Directional anisotropy of friction in microscale superlubric graphite/$h\mathrm{BN}$ heterojunctions

Structural superlubricity is one of the most fascinating tribological concepts, describing the state of almost vanishing friction, which arises from the cancellation of the lateral interactions between incommensurate rigid surfaces. In this paper, the dependence of friction on sliding directions is experimentally studied under ambient conditions using state of the art friction force microscopy of misaligned microscale heterogeneous contacts between the single crystalline $h\mathrm{BN}$ and graphite. The observed minor frictional anisotropy (≤20%) demonstrates that the microscale superlubric translational motion is maintained along all sliding directions. With atomistic simulations, we further show that the frictional anisotropy results from a change in the corrugation of the sliding potential energy surface with changing the sliding direction. Our results validate the robustness of the structural superlubricity against the sliding direction, and provide an effective approach to precise friction control.

Figure 1

Schematic diagram of the experimental system to measure the friction in microscale graphite/$h\mathrm{BN}$ junctions and AFM characterization of the single crystalline surfaces. (a) A $\mathrm{Si}{\mathrm{O}}_2$ capped graphite flake was placed atop a $h\mathrm{BN}$ substrate, which was rigidly fixed at the AFM stage. Normal load and lateral force were applied via a visible AFM probe, which was put in contact with the central area of the $\mathrm{Si}{\mathrm{O}}_2$ cap. The relative movement of the sliding flake with respect to the $h\mathrm{BN}$ substrate was monitored using an objective lens coupled to the AFM head. (b), (f) Schematic diagrams of the upward-facing graphite flake and $h\mathrm{BN}$ substrate, respectively. The three $4\times 4\mathrm{n}{\mathrm{m}}^2$ spots were used to verify the surface monocrystallinity. (c)–(e), (g)–(i) Atomically resolved AFM images of the six spots shown in panels (b), (f), respectively. The identical crystal orientation of all spots on the graphite and $h\mathrm{BN}$ surfaces, respectively, indicates the monocrystallinity of the graphite surface and of the smooth substrate region used in the friction measurements.

Figure 2

Dependence of the frictional force on the sliding direction measured for monocrystalline graphite/$h\mathrm{BN}$ heterojunction under ambient conditions (temperature of $27.41{}^{\circ }\mathrm{C}\pm 0.02{}^{\circ }\mathrm{C}$ and relative humidity of 9.83% ± 0.09%), which clearly demonstrates the anisotropy and sixfold symmetry of the measured friction.

Figure 3

Simulated directional anisotropy of friction for graphite/$h\mathrm{BN}$ heterojunction. (a) Schematics of the simulation setup. (b) Sliding potential energy surface of the whole system. (c), (d) Variation of the sliding potential energy along two different directions based on graphene/$h\mathrm{BN}$ interlayer potentials (ILPs) and Lennard-Jones (LJ) potential.