Negative Differential Friction Predicted in 2D Ferroelectric In2Se3 Commensurate Contacts

At the macroscopic scale, the friction force (f) is found to increase with the normal load (N), according to the classic law of Da Vinci-Amontons, namely, f = mu N, with a positive definite friction coefficient (mu). Here, first-principles calculations are employed to predict that, the static force f, measured by the corrugation in the sliding potential energy barrier, is lowered upon increasing the normal load applied on one layer of the recently discovered ferroelectric In2Se3 over another commensurate layer of In2Se3. That is, a negative differential friction coefficient mu can be realized, which thus simultaneously breaking the classic Da Vinci-Amontons law. Such a striking and counterintuitive observation can be rationalized by the delicate interplay of the interfacial van der Waals repulsive interactions and the electrostatic energy reduction due to the enhancement of the intralayer Se-In ionic bonding via charge redistribution under load. The present findings are expected to play an instrumental role in design of high-performance solid lubricants and mechanical-electronic nanodevices.

Automated summary

By employing first-principles calculations, this study reveals that the friction force can be reduced and a negative differential friction coefficient can be realized upon increasing the normal load on one layer of ferroelectric In2Se3 over another commensurate layer of In2Se3, breaking the classic Da Vinci-Amontons law. This counterintuitive observation is attributed to the delicate interplay of interfacial van der Waals repulsive interactions and the electrostatic energy reduction due to enhanced Se-In ionic bonding under load, providing insights for the design of high-performance solid lubricants and mechanical-electronic nanodevices.