Superlubricity of Ti₃C₂T_x MXene at the Nanoscale—Effect of Layer Thickness and Aging

Due to the increasing specialization of space and earth-bound mechanical systems, the use, study, and synthesis of solid lubricants has gained attention over the last decade. The discovery of two-dimensional (2D) and hexagonal layered solid materials such as graphene, and members of the metal dichalcogenides family revolutionized the solid lubricants’ field with their ability to reduce the friction and applicability to extreme conditions of space. Very recently, it was shown that early transition metal carbides and nitrides (MXenes) as a new class of 2D layered material also offer promising solid lubrication properties at the macro- and nanoscales. Nevertheless, since the first titanium carbide MXene, Ti₃C₂T<i>_x</i>, was synthesized in 2011, friction property of Ti₃C₂T<i>_x</i> has not been fully understood. Lateral force microscopy (LFM) is an ideal technique to investigate the friction of Ti₃C₂T<i>_x</i> at the nanoscale under controllable environmental conditions. Here, LFM was employed to scrutinize the effect of layer thickness on the friction property of Ti₃C₂T<i>_x</i>MXene flakes on silicon substrates at the nanoscale in dry nitrogen environment. We have observed that a reduction of the friction coefficient to the superlubric regime could be achieved at the nanoscale, and the decrease became more pronounced with the increase of the thickness of Ti₃C₂T<i>_x</i>MXene flakes from single- to trilayers. Even though the further increase of the layer thickness slightly increased the friction coefficient, the superlubricity was preserved for bulk Ti₃C₂T<i>_x</i>MXene flakes. Moreover, the friction behaviors of Ti₃C₂T<i>_x</i>MXene flakes with different thicknesses were investigated at different time intervals, and a drastic deterioration from superlubricity to solid lubricity regime was observed as a consequence of oxidative degradation on Ti₃C₂T<i>_x</i>MXene flakes.