Robust Structural Superlubricity under Gigapascal Pressures

Structural superlubricity (SSL) is a state of contact between two sliding surfaces with no wear and ultralow friction. SSL has been characterized at contacts with van der Waals (vdW) layered materials, while its robustness under high pressure has not been assessed. Here we report experimental results showing that incommensurate vdW interfaces between graphite can maintain the state of SSL under the pressure of no lower than 9.45 GPa, and the vdW contact between a tungsten tip and graphite substrate remains stable up to 3.74 GPa, even higher than the strength of most solids. Beyond this critical pressure, wear is activated, signaling the breakdown of vdW contacts and SSL. This unexpected strong pressure resistance and wear-free feature of SSL signals a wear mechanism beyond the Archard law. Atomistic simulations reveal the microscopic processes of SSL breakdown, which originates from lattice destruction at the vdW contact by pressure-assisted bonding, triggering wear through shear-induced tearing of layered vdW materials. The correlation between the breakdown pressure and material parameters for a number of vdW contacts was analyzed. The results show that the bulk modulus and the first ionization energy, are the most relevant factors, which indicate the combined structural and electronic effects. The critical pressures of several metal/graphite contacts could exceed the strength of metals, demonstrating the robustness of SSL. These findings offer fundamental understandings of wear at the vdW contacts, and could guide the design of SSL-derived applications.