Negative differential friction predicted in two-dimensional electride commensurate contacts: Role of the electronic structure

In recent decade, structural superlubricity has been established as one of the most effective methods to achieve extremely low friction when two crystalline surfaces slide over each other in dry incommensurate contact, which however may be blocked to commensurate configurations during the sliding and thus lead the failure of superlubricity. Here, our first-principles calculations predict negative differential static friction coefficient in the commensurate contact of bilayer two-dimensional (2D) electride (such as ${\mathrm{Ca}}_{2}N$, ${\mathrm{Sr}}_{2}N$, and $Y_{2}C$), which was essentially sustained by the concept of electronic lubricity, where the lubricity was dominated by the electronic structures, rather than the structural effect. Specifically, it is demonstrated that, in the range of 0–10 GPa, the pressure-enhanced charge transfer from the vicinity of surface Ca and interfacial Ca atoms to the uniformly distributed interstitial anionic electron regime collectively screens the corrugation of the sliding potential energy surface (PES) and thus leads to negative differential friction coefficient μ. However, beyond 10 GPa, the accumulated interstitial anionic electrons become significantly localized when sliding to the saddle point of the PES, thus leading to much enhanced electronic kinetic energy, leading to positive μ. The present findings on electronic lubricity are expected to play an instrumental role in developing high-performance solid lubricants.