Approaches for Achieving Superlubricity in Two-Dimensional Materials

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Approaches for Achieving Superlubricity in Two-Dimensional Materials Diana Berman, Ali Erdemir, and Anirudha V. Sumant ACS Nano, Just Accepted Manuscript • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Approaches for Achieving Superlubricity in Two-Dimensional Materials Diana Berman*1, Ali Erdemir2, and Anirudha V. Sumant*3 1

Materials Science and Engineering Department, University of North Texas, Denton, Texas, 76203, United States 2

Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

3

Nanoscience and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, United States *

Corresponding authors: Anirudha Sumant ([email protected]), Diana Berman ([email protected])

Abstract Controlling friction in and reducing wear of moving mechanical systems are great challenges in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to designing materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and others opened an era for conformal, atomically thin protective coatings. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating these mechanisms by introducing 2D materials. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values—superlubricity—across scales from nano- up to macroscale contacts.

Keywords Superlubricity, 2D materials, graphene, friction, wear, solid lubricants, sliding interfaces, energy dissipation, nanoscale, macroscale

Vocabulary Superlubricity—Reduction of coefficient of friction to near-zero values, below 0.01. 2D materials— Crystalline layered structures in two dimensions consisting either single or few layers (up to 10) of atoms (e.g. graphene, hexagonal boron nitride (1 layer), and molybdenum disulfide (3 layers).

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Graphene—One atom thick single sheet of carbon atoms in a hexagonal arrangement. Coefficient of friction—Force acting against sliding, friction force, divided by the normal load. Wear—Physical loss of materials due to friction during sliding. Solid lubricants—Solid materials used to reduce friction. Sliding interfaces—Two surfaces that are in physical contact during sliding.


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1. Need for Atomically Thin Lubricants 2. Mechanisms of frictional energy dissipation and the methods for controlling them 2.1 Wear related 2.2 Molecular deformation 2.3 Thermal effects 2.4 Electronic effects 2.5 Bonding (Chemical bond formation and breaking) 2.6 Phonon effects (mass effect) 2.7 Environmental/Ambient Chemistry Effects 2.8 Structural effects 3. Going beyond the Limits of Friction 3.1 Superlubricity at the single asperity contact 3.2 Incommensurability of Surfaces for Nanoscale Superlubricity 3.3 Efforts toward Macroscale Superlubricity 3.4 Macroscale Superlubricity Observations in 2D Materials 3.5 Making a mesoscopic link 4. Conclusions and Prospects


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Need for Atomically Thin Lubricants Friction is an integral part of our everyday life.1-2 Every moving mechanical system around us consumes huge amounts of energy due to friction. Therefore, researchers have been looking for a long time for innovative ways to minimize adverse impacts of friction (and hence energy losses) even down to negligible levels (near-zero friction).3 This is referred to as superlubricity.4-7 Despite all these research efforts and fundamental approaches—which included many kinds of solid and liquid lubricants8—superlubricity has seldom been achieved at engineering scales or implemented in practical systems.9-11 Much of the difficulty is often due to the very complex physical, chemical, and mechanical interactions that occur simultaneously at the sliding interfaces of mechanical systems.12-13 Most existing studies on superlubricity focus on practical ways to diminish friction by controlling structural anisotropy in lamellar materials. However, the fundamental understanding of frictional energy dissipation due to surface chemical and physical effects has been somewhat neglected in previous studies.14-16 The widely accepted belief is that, without a clear grasp of the friction mechanism at the atomic level, predicting and controlling the tribological behavior of mechanical systems (whose applications range from nano- to macroscale) would be impossible.17-20 Interestingly, the discovery of graphene21 and similar two-dimensional (2D) materials such as molybdenum disulfide,22-23 boron nitride,24-25 and alike26 has reignited excitement in the tribology community, mainly because the excellent tribological properties of these materials lower friction to superlubric regimes.27-28 Although superlubricity at the atomic or molecular scale is more or less provided by desirable structural orders or orientations of graphite, MoS2, and other materials,29-31 transitioning such superlubric behaviors to macro- or mesoscale engineering systems has been very challenging.32 This review highlights recent advances in the fundamental understanding of the superlubricity regime in lamellar materials,7 including experimental efforts toward achieving macroscale applicability. We focus here on efforts directed toward achieving superlubricity in 2D materials because these materials present the greatest possibilities for practical applications; they control structural effects while limiting chemical effects due to reactivity with the surrounding environment.21, 33-35 Mechanisms of Frictional Energy Dissipation and Methods for Controlling Them In general, friction between two macroscale surfaces, when macroscopic objects touch each other at a multiple asperity contact,36-37 is proportional to normal force and independent of surface roughness, contact area, and sliding velocity.38 This simple explanation of friction at the macroscopic level is described by Amontons’ law.39 However, at the nanoscopic level, friction becomes far more complicated40; different processes contribute to energy losses during sliding and thus lead to friction. Figure 1 illustrates some possible mechanisms of frictional energy dissipation and highlights the frictional response of the system. These include (a) wear (energy is dissipated due to shear and removal of material from sliding surfaces); (b) molecular deformation (an effect of the elastoplastic surface deformation); (c) thermal effects (thermally activated energy barriers to overcome


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during sliding); (d) electronic effects (electrostatic charge generation, transfer, and discharge); (e) bonding (formation of the chemical bonds—at atomic/molecular levels—between sliding surfaces or top layer and substrate); (f) phonons (energy dissipation through lattice vibrations); (g) environment/chemistry (surface functionalization and contribution of the surrounding atmosphere during sliding); and (h) structural effects (locking-in of atoms on commensurate surfaces).

