The evolving quality of frictional contact with graphene

Abstract

Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick-slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

“…[3] Although the term "tribology" was only coined in the 1960s, [4] tribological technologies date back to antiquity. [9,10] Transportation of the massive stone building blocks of the pyramids required lubricated contacts.…”

Macroscale Superlubricity Enabled by Graphene‐Coated Surfaces

“…The four‐layer graphene had a thickness of 1.035 nm, and the interlayer distance was 0.3445 nm. The interactions of CC, SiSi, and CSi covalent bonds were simulated by the adaptive intermolecular reactive empirical bond order (AIREBO), Tersoff and 6–12 Lennard‐Jones (LJ) potentials, respectively . For the LJ potential, the total energy U was calculated where ε is the depth of the potential well, σ is the distance in which the interparticle potential is zero, and r is the distance between pairs atoms .…”

“…For the LJ potential, the total energy U was calculated where ε is the depth of the potential well, σ is the distance in which the interparticle potential is zero, and r is the distance between pairs atoms . Herein, ε C‐Si , σ C‐Si , and r C‐Si were 0.0096 eV, 3.0 Å, and 10 Å, respectively . The simulations were carried out at 300 K using the Nosé‐Hoover thermostat and LAMMPS code.…”

“…The dynamic trapping process of GNS was displayed in Movie S7 in the Supporting Information. The atomic interactions in graphene and amorphous SiO 2 were calculated by AIREBO and Tersoff potentials, respectively . A typical 6–12 LJ potential was employed to describe the van der Waals adhesive interaction between the MLG and ball, MLG and NS, and between graphene layers.…”

Modification of the contact surfaces for improving the puncture resistance of laminar structures

“…These results are inconsistent with the traditional velocity strengthening mechanism and require further investigation. At the microscopic level, multilayered nanocomposites have received increasing attention with the evolution of frictional characteristics 22–24 and dynamic responses 25 . Multilayered graphene has prompted substantial research interest owing to its excellent specific energy dissipation under hypervelocity impacts 1, 26, 27 , which is related with the intrinsic strength and fracture toughness of graphene layers 28, 29 .…”