The Effects of the Temperature and Termination(-O) on the Friction and Adhesion Properties of MXenes Using Molecular...

Two-dimensional transition metal carbides and nitrides (MXenes) are widely applied in the fields of electrochemistry, energy storage, electromagnetism, etc., due to their extremely excellent properties, including mechanical performance, thermal stability, photothermal conversion and abundant surface properties. Usually, the surfaces of the MXenes are terminated by -OH, -F, -O or other functional groups and these functional groups of MXenes are related surface properties and reported to affect the mechanical properties of MXenes. Thus, understanding the effects of surface terminal groups on the properties of MXenes is crucial for device fabrication as well as composite synthesis using MXenes. In this paper, using molecular dynamics (MD) simulation, we study the adhesion and friction properties of Ti₂C and Ti₂CO₂, including the indentation strength, adhesion energy and dynamics of friction. Our indentation fracture simulation reveals that there are many unbroken bonds and large residual stresses due to the oxidation of oxygen atoms on the surface of Ti₂CO₂. By contrast, the cracks of Ti₂C keep clean at all temperatures. In addition, we calculate the elastic constants of Ti₂C and Ti₂CO₂ by the fitting force-displacement curves with elastic plate theory and demonstrate that the elastic module of Ti₂CO₂ is higher. Although the temperature had a significant effect on the indentation fracture process, it hardly influences maximum adhesion. The adhesion energies of Ti₂C and Ti₂CO₂ were calculated to be 0.3 J/m² and 0.5 J/m² according to Maugis-Dugdale theory. In the friction simulation, the stick-slip atomic scale phenomenon is clearly observed. The friction force and roughness (Ra) of Ti₂C and Ti₂CO₂ at different temperatures are analyzed. Our study provides a comprehensive insight into the mechanical behavior of nanoindentation and the surface properties of oxygen functionalized MXenes, and the results are beneficial for the further design of nanodevices and composites.