Friction between MXenes and other two-dimensional materials at the nanoscale

Two-dimensional (2D) materials have unique friction properties that are different from their 3D counterparts. MXenes, a large family of 2D transition metal carbides, carbonitrides, and nitrides, have recently attracted attention as solid lubricants on the macroscale. Here we experimentally investigate the friction of Ti3C2Tx and Ti2CTx MXenes against Ti3C2Tx and Ti2CTx MXenes, graphene, and MoSe2 at the nanoscale, using atomic force microscopy (AFM). Ti3C2Tx and Ti2CTx MXene coated SiO2 spherical AFM probes were used to slide against films of these materials with varying number of monolayers deposited on the substrate. We have found that over the 15 nm sliding distance the measured nanoscale coefficients of friction (COFs) for all the investigated 2D interfaces are below 0.01. At the same time, hydrogen bonding involving -O- and –OH surface terminating groups on MXenes induced higher friction forces for the MXene/MXene contacts compared to other contacts. Moreover, a less significant number-of-monolayers effect on the friction was observed for MXenes due to their thicker monolayers compared with other 2D materials in this study. These results further contribute to our understanding of the mechanisms of friction of MXenes and reveal the potential of MXenes as tunable lubricants at the nanoscale.