Figure 1. Representative schematics of possible mechanisms for energy dissipation during sliding: (a) wear, (b) molecular deformation, (c) thermal effect, (d) electronic effect, (e) bonding, (f) phonons, (g) environment/chemistry, and (h) structural effect. All of these mechanisms are discussed further in this paper.

The recent popularity of 2D materials and their usefulness for multiple applications, as well as their use in moving or sliding parts, has intensified the need to understand their mechanical and tribological properties.41 In atomically thick materials, fundamental frictional mechanisms should be assessed from another point of view. Below we review the possible frictional energy dissipation mechanisms that are most relevant to 2D materials and highlight studies that contribute to understanding of these mechanisms.

Wear Related Often, frictional energy dissipation is accompanied by physical damage to a material. This can take the form of either structural deformation and fatigue, or crack initiation and propagation that leads to the formation of loose wear debris. Such friction-induced wear often requires a certain amount of energy to be consumed or dissipated during sliding, so wear events can often be


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captured through changes in the frictional behavior of a tribological system. As wearing surfaces become rough and highly reactive (due to the creation of many defects and nascent surface atoms), frictional losses increase and therefore cause much higher energy dissipation. The results from wear-induced frictional losses are more typical of macroscale sliding, especially under conditions of high load, speed, and environmental effects. Graphene demonstrated the effectiveness in reducing friction42 and wear-related losses in steel43-44 and gold45 surfaces. Specifically, using a macroscale pin-on-disk tribometer, Berman et al.44 demonstrated that in addition to providing easy shearing at the sliding interface, graphene suppressed abrasive, adhesive, and corrosive degradations very effectively; prevented wear; and reduced friction. Figure 2 illustrates the dramatic improvement in the wear and friction performance of steel surfaces when graphene flakes are used on sliding surfaces.

Figure 2. (a) Coefficient of friction for steel sliding against steel in a nitrogen environment with and without graphene. (b, c) Ball and flat wear for steel sliding against steel in a nitrogen environment without graphene and (d, e) with a single layer of graphene. Reprinted from [44] with permission from Elsevier. As graphene flakes wear out or agglomerate into clusters, friction increases substantially46. Eventually, this triggers a high-wear regime due to the creation of many surface defects, wear debris, and a very rough surface finish. Molecular Deformation The second mechanism of friction is associated with the deformation of large molecules present on or near the surface of sliding counterparts. Burns et al. 47 demonstrated that tensile deformation and collective motion of the thiol chains in densely packed self-assembled alkanethiol monolayers provide additional channels for energy dissipation and cause friction to


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increase. Such molecules may also trigger adhesive interactions between mating surfaces, depending on their chemical and/or physical affinity toward the surface. For 2D materials, the effect of molecular deformation is not straightforward. The structurally defect-free and highly chemically inert nature of thin membranes precludes sources of molecular deformation and gross adhesive interactions. However, in this case bending and stretching in the lattices act as energy dissipation mechanisms. For example, the atomic force microscopy (AFM) measured friction reduction in 2D materials with increasing number of layers48 was thought to be associated with changes in the viscoelastic properties of the graphene flakes,49 mainly because the multiple layers can reduce the elastic deformation of supported materials. Furthermore, for free-standing membranes, bending flexibility largely affects frictional behavior even at negative loads, when the AFM tip is pulled off the sample.50 In a representative study of out-of-plane deformation on friction behavior, Deng et al.51 demonstrated that delamination of the topmost layer in a graphite sample with a normal load decrease increases the lateral stiffness of graphene, and so does friction. Figure 3 illustrates the friction force response for an ultra-nanocrystalline diamond AFM tip sliding on a stack of graphene layers. This highlights the schematic of the delamination process taking place during the pull-off regime.

Figure 3. (a) Friction-load curve indicating the negative friction trend for retracting regime. (b) Schematic representation of approach-retract hysteresis illustrates the deformation of several surface and subsurface layers. (Reprinted with permission from Macmillan Publishers Ltd: 51, copyright (2012)). Thermal Effect Another important mechanism that influences the frictional properties of all tribosystems can be attributed to the thermal activation of the atoms to move around and across the interface as contacts are formed. These atomic motions may occur spontaneously at a certain rate for any given temperature. In addition to the spontaneous jumps due to ambient heat, external and/or frictional heating due to sliding may further increase thermally activated processes on the surface. These two effects may dominate over each other for the different ranges of sliding


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velocities. Riedo et al.52 investigated the correlation between friction force and thermally activated atom jumping in real contact spots using an AFM when a silicon tip sliding on a mica surface. The study revealed that the corrugation of the atoms at contact determines the frictional response of the system. When the interaction potential at the sliding interface is more corrugated at a fixed velocity of sliding, energy dissipation—or friction—is lower due to the strong effect of the thermally activated hopping process, which eases sliding. Therefore, this thermal effect on friction results in a decrease in sliding friction as temperature increases in standard threedimensional materials. However, in 2D materials, the trend toward friction dependence on temperature can be reversed completely. Theoretical studies53 demonstrated that, when the temperature of the sliding system increases, thermally induced dynamic rippling of free-standing graphene membranes can lead to higher friction between graphene and a tip made of a single-wall carbon nanotube. Figure 4 illustrates the positive change in friction response with temperature for various tip loading. This dependence is believed to arise due to increased bending stiffness of the membrane, which produces high dynamical corrugation of the surface at elevated temperatures. As a result, during lateral scanning, the tip interacts not with the atomically flat surface of graphene, but with the thermally excited ripples of the 2D material.


