Humidity-Dependent\nTribological Performance of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>/MoS<sub>2</sub> Nanosheet-Based Hybrid Coatings

Two-dimensional (2D) materials are\nknown to induce excellent\nsolid\nlubrication performance. However, their performance largely depends\non the ambient conditions (working atmosphere), since moisture and\noxygen deteriorate the frictional properties of MoS<sub>2</sub> and\nMXenes. Currently, little knowledge is available regarding the humidity-dependent\ntribological performance of MXene coatings. Therefore, our contribution\naims at experimentally evaluating the tribological performance of\nmultilayer Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>/MoS<sub>2</sub> hybrid coatings used as solid lubricants dependent\non the relative humidity (10, 35, and 70%). For this purpose, mixed\n(50% multilayer Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> and 50% MoS<sub>2</sub>) and bilayer (bottom Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> and top layer MoS<sub>2</sub>) hybrid coatings were deposited via spray-coating on chromium steel\ndiscs to be tested under dry reciprocating conditions using ball-on-disk\ntribometry. Our results show that both hybrid coatings performed best\nunder low relative humidities, thus notably improving the resulting\nfriction and wear performance. The favorable tribological mechanism\nwas mainly attributed to the low-shear stress between sulfides derived\nfrom MoS<sub>2</sub>. At higher relative humidities, the formation\nof oxides detrimentally affected lubrication, thus causing friction\nto increase and leading to more pronounced wear. This study sheds\nlight on the underlying mechanisms governing friction and wear for\nhybrid MXene/MoS<sub>2</sub> coatings tested at different humidity\nlevels.