Effective stick-slip parameter for structurally lubric two-dimensional interface friction

J Wang and A Vanossi and E Tosatti, PHYSICAL REVIEW B, 109, 134102 (2024).

DOI: 10.1103/PhysRevB.109.134102

The wear -free sliding of layers or flakes of graphene-like 2D materials, important in many experimental systems, may occur either smoothly or through stick -slip, depending on driving conditions, corrugation, twist angles, as well as edges and defects. No single parameter has been so far identified to discriminate a priori between the two sliding regimes. Such a parameter, eta , does exist in the ideal (Prandtl-Tomlinson) problem of a point particle sliding across a 1D periodic lattice potential. In that case eta > 1 implies mechanical instability, generally leading to stick -slip, with eta = 2 pi U-2(0)/K(p)a(2) , where U-0 is the potential magnitude, a the lattice spacing, and K-p the pulling spring constant. Here we show, supported by a repertoire of graphene flake / graphene sliding simulations, that a similar stick -slip predictor eta(eff) can be defined with the same form but suitably defined U-eff , a(eff) , and K-eff . Remarkably, simulations show that a(eff) = a of the substrate remains an excellent approximation, while K-eff is an effective stiffness parameter, combining equipment and internal elasticity. Only the effective energy barrier U-eff needs to be estimated in order to predict whether stick -slip sliding of a 2D island or extended layer is expected or not. In a misaligned defect -free circular graphene sliding island of contact area A , we show that U-eff , whose magnitude for a micrometer size diameter is of order 1 eV, scales as A(1/4), thus increasing very gently with size. The PT -like parameter eta(eff) is therefore proposed as a valuable tool in 2D layer sliding.

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