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Figure 4. Dependence of friction force on temperature at various loads for (a) supported graphene and (b) suspended graphene normalized with respect to the value at T = 2K. (c) Schematic representation of the rippling process for (d) supported and suspended sample configurations. Adapted from [53] with the permission of the Royal Society of Chemistry. As expected, the thermal-rippling effect on friction diminishes for supported graphene (Figure 4b), when thermal smoothing of the graphene-tip energy profile causes friction to decrease, similarly to bulk material. In addition, the data demonstrate that friction trends inversely to temperature dependence for the restrained graphene.

Electronic Effects Another mechanism that contributes to friction also corresponds to electronic effects. In particular, static electricity buildup can influence friction between sliding surfaces, so electrostatic forces can increase friction. Dayo et al.54 reported that the coefficient of friction dropped by a factor of 2 for ultra-thin lead samples sliding across frozen nitrogen layers below the lead superconducting transition temperature. In this case, lead-coated quartz crystal microbalance (QCM) electrodes oscillate in the shear mode, and the electronic contribution to friction is eliminated during the superconductivity transition. In general, theoretical studies have predicted the electronic contributions to friction from 2D materials.55-57 Using density functional theory calculations, Wang et al. revealed that there is lower friction between hydrogenated graphene layers sliding against each other than between pristine graphene layers.56 The calculated coefficient of friction values for hydrogenated graphene sheets ranged from 0.01 to 0.05 under applied load variation; these values were five times lower than for pristine graphene sheets under the same loads (Figure 5). The reduction of friction is thought to originate from the accumulation of charge between carbon and hydrogen atoms, which results in electrostatic repulsion of the hydrogenated graphene sheets as they slide against each other.


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Figure 5. Side view of total charge distributions: (a) graphene and (b) single-side hydrogenated (SSH) graphene. The large spheres are carbon atoms, and the small gray spheres are hydrogen atoms. In comparison to pristine graphene, charge redistribution in hydrogenated graphene films results in lower friction when sliding against each other. (c) Coefficients of friction for SSH graphene as a function of the loads. The coefficients of friction for clean graphene are also shown for comparison. Adapted from [56] with permission from Springer. Note that for 2D materials, especially at macroscale friction regime, the majority of the electronbased contribution to friction arises from van der Waals and electrostatic interactions between the 2D layers. These forces are relatively weak and allow the layers of 2D materials to shear easily or separate, and thus they readily slide against one another. The 2D nature of these materials limits interactions between them when the planes are in an incommensurate state. In addition, in some of the 2D materials (in particular graphene), high electrical conductivity works against electrostatic charge buildup and curtails electronic contributions to friction. Bonding (Chemical bond formation and breaking) One of the most important issues associated with an increase in friction is related to chemical interactions at the asperity contacts. The formation and breaking of bonds during relative motion can account for this. Chemical bond formation often acts as an adhesion modulator. A recent study58 demonstrated that 2D materials—in particular, graphene—are capable of reducing the effect of surface functionalization on adhesion between sliding surfaces. In an AFM study involving friction between a silicon tip and graphene layers deposited on rough surfaces, when the rough surface (made of 20-nm-diameter silica nanoparticles, or NPs) was chemically modified to increase either hydrophobicity or hydrophilicity of the surface, there were substantial differences in the frictional behavior of graphene as a result. Figure 6 presents friction force maps for graphenecoated nanoparticle layers. In absence of graphene, friction of bare silica nanoparticles is larger than the friction for octadecyltrichlorosilane (OTS) coated nanoparticles. Such a difference is attributed to a hydrophobic nature of the OTS coating. However, once the graphene is introduced on top of the nanoparticles, the friction change is reversed.


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Figure 6. Contact mode AFM topography and lateral force images of graphene films on ((a) and (b)) hydrophilic surface of clean silica nanoparticles and ((d) and (e)) hydrophobic surface of OTS modified silica nanoparticles. (c) The friction signal of graphene on hydrophilic surface is normalized to demonstrate the different trend in friction as a function of number of layers for sharp and blunt probes. (f) The effect of graphene coverage on hydrophilic and hydrophobic surfaces. Reproduced from [58] with permission from the Royal Society of Chemistry. The introduction of a second body (i.e., graphene) results in competition between two kinds of adhesion mechanisms involving the graphene substrate and the graphene tip; in general, the chemical modification of the original surface determines which adhesion mechanism dominates and drives the resulting friction values. Specifically, when graphene coats hydrophobic OTS NPs, low adhesion between graphene and underlying NPs allows more flexibility in graphene’s movement and leads to formation of larger contact area between the graphene surface and the tip, thus leading to higher friction. In contrast, if the NPs are made hydrophilic by means of a silanol treatment before graphene is applied to the surface, friction decreases as graphene’s out-of plane deformation is ceased by the adhesion between the graphene and the substrate. Minimizing the friction by introducing a weakly interacting molecular layer was demonstrated as a possible approach for efficient lubrication of 2D materials in liquids59. The chemical effect on the tip-sample adhesion, and thus on friction, in 2D materials was highlighted further by Deng et al. 51. By exposing the freshly-cleaved graphite samples to oxygen environment for different duration, the authors modified the adhesion between the sample and the ultrananocrystalline diamond AFM tip; leading to partial delamination of the top-most graphene layer as the adhesion energy was higher than the exfoliation energy for graphite. Interestingly, they have observed “negative friction”(increase in friction with decreasing load)


