Abstract
Structural superlubricity is a theoretical concept stating that the friction force is absent between two rigid, incommensurate crystalline surfaces. However, elasticity of the contact pairs could modify the lattice registry at interfaces by nucleating local slips, favoring commeasure. The validity of structural superlubricity is thus concerned for large-scale systems where the energy cost of elastic distortion to break the lattice registry is low. In this work, we study the size dependence of superlubricity between single-crystal graphite flakes. Molecular dynamics simulations show that with nucleation and propagation of out-of-plane dislocations and strained solitons at Bernal interfaces, the friction force is reduced by one order of magnitude. Elastic distortion is much weaker for non-Bernal or incommensurate ones, remaining notable only at the ends of contact. Lattice self-organization at small twist angles perturbs the state of structural superlubricity through a reconstructed potential energy surface. Theoretical models are developed to illustrate and predict the interfacial elastoplastic behaviors at length scales beyond those in the simulations. These results validate the rigid assumption for graphitic superlubricity systems at microscale, and reveal the intrinsic channels of mechanical energy dissipation. The understandings lay the ground for the design of structural superlubricity applications.
Highlights
The notion of structural superlubricity has recently drawn notable interest for its practical implications in engineering, holding great promises in applications that are friction-free, dissipation-free, and even wearless [1,2,3,4,5]
Efforts were made by exploring the amplitude of friction force or mechanical energy dissipation of a graphite mesa sliding on a graphitic substrate, demonstrating the rotation-angle dependence and six-fold symmetry of the potential energy landscape, where graphite flakes are considered as rigid plates and elastic deformation is neglected [2, 10]
Our simulation results show that for L = 10 nm, typical stick-slip phenomena are observed for the graphitic contact, where the whole mesa slides over the substrate (Fig. S5 in the ESM), while for L = 200 nm, spatial patterns of the friction characteristics emerge (Fig. 1)
Summary
The notion of structural superlubricity has recently drawn notable interest for its practical implications in engineering, holding great promises in applications that are friction-free, dissipation-free, and even wearless [1,2,3,4,5]. Efforts were made by exploring the amplitude of friction force or mechanical energy dissipation of a graphite mesa sliding on a graphitic substrate, demonstrating the rotation-angle dependence and six-fold symmetry of the potential energy landscape, where graphite flakes are considered as rigid plates and elastic deformation is neglected [2, 10]. This process can be captured in the Prandtl–Tomlinson (PT) model [11], which treats the mesa as a particle and the substrate contact through a fixed potential surface defined by the mesa–substrate interaction. Forces on atoms cancel out, resulting in a frictionless contact in the state of structural superlubricity [12,13,14]
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