[49, 63] The second one is the applied normal load on the friction interface, which obviously leads to the increase of friction when it exceeds a critical value. It was also found that the edge atoms of a graphene flake in the incommensurate contact are most prone to the effects of an increased normal loads, and 62

may lead to the failure of superlubricity via their pinning to the underlying surface.[74] Furthermore, the dynamic reorientations of the sliding surfaces, which can lock the system into the commensurate contact, limit the realization of superlubricity to short timescales,[173] and the elastic deformation has also been predicted to cause the enhancement of friction in the incommensurate contact.[174] However, there are still several limitations in the simulations and calculations, such as the time- and length-scales of computational models are several orders of magnitude smaller than the friction force experiments while the shear velocities are much higher than the experiments. Therefore, it is a challenge to establish the accurate relationship between the superlubricity mechanism in simulation and superlubricity behavior in experiments at present.

As for DLC film, the origins of superlow friction are mainly based on two effects including

termination-atom-induced

surface

passivation

and

tribo-induced

phase

transformation with in-situ growth of a lubricating tribolayer. As revealed by MD simulations and tight-binding quantum chemistry calculations of bond population at the sliding interface,[84] antibonding interactions exist at the H- and F-terminated interfaces, indicating no covalent bonds are formed between the self-mated surfaces. More specifically, the F-passivated DLC surfaces exhibit even lower friction than H-passivated surface on account of stronger repulsive Coulombic force between fluorine atoms in contrast to the weaker van der Waals interactions between paired hydrogen atoms at the sliding interface. Besides, the possible presence of tribo-induced generation of hydrogen molecules between the contact surfaces exerts additional steric effect to remove the load from the substrate, prohibiting the 63

formation of C-C covalent bonds.[175] However, in some cases surface passivation is not sufficient, and tribo-induced carbon phase transformation of sp3-to-sp2 is another prevailing material behavior during rubbing, in which contact pressure is usually a remarkable factor determining the degree of structural change of hydrocarbon matrix and the stability of the sliding interface. This process is accompanied by the formation of a nanostructured tribofilm with an increased amount of sp2-bonded carbon as compared to the pristine film. As clarified by MD simulations,[176] the rehybridization of carbon is tribomechanically driven by the local fluctuations in contact stress that propagates from the continuously-sheared near-surface region into the stationary bulk of the film. Further sliding leads to the shear localization within this newly-grown sp2-rich tribofilm, which may effectively trigger the occurrence of more ordered nanostructures along the sliding interface and hence the acquisition of superlow friction. However, it should be emphasized here that in all these attempts, robust superlubricity was mostly achieved for the a-C:H films, which highlights the pivotal role of hydrogen in anti-friction events. On one hand, the formation of a nanoscale sp2-C-rich soft tribolayer with locally ordered nanostructures first guarantees the easy-shear capability; on the other hand, the presence of sufficient hydrogen in the tribolayer, especially the preferential aggregation in the near-surface area further allows the shielding of any chemical interactions between contact asperities. In common, the leading cause of superlow friction in UNCD films is ascribed to the surface passivation effect. As demonstrated by ab initio MD simulations,[148] in ambient environment the load-induced confinement is able to trigger the surface passivation of diamond by dissociation of water molecules. Specifically, the clean diamond surface is not favored for the appearance of low friction as the passivated surfaces 64

are more stable than the bare ones. There is a thermodynamic equilibrium of the adsorbates such as hydroxylation, H2O termination, hydrogenation and oxygenation with the environmental water vapor.[177] The preferential occupation by a specific termination is highly dependent on the relative portion of each component and its vapor pressure. The low sliding friction forces originate from the combination of a highly repulsive interaction at short range and a weak attraction at long range between the passivated surfaces.

There is also necessary to address the pros and cons between layered and spherical-like sp2-bonded carbon structures. The weak van der Waals forces between the sp2-C layers endow the layered carbon structure with easy-shearing nature during sliding, which is the major argument for the low or even superlow friction achieved for these lubricants. Robust superlubricity is more easily achieved for small-sized sp2-bonded structure such as graphene in nanoscale contact configuration rather than in macroscale regime, as the high-quality layered structure without defects and contaminants can be more readily guaranteed at this scale. Meanwhile, the established superlubricity system can bear high contact pressure up to the level of GPa. However, in macroscale, the random distribution of sp2-C layers and the possible presence of structural defects and environmental contamination result in rapid degradation of the lubricity and the relatively poor load capacity of the layer-like carbon, which is the case observed for graphene in most reported macroscale experiments. In comparison, spherical-like sp2-bonded carbon structure provides another lubricating pathway to avoid the direct contact between the sliding surfaces. In some extent, rolling is more effective to lower the interfacial friction as the force required to overcome the friction 65

moment is much smaller than the sliding mode. In addition, it is more feasible to reduce the contact area in rolling and suppress the intensive adhesive contact between surface asperities. Meanwhile, the possibility to establish an incommensurate contact is also significantly improved owing to the continuously-evolved interfacial structural features during rolling. However, in practice, it is quite difficult to synthesize complete spherical sp2-carbon structures, and lots of cross-linked network, dangling bonds and active sites are present within the bulk or on the surface of the OLC/FLC films. This drawback to the large extent limits the full exploiting of the potential of spherical-like carbon structures as anti-friction lubricants, and sometimes exposes a huge obstacle to realize superlubricity.