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while retracting the tip away from the sample. The higher adhesion of the oxygenated graphene resulted in pulling the graphene upwards with the tip during tip retraction with larger tip-surface contact area and thus increasing the friction. This experiment nicely demonstrate how surface coverage of the oxygen critically affect tip sample interaction impacting the nanoscale friction. Phonon Effects (Mass effect) One of the scientific wonders associated with friction is when heat is generated during sliding in the absence of wear. Theoretical predictions associated with this phenomenon rely on the atomiclevel dissipation of energy through lattice vibrations (so called phonons), optical excitations (photons), electronic excitations (exoelectrons) and the like at or near the surfaces of materials during sliding.60-63 Exoelectron emission was detected from a graphite sample during a vacuum pin-on disk tribotest64 due to material fracturing65, while optical excitations, or triboluminescence, were observed during peeling of an adhesive tape due to breaking of the chemical bonds. Among other excitation mechanisms, lattice vibrations are considered the dominant frictional energy dissipation mechanism and thus discussed in more details. In particular, the general expectation was that the surface monolayer should have acted as a kinetic energy-transferring medium, and that the rates of energy dissipation are proportional to the vibrational frequency of the atoms, resulting in higher friction for lighter atoms than for heavier atoms.66 When the tip slid over the surface, part of the tip’s kinetic energy translated to the surface atoms being transferred back to the tip, which helps to overcome the energy barrier for the tip to slide to the next position. However, for lighter atoms, the translated energy very easily dissipates through interactions between surface atoms; thus, much smaller amounts can transfer back to the tip. Cannara et al. 67 demonstrated the effect of lattice vibrations on friction. In this study, singlecrystal diamond and silicon substrates were terminated with hydrogen and deuterium atoms using a hot-filament process, which resulted in greater mass changes for deuterium atoms than for hydrogen atoms, although the chemical composition of the surface atoms remained the same. For both diamond and silicon substrates, the results confirmed that friction decreased after the surfaces terminated with the heavier atoms, such as deuterium, as compared to the hydrogenterminated ones. De Mello et al.68 further analyzed the contribution of phonon dissipation in nanoscale friction when light or heavy atoms are incorporated into bulk material. Continuous incorporation of the deuterium heavy atoms into amorphous carbon (a-C:D/H) films as replacements for hydrogen atoms resulted in decreased friction for a diamond spherical dome sliding on the a-C:D/H surface. For 2D materials, the phonon energy dissipation effect was demonstrated on MoS2 surfaces.69 The study used a combined AFM with a QCM system to probe the changes in friction between a silicon nitride tip of AFM and a MoS2 flake transferred and aligned on the QCM electrode (Brillouin zone Γ to K or Γ to M directions are aligned with the oscillation direction) as a function of the QCM oscillation amplitude. Lattice strain was introduced by a tip contacting the MoS2 surface. The acoustic phonons were generated and propagated through lattice strain relaxation with a frequency of approximately 5 MHz, which allowed the energy dissipation to be tuned by adjusting the QCM amplitude to fit the strain. When the lattice strain decreased, the


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released dissipation energy increased, indicating higher frictional losses and lower quality of the system (Figure 7). This dissipation was invariant of the applied load; it strongly depended on the direction of shearing. In the Γ to K direction, one-dimensional lattice strain induces longitudinal acoustic (LA) phonons, while in the Γ to M direction, 2D lattice strain induces transverse acoustic (TA) phonons.

Figure 7. Phonon effect on friction in MoS2 film. (a) Schematic of experimental setup used for MoS2 aligned with (b) Γ to K and (c) Γ to M directions along the shearing direction. Changes in energy dissipation for (d) Γ to K and (e) Γ to M directions correlate with one-dimensional and


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two-dimensional phonon coupling. Reprinted with permission from American Physical Society.

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. Copyright (2016) by the

Prasad et al. measured the role of phononic coupling in frictional dissipation using molecular dynamics (MD) simulations for carbon nanotube (CNT) oscillators.70 The authors evaluated the effect of contact area, chirality, CNT’s end configuration, and commensurability on phonon coupling between two coaxial CNTs sliding against each other. By looking at phonon scattering dynamics, they demonstrated that phononic friction is a net effect of two distinct coupling mechanisms for longitudinal and transverse phonon modes. Friction is affected by LA phonon scattering along the contact length, and incommensurability of the contacting surfaces decreases LA scattering. For transverse phonons, friction is affected by severe scattering at the ends of the CNTs where the translational symmetry is broken. Environmental/Ambient Chemistry Effects In previously highlighted friction mechanisms, the majority of the effects influencing friction were due primarily to the interactions between solid surfaces. Such assumptions can be true for ultra-high vacuum conditions. However, in many cases the surrounding environment, which contains specific gas chemistry, could significantly affect friction and wear by modifying the surface chemistry of the sliding surfaces. It is important to note that shearing of solid/solid interfaces under high contact pressures may give rise to heating-related problems and mechano-chemical reaction-induced changes in the materials at the shearing interface. Walker et al.71 demonstrated that an increase in the speed of QCM oscillations with adsorbed krypton layers sliding on graphene resulted in heating at the interface, which affected the frictional response of the whole system. Such an effect may be important in low-temperature sliding experiments and needs to be considered. It is important to account for the effects of the test environment when observing tribological pairs sliding against one another. Specific mechanisms of environmental effects could include water adsorption on the surface of interest or an environmentally induced tribochemical reaction. In the first case, the presence of water leads to an increase of capillary forces acting on the probe during AFM lateral force measurements72 or suppresses the formation of low-friction structures at the sliding contact interface.73-74 In the second case, environmental factors can lead to the formation of chemical and structural states that could be more or less tribologically favorable. For 2D materials, the influence of different environments on sliding has been demonstrated when graphene used as a solid lubricant for sliding steel surfaces in macroscale pin-on-disk tests.75 Specifically, in the tests performed, single-layer graphene sheets grown via chemical vapor deposition (CVD) were transferred onto a flat steel sample, and then their friction behavior was monitored against a steel ball counterpart. Graphene reduced the friction of steel against steel, independent of environment, but the lifetime of such a thin protective coating varied. When the tests were performed in a nitrogen environment, a single-layer film of graphene survived for only 500 cycles (approximately 47 meters). In contrast, when the environment consisted of hydrogen gas, graphene lasted for 6,500 cycles (612 meters), demonstrating that graphene is an excellent solid lubricant. Replacing single-layer graphene with few layers of graphene flakes led to an


