Static friction boost in edge-driven incommensurate contacts

We present a numerical investigation of the size scaling of static friction in incommensurate two dimensional contacts performed for different lateral loading configurations. Results of model simulations show that both the absolute value of the force $F_s$ and the scaling exponent $γ$ strongly depend on the loading configuration adopted to drive the slider along the substrate. Under edge-loading a sharp increase of static friction is observed above a critical size corresponding to the appearance of a localized commensurate dislocation. Noticeably, the existence of sublinear scaling, which is a fingerprint of superlubricity, does not conflict with the possibility to observe shear-induced localized commensurate regions at the contact interface. Atomistic simulations of gold islands sliding over graphite corroborate these findings suggesting that similar elasticity effects should be at play in real frictional contacts.