Friction behaviors of two-dimensional materials at the nanoscale

Two-dimensional (2D) layered materials such as graphene, hexagonal boron nitride, molybdenum disulfide and alike possess unique frictional properties, making them promising candidates as (additives to) lubricants for friction reduction and wear protection, especially in micro- and nano-electromechanical systems. To truly apply these materials, it is essential to understand friction at the nanoscale level. In recent decades, with the development of atomic and friction force microscopy and atomistic simulation tools, our understanding of the friction of 2D materials has substantially increased. Herein, we summarize the essential friction behaviors of 2D materials as well as the underlying physical mechanisms explored by friction force microscopy. In particular, the effects of atomic structures and external factors on friction, and the strategies to realize the modulation of friction are discussed. Finally, the challenges in practical applications of 2D materials as atomically thin lubricants and the perspectives for future progression are provided.