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even more substantial increase in the lifetime of the coating in the hydrogen environment; it lasted up to 47,000 cycles (approximately 4,500 meters). The prolonged lifetime of the graphene coating resulted in a substantially lower wear rate for sliding steel surfaces. The role of hydrogen is believed to be as follows: as a reactive gas it reduces or removes oxygen from the steel surfaces, thus suppressing the formation of iron oxides.76-77 It also passivates the dangling carbon bonds of ruptured graphene during sliding, preserving its integrity and stabilizing it on the steel surface for a much longer time (see Figures 8a–8c).78-79 Because nitrogen gas is very stable and thus does not offer this stability or opportunity for repair, the graphene layers were fragmented and destroyed. This shortened the time to failure or removal from the surface (see Figures 8d–8f).

Figure 8. Coefficient of friction and wear of the graphene-coated steel counterparts sliding (a and b, respectively) in a hydrogen environment and (d and e, respectively) in a nitrogen environment. Molecular dynamics simulation results demonstrate (c) passivating of graphene dangling bonds in a hydrogen environment and (f) no preferential sites for physisorption in a nitrogen environment. Adapted with permission from Ref. 75. Copyright 2014. Wiley-VCH. The observed effect of surface chemical modification on friction was reported for fluorinated graphene in comparison to pristine graphene.80 Even though the adhesion between the AFM tip and the 2D film decreased during fluorination, lateral force measurements indicated an increase of up to 6 times more friction. The density functional theory calculations demonstrated higher bending stiffness of the fluorinated graphene, which results in additional resistance to the movement of the tip along the load-deformed graphene film (Figure 9).


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Figure 9. Increase in the friction observed for graphene upon fluorination due to increased outof-plane stiffness. Reprinted with permission from [ 80]. Copyright (2012) American Chemical Society.

Structural Effects The last effect, which requires a separate discussion due to its particular importance for 2D materials, is the so-called structural effect, which is solely dependent on the commensurability/incommensurability of the surfaces. When the lattices of two ordered materials perfectly match one another, and are aligned in the direction of sliding, a commensurate state of contact prevails. Such surfaces will experience atomic-level interlocking (Figure 10a) and thereby strong adhesion and friction. Mismatching of the lattices, or creating a state of incommensurability between the sliding surfaces (Figure 10b), prevents interlocking and thus dramatically reduces friction to near zero (this is often referred to as structural superlubricity). In the real applications, the surface structure is not completely uniform (due to the presence of many grains with different crystallographic orientations); therefore, when two surfaces are in contact, they will suffer from local commensurability, and higher friction will ensue.


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Figure 10. Schematic representing (a) commensurability and (b) incommensurability of two surfaces in contact. (c) Commensurability effect demonstration in graphene planes. (Reprinted with permission from 30. Copyright (2004) by the American Physical Society.) Bylinski et al.81 presented a very good example of the importance of incommensurability between the atomic planes of crystalline materials. The authors created an ionic-crystal nanofriction simulator by implementing a sliding interface between a laser-cooled Coulomb crystal of individually controlled ions and a periodic light-field potential. The mechanism considered for the friction increase is associated with the energy needed to overcome the energy barrier. When the ions match in accordance with the periodicity of the energy barriers, the total energy needed to move the system of the ions across the energy barrier network is composed of the sum of small energy jumps. When the lattice is mismatched, as it is in incommensurable graphene surfaces, only a limited number of ions are trapped at any given moment, so less total energy is required to initiate movement; this is similar to “caterpillar movement.” Interestingly, the formation of small defects on the sliding surfaces, as demonstrated by the propagation of kinks and antikinks, becomes energetically favorable for overcoming surface commensurability and reducing the resulting friction during sliding.82 The discovery of surface commensurability/incommensurability in 1991, and its effects on friction,83 has led to multiple efforts to transfer this theoretical prediction to real experimental observations. 84 Importantly, the clear evidence for the effect of incommensurability was demonstrated with 2D materials 29-30 indicating its major role in atomically thin layers. Structural ordering of single-domain 2D materials, as in case of graphene, allowed precise control of a degree of commensurability between two sliding surfaces. In the experiment,29 sliding a tungsten tip against a graphite substrate caused the graphene flake to transfer onto the tip. Consequently, all the following measurements were attributed to graphene versus graphene sliding. Controlling the rotational angle of two graphitic surfaces demonstrated a frictional symmetry of 60 degrees; this can be attributed to the symmetry of the graphene lattice (Figure 10c). When there is no rotation between two planes of graphene, the surfaces are perfectly commensurate, which should cause them to interlock during sliding. A small degree of rotational misplacement leads to the loss of commensurability, which results in ultra-low friction. Structural lubricity was also achieved for 2D materials in combination with other structures, such as a gold-graphene interface.85 Notably, apart from the well-known commensurability effect, that can give rise to friction, some nanoscale morphological corrugation have also shown to affect friction. Shi et al. demonstrated an unusual stick-slip instability for friction force microscopy measurements performed for graphene grown on ruthenium Ru (0001) substrate84. Instead of smooth friction signal, the large sliding energy barrier arising from the graphene’s morphological corrugation leading to longrange stick-slip behavior, which presents another interesting approach to manipulate friction. The chemical inertness, atomically flat nature, and excellent mechanical properties of 2D materials along with their ability to shear and constantly readjust with respect to the sliding counterface make 2D materials the perfect candidates for friction manipulation and wear reduction. Therefore, these 2D materials are very attractive for studies related to structural superlubricity.


