The (Grain) Boundaries of Structural Superlubricity

Structural superlubricity, a state of ultra-low friction and wear arising from incommensurability between contacting surfaces, is an intriguing physical phenomenon that holds promise for the significant reduction of energy loss and material damage in mechanical systems. One of the most prominent realizations of superlubric motion is demonstrated for nano- and micro-scale heterogeneous layered material contacts and their twisted homogeneous counterparts. On the route to scaling up superlubricity stand a few obstacles. In this chapter, we focus on the effect of grain boundaries, which inevitably emerge in large-scale layered material contacts, on their frictional properties. New frictional mechanisms associated with grain boundaries, such as shear induced buckling and unbuckling of corrugated dislocations and moiré superstructure scattering, are discussed. These, in turn, are characterized by unique frictional behavior, including nonmonotonic dependence on normal load, sliding velocity, and temperature that can be harnessed to restore structural superlubricity at increasing length-scales.