Formation of intermittent covalent bonds at high contact pressure limits superlow friction on epitaxial graphene

Epitaxial graphene on SiC(0001) exhibits superlow friction due to its weak out-of-plane interactions. Friction-force microscopy with silicon tips shows an abrupt increase of friction by one order of magnitude above a threshold normal force. Density-functional tight-binding simulations suggest that this wearless high-friction regime involves an intermittent $s{p}^3$ rehybridization of graphene at contact pressure exceeding 10 GPa. The simultaneous formation of covalent bonds with the tip's silica surface and the underlying SiC interface layer establishes a third mechanism limiting the superlow friction on epitaxial graphene, in addition to dissipation in elastic instabilities and in wear processes.

Figure 1

Friction force as function of increasing applied normal load recorded between an oxidized silicon tip and epitaxial graphene grown on a SiC(0001) substrate. Results in (a) and (c) were recorded with the tips subsequently imaged by transmission electron microscopy in (b) and (d). Atomic planes of the single-crystal structure of the silicon tips appear as lines, the surface oxide at the top apex as amorphous layers.

Figure 2

(a) Simulation setup of silicon carbide with graphitic interface layer, graphene, and one of five amorphous silicon oxide configurations. Colors signify silicon (yellow), oxygen (red), carbon (gray), and hydrogen (white); sticks between balls indicate a chemical bond. Snapshots are taken from the middle of the 0.3-ns simulation period for a contact pressure of 1, 10, and 15 GPa (temperature $T=300$ K and sliding velocity $v=100$ m/s). Green data points report the sliding velocity as function of the normal coordinate and indicate the location of the shear plane. (b) Simulated shear stress vs contact pressure for five different silicon oxide configurations which are represented by different symbols. Red symbols indicate absence of chemical bonds between graphene and silicon oxide atoms; blues symbols indicate the formation of at least one such chemical bond between a carbon atom and a silicon or oxygen atom (temperature $T=300$ K). Shear stress is calculated for the last 10% of 3-ns simulated sliding time at 10 m/s and of 0.3-ns sliding time at 100 m/s.

Figure 3

Shear stress as function of the number of chemical bonds. Yellow symbols represent C–C bonds between graphene and SiC, purple symbols C–O and C–Si bonds between graphene and silicon oxide. Data represent simulation results for five different silicon oxide configurations at a sliding velocity of 10 (left) and 100 m/s (right) (temperature 300 K, contact pressures between 1 and 40 GPa as indicated in Fig. 2. Note that the formation of C–C bonds alone does not necessarily lead to high shear stress.