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Going beyond the Limits of Friction The previous section highlights the major mechanisms of energy dissipation that contribute to friction. The class of 2D materials—with their ultra-thin layered structure and ability to shear easily, and their excellent mechanical properties—opens possibilities of controlling friction, and thus provides an ideal material to achieve low-friction regimes. The structural composition of 2D materials produces higher mechanical stability and strength in 2D lattices, and both of these qualities reduce wear-related friction. In addition, the impermeability of 2D materials, especially graphene, helps to prevent tribo-corrosion by passivating the surface and locking the surface chemistry. The easy shearing of the 2D flakes on the underlying substrate helps reduce friction, and more importantly changes the boundary conditions constantly at the tribological interface as the counterface geometry changes with wear; this process maintains a kind of dynamic equilibrium that no other tribological coating can provide. The major missing component in proceeding to the superlubricity regime is establishing perfect incommensurability between two surfaces. In the following sections, we summarize the recent progress in overcoming the scale effects on achieving superlubricity with 2D materials, from nanoscale lattice mismatches up to macroscale phenomena. Our goal is not to repeat the specific cases that were covered in the previous section and the other studies that deal with the classical frictional behavior of 2D materials. Instead, we highlight the most representative cases that successfully demonstrate superlubricity at various length scales, and we outline the major mechanisms responsible for such observations from nanoscale up to macroscale superlubricity.

Superlubricity at Single Asperity Contacts Traditionally, the single asperity configuration is used as a representative model for AFM lateral force measurements, when the actual contact during sliding is determined by nanoscale contact between the tip and the substrate. For single asperity contacts with 2D materials, the great majority of tribological effects are predefined by the contact areas between the tip and the substrate, with 2D materials in between. Experimental studies demonstrated that the single asperity friction of 2D materials is highly dependent on the number of layers,48 and that it approaches minimal values when the thickness of the graphene, boron nitride, and molybdenum disulfide films is higher (Figure 11). The origin of this effect was correlated to puckering of the 2D material around the tip during scanning. Recent theoretical studies also suggest that evolution of the contact over time provides an additional means for controlling friction.86 Specifically, created at the beginning of sliding, contact load-induced deformation results in creep and hence contact area increase87. This regime is accompanied by an increase in friction.


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Figure 11. (a) Optical and (b) AFM images of 2D materials analyzed with (c) lateral force microscopy. (d) Friction decreases as the number of layers increases for all four types of materials: graphene, MoS2, NbSe2, and hexagonal boron nitride. Reprinted from [48] with permission from AAAS. The main friction contribution during AFM measurements is attributed to the flexibility of 2D materials to move in an out-of-plane direction,88 when adhesion of films to the supporting substrate is weak.89 During the scanning, this leads to ability of 2D materials to coat the sliding tip and results in large contact area and friction. Increased adhesion to the substrate is key because it reduces out-of-plane puckering. This, in turn, decreases the friction: increased adhesion can be achieved through either deposition procedure 90 or the use of a hydrophilic substrate.89 AFM lateral force measurements demonstrate that CVD-grown graphene can exhibit ultra-lowfriction behavior at the nanoscale.91 In this case, the impact of friction modification is attributed to the strong adhesion between graphene and the underlying substrate, which consequently leads to reduced friction, even when compared to the previously observed highly oriented pyrolytic graphite (HOPG) near zero friction (Figure 12). Specifically the average lateral force, calculated


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as half of the difference between forward and reverse direction measurements, is substantially reduced for various AFM tips sliding against graphene in comparison to bare silica substrate (the lateral force for silica is ~18 nN, while for HOPG and for CVD graphene it is 1 nN and 0.5 nN correspondingly).

Figure 12. (a) Normalized friction response with (b) actual friction values for three different tips sliding on SiO2, HOPG, and CVD-grown graphene. Lateral force images and the detailed forward and backward lateral force signal for (c) SiO2, (d) HOPG, and (e) CVD-grown graphene when the diamond-like carbon (DLC) coated tip was used. Line-scan lateral force profiles included at the bottom of 12(c), 12(d) and 12(e) highlight reduction in the friction from 18 nN for SiO2 to 1 nN for HOPG and to 0.5 nN to CVD-grown graphene. A 25-nN normal load was used for these measurements. Reprinted from [91] with permission from Elsevier.


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Importantly, 2D films should demonstrate high quality and minimum defects in order to exhibit low friction.91 Oxygenation of the graphene surface dramatically affects the properties of materials and leads to high friction values.

Incommensurability of Surfaces for Nanoscale Superlubricity Although single asperity contact sliding enables controlled surface interactions and structural uniformity of the materials, conversion to real-scale applications often involves multiple-asperity contact. In this case, friction minimization is widely believed to be associated with the surface’s incommensurability. The effect of incommensurability on achieving superlubricity in 2D films has been demonstrated extensively in the literature. The first-ever experimental demonstration of structural superlubricity by Dienwiebel et al.29-30 reported that lattice mismatch between two sliding graphite flakes was confirmed by the measurements of the flakes’ rotation effect and resulted in ultra-low-friction behavior. Koren et al. highlighted a similar mechanism92 for sheared graphite structures. In this case, Pd and Au metal masks are lithographically fabricated on the freshly cleaved HOPG sample and cold-welded to the Pt/Ir tip. After dry oxygen plasma etching, the stacks of graphene with metal on top can be sheared against the bottom HOPG surface. The measured shear force required to move the tip with the attached stack at zero applied normal load reflects the tendency of the system to reach superlubric perfect alignment of two graphitic planes over each other (Figure 13).


