Shape-dependent friction scaling laws in twisted layered material interfaces

Static friction induced by moiré superstructures in twisted incommensurate finite layered material interfaces reveals unique double periodicity and lack of scaling with contact size. The underlying mechanism involves compensation of incomplete moiré tiles at the rim of rigid polygonal graphene flakes sliding atop fixed graphene or h-BN substrates. The scaling of friction (or lack thereof) with contact size is found to strongly depend on the shape of the slider and the relative orientation between its edges and the emerging superstructure, partially rationalizing scattered experimental data. A phenomenological analytical model is developed, which agrees well with detailed atomistic calculations. By carefully considering the edge orientation, twist angle, and sliding direction of the flake relative to the substrate, one should therefore be able to achieve large-scale superlubricity via shape tailoring.