Effective usage of 2D MXene nanosheets as solid lubricant - Influence of contact pressure and relative humidity

Newly emerging Ti3C2Tx-nanosheets (MXenes) have attracted considerable attention in energy storage, catalysis and, more recently, tribology. MXene nanosheets are characterized by a weakly-bonded multi-layered structure with self-lubricating ability, making them a suitable candidate for solid lubrication. This contribution aims at addressing for the first time their application in higher loaded steel/steel dry sliding contacts by investigating the influence of contact pressure and relative humidity on friction and wear performance. Compared to the uncoated reference, a 2.3-fold friction reduction and a 2.7-fold reduction of the wear volume were verified for MXene-coated specimens for moderate contact pressures and low relative humidity. This was due to the in-situ formation of a compacted tribo-film consisting of densified Ti3C2Tx-nanosheets. In contrast, too high pressures induced a partial rupture of the wear-protecting tribo-layer, thus reducing its beneficial effects. Additionally, no beneficial effects regarding friction and/or wear were found at higher relative humidities, which was correlated with the expansion of the basal spacings. Therefore, this study summarizes favorable operating conditions for MXene nanosheets when used as solid lubricant to improve friction and/or wear thus making them excellent candidates for advanced, next-generation solid lubricants