Macroscale superlubricity enabled by graphene nanoscroll formation

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Science  14 May 2015:
DOI: 10.1126/science.1262024


Friction and wear remain as the primary modes of mechanical energy dissipation in moving mechanical assemblies, thus it is desirable to minimize friction in a number of applications. We demonstrate that superlubricity can be realized at engineering scale when graphene is utilized in combination with nanodiamond particles and diamond-like carbon (DLC). Macroscopic superlubricity originates because graphene patches at a sliding interface wrap around nanodiamonds to form nanoscrolls with reduced contact area that slide against the DLC surface, achieving an incommensurate contact and significantly reduced coefficient of friction (~0.004). Atomistic simulations elucidate the overall mechanism and mesoscopic link bridging the nanoscale mechanics and macroscopic experimental observations.

Macroscopic friction and wear remain the primary modes of mechanical energy dissipation in moving mechanical assembles such as pumps, compressors, turbines etc. leading to unwanted material loss and wasted energy. It is estimated that nearly 1/3 of the fuel used in automobiles is spent to overcome friction, while wear limits mechanical component life. Even a modest 20% reduction in friction can significantly impact cost economics in terms of energy savings and environmental benefits (1). In that context superlubricity is desirable for various applications and therefore an active area of research. To date, superlubricity has been primarily realized in a limited number of experiments involving atomically smooth and perfectly crystalline materials (25) and supported by theoretical studies (6, 7). Superlubricity has been demonstrated for highly oriented pyrolytic graphite (HOPG) surfaces (8), as well as for multiwalled carbon nanotubes (MWCNT), when the conditions for incommensurate contacts are met in a dry environment (9). Since these conditions are due to the incommensurability of lattice planes sliding against each other, they are referred to as structural lubricity and restricted to material interactions at the nanoscale. At the macroscale, this structural effect (hence, superlubricity) is lost due to the structural imperfections and disorder caused by many defects and deformations.

Low friction has been recently observed in centimeter long double-walled carbon nanotubes with perfect atomic structures and long periodicity (10). Ultra-low friction in disordered solid interfaces such as self-mated DLC films (1114) and in fullerene-like nanoparticles such as molybdenum disulfide (MoS2) (15) has been observed under specific environmental and sliding conditions. However, the exact superlubricity mechanism in above cases is still debatable and is not realized for industrial applications. In recent studies at nano and macroscale, graphene has shown a potential to substantially lower friction (1618) and wear (1921) under specific conditions. However, sustained macroscale superlubricity, particularly at engineering scales, has yet to be demonstrated.

We demonstrate our observation of stable macroscale superlubricity while sliding graphene coated surface against DLC-coated counterface. Our initial assumption was that the random network of mixed sp3/sp2 bonded carbon in DLC might provide perfect incommensurate surface needed for the ordered graphene flakes to slide against DLC with least resistance. This was indeed proved to be true; however, the coefficient of friction (COF) values for graphene sliding against DLC in dry environment were not in the superlubric regime (COF~0.04, as shown in Fig. 1B). Initial observation of the wear debris revealed formation of graphene nanoscroll in the wear track. This prompted us to use nanodiamond as an additive, which may act as nano ball bearings when covered by graphene providing extra mechanical strength. We saw a dramatic reduction in friction reaching the superlubric state (in Fig. 1B and inset the COF dropped, to near zero, i.e., 0.004) when we introduced nanodiamond in combination with few layer (3-4 layers) graphene flakes on the silicon dioxide (SiO2) substrate by a solution process method (figs. S1 and S2), providing a partial coverage on the SiO2 surface (22). The observed superlubricity was demonstrated in dry environment (Fig. 1B). The observed wear marks on the flat (Fig. 1C) and ball sides were minimal and produced primarily by the contact pressure during sliding tests (fig. S3). Raman analysis of the wear track (inset to Fig. 1C) showed modification of graphene inside the wear track as observed by a decreased 2D peak (at ~2660 cm−1) and an increased D peak (at ~1330 cm−1) in comparison to initial graphene’s Raman signature, indicating gradual loss of crystallinity of few-layer graphene and increase in defects due to possible tearing of graphene under constant sliding at high contact pressure (0.3 GPa). The Raman spectrum indicates no DLC transfer in the wear track during sliding and thus confirming that the superlubricity regime is not connected with the previously observed low-friction performance of DLC against DLC (fig. S14) (12).

Fig. 1 Experimental demonstration of the superlubricity regime.

(A) presents not-to-scale schematic of the superlubricity test. (B) The COF for DLC ball sliding in dry nitrogen environment against: 1) graphene-plus-nanodiamonds (superlubricity state with COF ~0.004+/−0.002), 2) graphene alone (COF~0.04+/−0.01), and 3) nanodiamond alone (COF~0.07+/−0.01). Inset shows a plot for superlubricity. (C) In the case of superlubricity the wear tracks on the flat side and on the ball side (fig. S3) are almost invisible. Inset shows a typical Raman signature of defective graphene. For graphene-plus-nanodiamond sliding against DLC ball in humid environment, (D) the COF reveals a high value of ~0.27+/−0.04 and (E) the corresponding wear track on the flat side is wide as seen in with inset showing typical Raman signature of DLC and disordered carbon. The tests perfomed at room temperature under 1N load and with 3 cm/s linear speed.

The importance of using graphene-plus-nanodiamonds in establishing the superlubricity is demonstrated in Fig. 1B. In particular, graphene or nanodiamond when used alone on a SiO2 substrate sliding against a DLC ball in dry environment display higher values of COF (0.04 and 0.07). Additionally, the erratic nature of COF indicates large wear debris formation.

Our experimental studies confirm that the stable superlubricity regime occurs over a wide range of test conditions; i.e., when the load was changed from 0.5 N to 3 N, velocity was varied from 0.6 cm/s to 25 cm/s, temperature increased from 20°C to 50°C (fig. S15), and substrate changed to nickel or bare silicon (fig. S16). The temperature and velocity range for maintaining stable superlubricity is further backed by theoretical simulations (tables S2 and S3).

For graphene-plus-nanodiamonds in an ambient humid environment (relative humidity ~30%), both COF and wear were comparatively large (Fig. 1, D and E). More importantly, a substantial amount of graphitized carbon debris were formed in the wear track, as shown by the optical images and Raman data (Fig. 1E and fig. S3). The substrate itself suffered from significant wear during sliding. The uniqueness of tribopair and dramatic dependence on the environmental conditions led us to further explore the underlyng mechanism for the observed superlubricity.

More detailed analysis of the wear track formed during the superlubricity regime in dry nitrogen was carried out by sampling and examining the wear debris by transmission electron microscopy (TEM). The TEM images in Fig. 2 show that a large fraction of the nanodiamonds were wrapped by graphene nano-scrolls (more detailed scrolls images are shown in fig. S4). Electron energy loss spectra (EELS) confirmed presence of diamond in the wear debris, as evident from the typical EELS signature for diamond. The π* peak (at ~285 eV) in the carbon K-edge represents a small fraction of sp2 bonded carbon due to the presence of the few layers of graphene wrapped around the nanodiamond, which is similar to disordered carbon shell observed previously in detonated nanodiamonds (23). For pure nanodiamonds, this π* peak should be absent (23, 24), while in case of pure graphene scrolls, this π* peak is higher (Fig. 2B, inset). Because of the random orientation of scrolls with diamond embeded inside, we had to focus the TEM differently to view clearly the diamond lattice and graphene layers; therefore, some of the scrolls in Fig. 2 do not show nanodiamonds inside.

Fig. 2 Graphene nanoscrolls formation.

(A and B) TEM images of the wear debris for DLC ball sliding against graphene-plus-nanodiamonds demonstrating superlow friction in the dry environment. Graphene scroll formation is observed. Insets show electron energy-loss spectra for both diamond and graphene in the wear debris.

To further explore the superlubricity mechanism, molecular dynamics (MD) simulations were performed (table S1) (22) and our simulations suggest that nanodiamonds can activate, guide, and stabilize the scrolling of initially planar graphene patches (fig. S5). During sliding in dry environment, nanodiamonds facilitate scroll formation via two mechanisms: 1) graphene platelets are highly reactive and easily attach to the dangling bonds present on the surface of nanodiamonds, initiating the scroll formation; 2) the sliding graphene patches encounter the 3-D structure of nanodiamonds, which act as obstructions (fig. S9). Additionally, the presence of topological defects (e.g., double vacancies, Stone-Wales) in graphene is expected to promote the scrolling behavior (fig. S12). Based on relative binding energetics between graphene-DLC and graphene-nanodiamond, we find that graphene prefers to wrap around the nanodiamond to promote higher surface contact (fig. S10). Once in a scrolled state, the final structures of graphene on diamond are well coordinated and stabilized by van der Waals forces.

Figure 3, A and B, shows scroll formation and evolution of the COF, respectively, for a single graphene patch in dry environment. At time t < 1.0 ns, COF values are high, ~0.2-0.4 since the graphene patch is in an extended or unscrolled state. The wrapping of a graphene sheet over the nanodiamonds begins at ~1.5 ns; this coincides with COF values dropping significantly leading to a superlubric state which is maintained until the end of the simulation. Once formed, these scrolls slide against randomly arranged DLC atoms which provide an incommensurate contact. This constant out-of-registry sliding translates into a superlubric regime. The COF also depends on contact area between formed graphene scrolls and DLC. The superlubricity is thus attributed to (a) reduction in the interfacial contact area (> 65%) and (b) incommensurability between DLC and graphene scrolls.

Fig. 3 Simulations of the single scroll formation.

Temporal evolution of nanoscale friction for DLC ball sliding against graphene-plus-nanodiamonds in dry (A and B) and humid (C and D) environments extracted based on the MD simulation trajectories. Graphene scroll formation over nanodiamonds is observed in a dry environment (the steady-state COF is 0.005+/−0.004), whereas ordered water layers above the graphene flakes prevent scroll formation in the humid environment (COF is 0.12 +/− 0.04).

At the molecular level, the observed superlubricity has its origin in graphene’s nanoscopic anisotropic crystal structure, consisting of strong covalent intra-layer bonding and weaker dispersive interlayer interactions. The structural contact between an incommensurate DLC ball and the graphene scrolls allows DLC to slide on top of the underlying graphene sheets by overcoming relatively small energetic barriers. In recent experiments, Dienwiebel et al. (8) observed friction reduction to vanishingly small values, depending on the degree of commensurability between the graphene flakes and the extended graphite surface. In an incommensurate state, the unit-cells in contact have to overcome much smaller barriers at any point in time, leading to considerably reduced resistance toward sliding. Consistent with the prediction of Mo et al. (25), we find that the friction force depends linearly on the number of atoms that chemically interact across the contact. The effective contact area between the graphene sheets and DLC decreases with time upon scrolling. Since friction is controlled by the short-range (chemical) interactions even in the presence of dispersive forces, scrolling induced reduction in nanoscopic contact is significant enough to lead to superlubric state.

Our experiments suggest that the humid environment increases the friction and wear of the ball side, as graphene layers remain strongly attached on the surface. We therefore performed MD simulations of the graphene-DLC-nanodiamond system in a humid environment (fig. S6). MD trajectories suggest formation of quasi two-dimensional ordered water layers between the nanoscopic contacts, i.e., DLC and graphene sheets (Fig. 3, C and D). These water layers prevent the scrolling of the graphene during sliding (fig. S7), the ordered two-dimensional water layers [based on calculated translational and tetrahedral orders (supplementary text) (22)] present a constant energy barrier for the DLC to overcome. These two effects result in little or no friction variation over time (Fig. 3D), and a nearly constant high friction condition is maintained (COF ~0.1). We have simulated the effects of surface chemistry and considered the role of defects (supplementary text) (22). We find that the presence of defects greatly facilitates the adsorption of water from the ambient atmosphere (fig. S13). Water preferentially adsorbs and stabilizes defective sites which further prevents the formation of scrolls.

To bridge the gap between the nanoscale mechanics and macroscopic contacts evident in our experiments, we performed a large-scale MD simulation for an ensemble of graphene-plus-nanodiamonds present between DLC and the underlying multi-layered graphene substrate (fig. S8). The mesoscopic link is crucial to explain how the formation of nanoscrolls translates from a nanosystem with a single graphene patch (nm2 area of sliding interface) into the observed superlubricity at the macroscale (mm2 area). We has evaluated the collective scrolling and tribological behavior of many individual graphene patches and a density distribution of their tribological state is created to assess their contribution to the observed friction. During the initial sliding period at time t = 0, the unscrolled graphene patches are in close contact with the interface. The contact area normalized with respect to the initial value at t = 0 is ~1 (22) as shown in Fig. 4C. The density distribution of COF values (Fig. 4B) shows a narrow distribution with a peak at ~0.6-0.7, suggesting that the system is in high friction state. With time (200-300 ps), the graphene patches increasingly scroll over nanodiamonds, and we observe a corresponding reduction in this peak intensity. The density profile shows a broader distribution and shifts prominently toward lower COFs (< 0.2). The contact area, which is proportional to the number of interacting atoms, reduces by 40-50% during (26) this period. During the latter stages (~500 ps), most of the graphene patches are scrolled. The density profile shows a shift in the distribution to COF values << 0.01. The effective contact area in the present case is reduced significantly, by ~65-70%, and the mesoscopic system has reached a superlubric state.

Fig. 4 Meso-scale MD simulations of superlubricity.

Meso-scale MD simulations demonstrating the time evolution in the distribution of COF values: (A) snapshot showing the 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, and (C) evolution of the corresponding contact area. Initially at t = 0 ps, the patches are mostly sheet-like and in close contact with DLC leading to an average COF of ~0.6-0.7. 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.

The tribological evolution of a single graphene patch at the nanoscale resembles that of a single asperity contact, whereas the mesoscopic behavior resembles a multiple asperity contact. The friction mechanism at the mesoscale for an ensemble of graphene patches is not different from nanoscale (single patch). The initial tribological state of the patches, as well as the configuration of the patch vs. nanodiamonds, dictates the dynamics of scroll formation, which, in turn, affects the dynamical evolution of COF for the mesoscopic system. The macroscopic contact in our experiments can be envisioned as comprising a much larger number of such smaller contacts or asperities, which explain the difference in time for the onset of the superlubric state in the experiments vs. simulated systems.

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S16

Tables S1 to S3

References (2745)

Movie S1

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
  2. Acknowledgments: The help in the TEM data collection by Yuzi Liu is greatly appreciated. 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. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used tribological test facilities of the Energy Systems Division supported by the Vehicle Technologies Program of the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. An award of computer time was provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This research used resources of the Argonne Leadership Computing Facility at Argonne National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-06CH11357. Experimental data and simulations are archived on servers at Argonne National Laboratory. Part of the experimental results are covered by a patent (US20140023864A1). Both D.B. and S.D. contributed equally in this work. D.B. performed the experiments and analyzed the data. S.D. and S.K.R.S. devised and performed the molecular dynamics simulations and performed all the related data analysis. A.V.S. conceived the idea, helped in the data analysis of experimental results and directed the project. A.E. co-directed the project and helped in the data analysis of tribological tests. SKRS guided the simulation effort. D.B., S.D., S.K.R.S., A.E., and A.V.S equally contributed to discussing the results and composing the manuscript.
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