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Figure 13. Measured shear force versus tip displacement. Black and blue curves denote the scan direction from left to right and from right to left, respectively, as indicated by the arrows. Inset: Enlarged view of the shear force for a trace and retrace scan showing the nonreversible friction contribution. L and R represent the scanning directions from left to right and from right to left correspondingly. (From [92]. Reprinted with permission from AAAS.) Minimizing friction due to lattice mismatch led to spontaneous effects of graphene-on-graphene sliding, when the incommensurate registry is induced either manually with the scanning tunneling microscope tip93 or by thermal fluctuations generated with the shear oscillations of the nanoindenter.94 In both cases, initial energy is still necessary to initiate the superlubric sliding of 2D materials in an incommensurate state. In addition, superlubricity is achieved due to the lattice mismatch of graphene nanoribbons on a single-crystal gold surface.95 Although graphite and graphene serve as model systems for manipulating commensurability, recent developments in 2D materials synthesis24 allow to transfer this mechanism towards other 2D materials. Ultra-low friction has long been observed for molybdenum disulfide films in an incommensurate state.96 A recent study used an in-situ scanning electron microscope to observe sliding of single-layer MoS2 flakes attached to a Si nanowire on the MoS2 film.96 The tests show that the friction coefficient decreased to 10-4 values as the incommensurability requirement was met. Dietzel et al.97, analyzed the limitations of structural superblubricity by measuring sliding friction of differently sized antimony particles on MoS2 and HOPG substrates. The nanomanipulation experiments revealed: when the particle radius is larger than the characteristic length scale of the Sb/MoS2 system, high chemical interaction energies lead to loss of incommensurability-driven superlubricity. In case of HOPG, the chemical interactions are weak and can be neglected. Further analysis of the effect sliding speed of the friction indicated regimes of so-called ballistic friction, when friction drops down to superlubricity due to preventing of the angular pinning during fast sliding motion.98 The effect was demonstrated for a gold cluster sliding on a graphene surface. Efforts toward Macroscale Superlubricity The importance of building perfectly incommensurable contact areas for achieving superlubricity, as demonstrated at the nanoscale, can hardly be overemphasized. At the same time, advances in materials development and fabrication will lead to a focused pathway of transitioning the near-zero friction regime to macroscale observations. Substantial changes in friction mechanisms when transitioning from nanoscale to large scales40 pose major challenges in observing superlubricity. Therefore, previous studies have concentrated on building ideal structured materials that have perfect crystallinity and are free from defects, on a larger scale. For example, in a representative study, Liu et al.99 focused on coating a microsphere with graphene and sliding it against graphene and hexagonal boron nitride (Figure 14). Observed ultralow friction (with a coefficient


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of friction as low as 0.003) originated from the lattice mismatch of 2D planes leading to overall incommensurability of the multi-asperity contact. This microscale superlubricity was demonstrated even under high contact pressure (up to 1 GPa) and in a high-humidity environment.90

Figure 14. Friction forces as a function of the applied normal load for graphene-coated microsphere sliding against (a) graphene film and (b) hexagonal boron nitride film. (Adapted from [99] with permission.) Ultralow-friction sliding has been also demonstrated for graphene rolled into one-dimensional structures known as carbon nanotubes. Zhang et al.100 focused on fabricating centimeter-long carbon nanotubes to demonstrate the nanoscale structural superlubricity effect at macroscale contacts. In this case, the inner and outer shells of double-walled carbon nanotubes were pulled against each other in an incommensurate state. Because of the perfectly incommensurable contact, friction that arose during inner sliding of two shells in a double-wall carbon nanotube diminished to nearly zero.

Macroscale Superlubricity Observations in 2D Materials In all previous studies, achieving an ultralow-friction regime relied heavily on structural perfection in 2D materials, which allowed the roughness of the substrate and defects in the membrane to be neglected. At the macroscale regime, however, controlling the ideal structure of the materials in sliding contact was almost impossible. This dependence of superlubricity on the uniformity of the 2D material has been the main reason previous efforts failed to demonstrate near-zero friction at the macroscale. Our recent study73 presents ways to overcome the effects of underlying surface non-uniformity by introducing three-dimensional materials, hard and spherical diamond nanoparticles, in combination with graphene. At the same time, providing a perfectly incommensurate surface in the form of DLC is essential in achieving superlubricity. We showed that during initial sliding in a dry nitrogen environment, graphene patches tend to wrap around tiny diamond nanoparticles


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and form nanoscrolls. This reduces the effective contact area and provides superlubricity against amorphous diamond-like carbon (Figure 15). Graphene, thanks to its chemical inertness, minimizes interactions with DLC in this case by enabling perfect incommensurability of the graphene-DLC interface. Figure 15 highlights the superlubricity mechanism when the coefficient of friction falls to 0.004. Transmission electron microscopy (TEM) analysis of the wear track indicates the formation of diamond wrapped in graphene structures.

Figure 15. Macroscale superlubricity achieved with nanoscroll formation. (a) Coefficient of friction (COF) during sliding of graphene combined with nanodiamonds against DLC in a dry nitrogen environment reaches ultralow-friction values (~0.004). (b) Observed superlubricity is attributed to the formation of graphene-around-nanodiamond nanoscrolls, as observed in TEM images. (Adapted from [73]. Reprinted with permission from AAAS.) The achieved superlubricity was demonstrated to be stable over a range of temperatures, sliding speeds, and contact pressures.73 However, there are still some important limitations for the observed near-zero-friction sliding. When the sliding occurs in humid air, water molecules present at the sliding interface prevent the formation of the scrolls and thus eliminate superlubric sliding.