Although the superlubricity mechanism has been studied in detail by the combination of experiments and simulations, there are still several limitations or problems that suppress the development of superlubricity technology. The first one is that the high resolution and accuracy measurement technique for near-zero friction coefficient or friction force was poorly developed. According to the definition principal of friction coefficient (friction force divided by the applied normal load), the key factor for precise measurement of superlow friction is to accurately detect the infinite friction force. In a macroscale superlubricity state established by a common tribometer, the normal load is usually in the range of several or dozens of Newtons, while the friction force vanishes to the level of a few mN, which reaches the sensitivity limit of most force sensors. It is a great challenge to detect the real value if the friction force further decreased to the extent of micro-Newton. Similarly, in the superlubricity state established by a common AFM contact, it is a difficulty to precisely measure the lateral 66

force below pN for most laser-related position-sensitive detectors. Therefore, it is urgent to effectively resolve the coupling of normal load and friction force to improve measurement accuracy especially when the friction force vanishes towards several orders of magnitude smaller than the normal load. The second one is that the time- and length-scales of computational models are much smaller than the friction force experiments. Therefore, the development of simulation technology which can shorten the time- and length difference between the actual conditions and the simulated ones would be a top priority. Besides, exploiting more versatile potentials with the capacity to vividly capture the real tribochemical activities during the rubbing process is also an essential strategy for understanding the mechanisms of superlubricity. Finally, there is a common problem for all superlubricity states achieved by these carbon nanostructures; that is, the superlubricity is closely dependent on the friction conditions and test environments, such as contact size, surface defects, contamination, humidity, atmosphere, and so on. For example, superlubricity of graphite would fail when the contact size is greater than ~10 µm scales, superlubricity of carbon nanotubes would fail when there are some redundant atoms adsorbed on the nanotubes, and superlubricity of DLC would fail in the high humidity conditions. Therefore, the achievement of robust superlubricity over a wide range of sliding speeds, contact size, surface deformations, contact pressures, and temperatures was absolutely imperative for the application of superlubricity at present. Moreover, most studies on superlubricity achieved with these carbon nanostructures focused on the friction, and few studies discussed the wear resistance during the superlubricity state, which is an important factor that influence the lifetime of superlubricity. In fact, the wear would inevitably occur at the macroscale due to 67

the high contact pressure, and meanwhile a deep investigation in the material durability nature will definitely favor the thorough understanding of superlubricity mechanisms regarding the energy dissipation and material loss. Therefore, the superlow friction and wear should be considered simultaneously for the application of superlubricity to the actual engineering lubrication.

Figure 20 Summary of proposed mechanism of superlubricity for different carbon nanostructures, including graphene, nanodiamond, amorphous carbon, carbon nanoscroll, carbon nanotube, and carbon nanostructures associated with liquid. Reproduced and modified with permissions from Refs. [55, 72, 80, 148, 167, 178].

To eliminate the restrictions of superlubricity of carbon materials in actual engineering conditions, many efforts have been made to improve the superlubricity properties during recently years, such as formation of graphene nanoscroll (extend the superlubricity from micro to macroscale), formation of heterojunction (prohibit forming commensurate contact, 68

and weaken the anisotropy of superlubricity), formation of Si-DLC (suppress the moisture-induced

instability

of

superlubricity),

formation

of

multiple

asperities

configurations (reduce the negative influence of large contact area on superlubricity), and formation of hydration layer by introducing a weakly interacting molecular layer on the graphene (help achieve superlubricity of carbon materials in aqueous environments). However, it is still a challenge to optimize the superlubricity of carbon nanostructures for engineering applications, as the engineering conditions are incredibly harsh, such as rough surface, extremely high pressure or speed, contamination, and great variation of temperature or humidity. Therefore, how to achieve robust superlubricity at the different scales and conditions with a long lifetime (no wear) by the design of specific surface structures and interfaces maybe the focus of study for superlubricity of carbon-based materials in the near future. It is believed that the studies on superlubricity would be developed very fast with the rapid development of experimental techniques and computer technologies, and the robust nano- and macroscale superlubricity system would be finally designed and applied on the actual mechanical sliding system, such as micro- and nano-electromechanical system, automotive and space vehicles, ship, and so on. Once the promise of superlubricity is realized for these mechanical systems, it can not only save great energy consumption and reduce the environmental pollution during sliding process, but it is also conductive to designing the super-machines with near-zero friction and wear.

69

Acknowledgement The work is financially supported by National Natural Science Foundation of China (51775295, 51975314, 51605247 and 51527901), and foundation from State Key Laboratory of Tribology (SKLT2019C01).

Author Information Corresponding Author: *E-mail: [email protected] Notes: The authors declare no competing financial interests.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: