Eliminating friction at the nanoscale

Macroscopic friction and wear remain the primary modes of mechanical energy dissipation in moving mechanical assemblies such as pumps, compressors, and turbines, leading to unwanted material loss and wasted energy. It is estimated that nearly one third 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 substantially affect cost economics in terms of energy savings and environmental benefits (1). In that context, superlubricity is desirable for various applications and therefore is 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 (2–5) 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 (MWCNTs), when the conditions for incommensurate contacts are met in a dry environment (9). Because 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 because of the structural imperfections and disorder caused by many defects and deformations.

Low friction has recently been observed in centimeter-long double-walled carbon nanotubes with perfect atomic structures and long periodicity (10). Ultralow friction in disordered solid interfaces, such as self-mated DLC films (11–14) and in fullerenelike nanoparticles such as molybdenum disulfide (MoS₂) (15), has been observed under specific environmental and sliding conditions. However, the exact superlubricity mechanism in the above cases is still debatable and is not realized for industrial applications. In recent studies at the nano- and macroscale, graphene has shown a potential to substantially lower friction (16–18) and wear (19–21) 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 a graphene-coated surface against a DLC-coated counterface. Our initial assumption was that the random network of mixed sp³/sp² bonded carbon in DLC might provide the 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 a 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 nanoscrolls 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 (0.004)] in a dry environment, when we introduced nanodiamond in combination with few-layer (three to four layers) graphene flakes on the silicon dioxide (SiO₂) substrate by means of a solution process method (figs. S1 and S2), providing a partial coverage on the SiO₂ surface (22). 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 (Fig. 1C, inset) showed modification of graphene inside the wear track as observed in a decreased 2D peak (at ~2660 cm⁻¹) and an increased D peak (at ~1330 cm⁻¹) in comparison with the initial graphene’s Raman signature, indicating a gradual loss of crystallinity of few-layer graphene and an increase in defects possibly due to 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, thus confirming that the superlubricity regime is not connected with the previously observed low-friction performance of DLC against DLC (fig. S14) (12).

 

Schematic diagram

Panel A depicts a pictorial representation of the experimental setup/sliding interface.

Questions

1. How does the coefficient of friction (COF) vary with the nanomaterial properties at the sliding interface in dry nitrogen?

2. Do the environmental conditions play a role in facilitating ultralow friction?

Experiments

Three different nanomaterials—graphene, nanodiamonds, and a combination of nanodiamonds and graphene—were used at the sliding interface of diamondlike graphene (DLC) and graphene-coated SiO₂ substrate.

Macroscale tribological tests were performed in both dry nitrogen and humid environments and at room temperature using a tribometer. The COF of DLC-coated balls during sliding against all three nanomaterials is recorded as a function of the number of cycles (see Figures 1B & 1D).

Answers

Question 1: A comparison of the COF obtained with all three nanomaterials in dry nitogen is shown in Figure 1B. The inset figure gives a clear view of ultralow friction achieved with the combination of graphene and nanodiamonds. Further, Figure 1C depicts the optical microscope image of the wear scar after testing with the hybrid material. No measurable wear can be seen. The Raman spectrum (see inset) is typical of graphene, implies that DLC coating remains intact after the sliding experiments. These results conclude that the combination of graphene and nanodiamonds as sold lubricants is found to be superior to graphene or nanodiamonds alone in facilitating superlubricity.

Question 2: Figure 1D shows the results obtained with the novel hybrid material—graphene+nanodiamonds—in a humid environment (in the presence of water). Higher-friction coefficients can be seen in this case, as opposed to that in a dry nitrogen atmosphere. Optical microscope imaging (Figure 1E) reveals considerable wear of the flat side and the Raman spectrum (see inset) shows the presence of DLC. Based on these observations, the authors concluded that the combination of graphene and nanodiamonds better perform under dry nitrogen conditions and superlubricity potential is deteriorated under humid conditions.

The necessity of using graphene-plus-nanodiamonds in establishing the superlubricity is demonstrated in Fig. 1B. In particular, graphene or nanodiamond when used alone on a SiO₂substrate sliding against a DLC ball in a dry environment displays 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; when the load was changed from 0.5 to 3 N, velocity was varied from 0.6 to 25 cm/s, temperature increased from 20°C to 50°C (fig. S15), and the substrate was 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). A substantial amount of graphitized carbon debris was formed in the wear track, as shown by the optical images and Raman data (Fig. 1E and fig. S3), and the substrate itself suffered from substantial wear during sliding. The distinctiveness of the tribopair and dramatic dependence on the environmental conditions led us to further explore the underlyng mechanism for the observed superlubricity.

We carried out more detailed analysis of the wear track that formed during the superlubricity regime in dry nitrogen by sampling and examining the wear debris with transmission electron microscopy (TEM). As shown in the TEM images in Fig. 2, a large fraction of the nanodiamonds were wrapped by graphene nanoscrolls (more detailed scroll images are shown in fig. S4). Electron energy-loss spectra (EELS) confirmed the 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 owing to the presence of the few layers of graphene wrapped around the nanodiamond, which is similar to the disordered carbon shell observed previously in detonated nanodiamonds (23). For pure nanodiamonds, this π* peak should be absent (23, 24), whereas in case of pure graphene scrolls, this π* peak is higher (Fig. 2B, inset). Because of the random orientation of scrolls with diamond embedded inside, we had to focus the TEM differently in order to view clearly the diamond lattice and graphene layers; therefore, some of the scrolls in Fig. 2 do not show nanodiamonds inside.

 

Figures 2A and 2B

Transmission electron microscope (TEM) images of the wear debris after the sliding experiment in dry nitrogen confirms the formation of graphene nanoscrolls. The round layers are the result of scrolled-up graphene sheets. The lattice corresponding to nanodiamonds can be seen as parallel straight lines inside the scrolled-up sheets. Inset shows the electron energy loss spectra (EELS) with energy loss plotted on the x-axis, which represents the lost energy of the incident electrons upon inelastic interaction with sample specimen atoms. Intensity is plotted on the y-axis, which corresponds to the number of electrons after the inelastic scattering. Since the EELS signal is sensitive to the atom type and its electronic state, each EELS spectrum is a signature of a particular material. Thus, EELS spectra taken at different locations of the sample confirm the presence of both diamond and graphene in the wear debris. A pictorial representation of graphene scrolled nanodiamond is provided for better visualization.

Electron Microscope

In 1986, the Nobel Prize in Physics was awarded to Ernst Ruska, Gerd Binnig, and Heinrich Rohrer for the development and design of the electron microscope.

Learn more: https://www.nobelprize.org/prizes/physics/1986/summary/

 

Check out this video of TEM instruments in action: https://www.youtube.com/watch?v=DTIlLeCU5JQ

To further explore the superlubricity mechanism, we performed molecular dynamics (MD) simulations (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 a dry environment, nanodiamonds facilitate scroll formation via two mechanisms: (i) Graphene platelets are highly reactive and easily attach to the dangling bonds present on the surface of nanodiamonds, initiating the scroll formation; and (ii) the sliding graphene patches encounter the three-dimensional (3D) structure of nanodiamonds, which act as obstructions (fig. S9). Additionally, the presence of topological defects (such as double vacancies or Stone-Wales) in graphene is expected to promote the scrolling behavior (fig. S12). On the basis of relative binding energetics between graphene-DLC and graphene-nanodiamond, we found 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.

Scroll formation and evolution of the COF are shown in Fig. 3, A and B, respectively, for a single graphene patch in a dry environment. At time t < 1.0 ns, COF values are high, ~0.2 to 0.4, because 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 substantially, leading to a superlubric state that 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 the contact area between formed graphene scrolls and DLC. The superlubricity is thus attributed to (i) reduction in the interfacial contact area (>65%) and (ii) incommensurability between DLC and graphene scrolls.

 

Panels A and B

Various steps involved in the simulation of the sliding DLC balls against graphene and nanodiamonds in a dry nitrogen environment are shown in Figure 3A. The single graphene patch at the bottom of the DLC ball is found to wrap around the nanodiamond during sliding. In Figure 3B, the friction coefficient during sliding is measured over a period of time. It is clear that the friction reduces to the superlubric regime after nearly 1.5 nanoseconds. The inset shows a clear view of the steady state ultralow friction, which corroborates the graphene nanoscroll mechanism.

Figures 3C and 3D

Simuation pathways of the sliding in a humid environment with time is shown. No nanoscroll formation is observed. The coefficient of friction in Figure 3D shows consistently higher values, indicative of poor tribological performance under humid conditions.

At the molecular level, the observed superlubricity has its origin in graphene’s nanoscopic anisotropic crystal structure, which consists of strong covalent intralayer 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 found 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. Because friction is controlled by the short-range interactions even in the presence of dispersive forces, scrolling-induced reduction in nanoscopic contact is substantial enough to lead to a superlubric state.

Our experiments suggest that the humid environment increases the friction and wear of the ball side because graphene layers remain strongly attached on the surface. We therefore performed MD simulations of the DLC-nanodiamond-graphene system in a humid environment (fig. S6). MD trajectories suggest formation of quasi-2D ordered water layers between the nanoscopic contacts, the DLC and graphene sheets (Fig. 3, C and D). These water layers prevent the scrolling of the graphene during sliding (fig. S7), and the ordered 2D 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 found 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 multilayered 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 (square-nanometer area of sliding interface) into the observed superlubricity at the macroscale (square-millimeter area). We evaluated the collective scrolling and tribological behavior of many individual graphene patches and created a density distribution of their tribological state in order to assess their contribution to the observed friction. During the initial sliding period at 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 to 0.7, suggesting that the system is in a high-friction state. With time (200 to 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 to 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 to 70%, and the mesoscopic system has reached a superlubric state.

 

Figure 4A

Simulation trajectories for a number of graphene patches and nanodiamonds sliding against DLC under dry nitrogen conditions are shown. Graphene sheets at different initial configurations get wrapped around the nanodiamonds during sliding and the final configuration displays the scrolled graphene sheets after 500 picoseconds of sliding.

Figure 4B

The 3D graph shows the evolution of two dependent variables—number density and coefficient of friction—over time. The color-coded bar on the side represent the number density scale, indicative of contact area. Higher-friction coefficients can be seen initially with a narrow density distribution and higher contact area, implying that most of the graphene patches are in an unscrolled state. As time passes by, a reduction in friction coefficients can be seen, finally reaching a superlubric state with very low friction values. The density distribution is getting broader and the color coding implies a reduced contact area at this stage.

Figure 4C

The Y-axis of the graph shows the ratio of the contact area (between DLC and graphene patches) at any given time (t) with respect to the initial contact area (when t=0) before sliding. On the X-axis is the simulation of time in picoseconds. The gradual reduction in the contact area from the beginning is the result of increased scroll formation during sliding. The steady state lower-contact area shows that most of the graphene patches end up in a scrolled state, implying a superlubric regime.

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 patches versus 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 explains the difference in time for the onset of the superlubric state in the experiments versus simulated systems.

Supplementary Materials

www.sciencemag.org/content/348/6239/1118/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S16

Tables S1 to S3

References (27–45)

Movie S1

References and Notes

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