In conventional systems, the coefficient of friction (COF) is typically positive, signifying a direct relationship between frictional and normal forces. Contrary to this, we observe that the load dependence of friction exhibits a unique bell-shaped curve when studying the frictional properties between graphite and $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ surfaces. As the applied normal force increases, the friction initially rises and then decreases. Finite element simulations reveal this behavior is due to edge detachment at the $\text{graphite}/\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ interface as the normal force approaches a critical value. Because friction in superlubric contacts predominantly arises from edges, their detachment leads to a decrease in overall friction. We empirically validate these findings by varying the radii of curvature of the tips and the thicknesses of graphite flakes. This unprecedented observation offers a new paradigm for tuning COF in superlubric applications, enabling transitions from positive to negative values.

• Received 25 September 2023
• Accepted 13 March 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.156201