Negative or Positive? Loading Area Dependent Correlation Between Friction and Normal Load in Structural Superlubricity

Abstract

Structural superlubricity (SSL), a state of ultra-low friction between two solid contacts, is a fascinating phenomenon in modern tribology. With extensive molecular dynamics simulations, for systems showing SSL, here we discover two different dependences between friction and normal load by varying the size of the loading area. The essence behind the observations stems from the coupling between the normal load and the edge effect of SSL systems. Keeping normal load constant, we find that by reducing the loading area, the friction can be reduced by more than 65% compared to the large loading area cases. Based on the discoveries, a theoretical model is proposed to describe the correlation between the size of the loading area and friction. Our results reveal the importance of loading conditions in the friction of systems showing SSL, and provide an effective way to reduce and control friction.

Keywords: friction; graphene; molecular dynamics simulation; normal load; structural superlubricity.

FIGURE 1

Simulation model and main results. (A) Schematic sketch of the simulation model. A hexagonal graphene flake (purple) on the strained graphene substrate (red). The area enclosed by the dashed hexagon is the loading area. L is the side length of the loading area of the hexagon. (B) Side view of the simulation model. (C–D) Dependence between the friction force f and the loading pressure P for (C) the small loading area and (D) the large loading area. The zero temperature results and room temperature results are shown in red and black respectively. The dashed curves are fitted with hook functions and linear functions.

FIGURE 2

(A) Schematic sketch of the position of atoms on the graphene flake (the bottom layer of the graphite flake as indicated by the dashed arrow) with different colors. (B) Side view of the simulation model illustrating the layer we focus and its height H. (C–D) Spatial distribution of the height H shown in Panel (B) vs. loading pressure with different L. (E–F) Spatial distribution of the standard deviation of the height $\Delta H$ vs. loading pressure for different L.

FIGURE 3

Load dependence of the frictional power dissipated at zero temperature for (A) small loading area case (L = 3 nm), and (B) large loading area case (L = 4 nm).

FIGURE 4

Simulation for applying the same total force in the loading area. (A) Numbers of atoms in the loading area N _L and pressure versus L. The product of N _L and pressure remains constant. (B) Friction force of the flake along y direction versus L at 0 K (red) or 300 K (black). The result obtained by our model at 0 K is shown in blue.