Making a Mesoscopic Link Various studies have explored the tribological performance of 2D materials at nanoscale and the methodologies for minimizing friction values. However, transitioning the nanoscale phenomenon to achieve superlubricity at the macroscale remained elusive for a long time because of the issues already mentioned in the previous section. The challenge of achieving superlubricity at true macroscale was finally unfolded by utilizing graphene combined with nanodiamond sliding against DLC73. A mechanism based on combining graphene and nanodiamond was demonstrated to achieve superlubricity at macroscale, eliminating restriction related to the lattice registry of the underlying sample. It was shown that incommensurability of nanoscrolls and counterpart DLC


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substrate allows to minimize friction at the sliding contact. However, the mechanism for maintaining superlubricity over an extended time lies in the nanoscale effect of wrapping and unwrapping of graphene nanosheets around nanodiamonds, which is dynamic and supports superlubricity at the macroscale. It is important to note that each individual nanodiamond wrapped with graphene represents single asperity contact, whereas, ensembles of these represents mesoscale contacts providing a mesoscale link to explain the friction behavior at macroscale. Mesoscale MD simulations provide further insight into the mechanism of this dynamic process, as shown in Figure 16. At any given time, only a certain amount of superlubric graphene-nanodiamond scrolls are formed. With time, the ensemble of nanoscrolls is changing by unwrapping old scrolls and creating new ones. As long as a large enough number of scrolls satisfies superlubricity conditions, it is possible to achieve and maintain superlubricity over long periods and wide ranges of test conditions. The demonstrated macroscale superlubricity showed that even a 65% reduction in the contact area, or approximately 65% of the scrolled graphene-diamond pairs, was enough to realize the effect at the macroscale. This demonstrates a possible route for transferring the nanoscale phenomenon to macroscale observations. More studies in this direction will provide further insights that will evolve our understanding in explaining these observations from nano to macroscale.


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Figure 16. Mesoscale MD simulations showing time evolution in the distribution of COF values. (a) Scroll formation on nanodiamonds for an ensemble of graphene patches when subjected to sliding. (b) Temporal evolution of COF distribution averaged over an ensemble of graphene patches. (c) Evolution of the corresponding contact area. Initially, at t = 0 ps, the patches are mostly sheetlike and in close contact with DLC, leading to an average COF of ~0.6 to 0.7. Sliding of DLC increases the probability of scroll formation by graphene patches, leading to a decrease in the average contact area, which manifests in the form of macroscopic superlubricity. The ensemble-averaged COF shifts to superlubric values at t = 500 ps, when most of the graphene patches are in a scrolled state. (From [73]. Reprinted with permission from AAAS.)


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Conclusions and Future Prospects This review identified various mechanisms of energy dissipation during sliding that are the major constituents for frictional losses and demonstrate the feasibility of using 2D materials as perfect lubricants in minimizing friction to unmeasurable values (Figure 17).

Figure 17. Summary of observed superlubricity received with 2D materials in different systems as summarized from highlighted earlier studies: Graphene-scroll-enabled macroscale superlubricity by Berman et al.73(Image credit: Argonne National Laboratory, USA); Microscale superlubricity enabled by graphene-coated microsphere by Liu et al.99; Superlubricity between MoS2 layers by Li et al.96 (adapted with permission from Ref. 96. Copyright 2014. Wiley-VCH.); Superlubricity of graphene nanoribbons on gold surfaces by Kawai et al.95(courtesy of E. Meyer from University of Basel); Spontaneous self-tearing of graphene nanoribbons by Annett et al.94 (courtesy of J. Annett, G. Cross and D. Malone from Trinity College); Ballistic nanofriction by Guerra et al.98 (reprinted by permission from Macmillan Publishers Ltd:, copyright (2010)). In particular: •

Eight different frictional energy dissipation mechanisms were discussed: wear, molecular deformation, thermal effect, electronic effect, bonding, environment and chemistry, phonons, and structural effect.


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• •

The case studies for observing the specific frictional energy loss mechanism using 2D materials were demonstrated. Different scales of superlubricity were reviewed. Nanoscale superlubricity mechanisms originating from the formation and frictionless sliding of graphene-nanodiamond scrolls can be successfully transferred to macroscale effects. Only around 65% of the total graphene and nanodiamond ensembles need to be in the scrolled state to demonstrate macroscale superlubricity.

The review suggest that research in 2D materials enabled realization of the superlubricity mechanisms across the scales, thus breaking old barriers that were restricting this effect to only nano and microscale. Discovery of combinations of 2D materials and structures and their excellent tribological properties sparked interest among researchers to explore these materials for manipulating friction and wear from nano to macroscales. Considering the extremely high interest in this area, from the prospective of the fundamental science as well as its potential in various applications in the industry, the field continues to evolve rapidly and will change the landscape of existing mechanical/rotating/sliding systems. Reducing friction to near zero in various practical applications will be game-changing across the length scales from tiny microelectromechanical systems that will never wear out or oil-free bearings to giant wind turbines scavenging energy even in low wind conditions. The opportunities are endless and will get even better with more research and active interest from the industrial world.

Acknowledgements The authors thank M. Salmeron for helpful discussions. They also acknowledge support from the Advanced Materials and Manufacturing Processes Institute (AMMPI) at University of North Texas. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.


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Two dimensional materials such as hexagonal boron nitride, graphene and molybdenum disulfide are new emerging solid lubricants 74x39mm (300 x 300 DPI)


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Approaches for Achieving Superlubricity in Two-Dimensional Materials

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