Tribological behaviour of Ti3C2Tx nano-sheets: Substratedependent tribo-chemical reactions

MXenes, a newly emerging class of layered two dimensional (2D) materials, are promising solidlubricants due to their 2D structure consisting of weakly-bonded layers with a low shear strength and abilityto form beneficial tribo-layers. This work aims at evaluating for the first time MXenes lubrication performanceand tribofilm formation ability on different metallic substrates (mirror-lapped Fe and Cu discs). After depositingMXenes via ethanol (1 wt%) on the substrates, pronounced differences in the resulting substrate-dependentfrictional evolution are observed. While MXenes are capable to reduce friction for both substrates after thefull evaporation of ethanol, MXenes lubricating effect on Cu is long-lasting, with a 35-fold increased lifetimecompared to Fe. Raman spectra acquired in the wear-tracks of the substrates and counter-bodies reveal notabledifferences in the friction-induced chemical changes depending on the substrate material. In case of Fe, theprogressive failure of MXenes lubrication generates different Fe oxides on both the substrate and the ball,resulting in continuously increasing friction and a poor lubrication effect. For Cu, sliding induces the formationof a Ti3C2-based tribofilm on both rubbing surfaces, enabling a long-lasting lubricating effect. This work boostsfurther experimental and theoretical work on MXenes involved tribo-chemical processes.