Humidity-Dependent Tribological Performance of Ti₃C₂T<i>_x</i>/MoS₂ Nanosheet-Based Hybrid Coatings

Two-dimensional (2D) materials are known to induce excellent solid lubrication performance. However, their performance largely depends on the ambient conditions (working atmosphere), since moisture and oxygen deteriorate the frictional properties of MoS2 and MXenes. Currently, little knowledge is available regarding the humidity-dependent tribological performance of MXene coatings. Therefore, our contribution aims at experimentally evaluating the tribological performance of multilayer Ti3C2Tx/MoS2 hybrid coatings used as solid lubricants dependent on the relative humidity (10, 35, and 70%). For this purpose, mixed (50% multilayer Ti3C2Tx and 50% MoS2) and bilayer (bottom Ti3C2Tx and top layer MoS2) hybrid coatings were deposited via spray-coating on chromium steel discs to be tested under dry reciprocating conditions using ball-on-disk tribometry. Our results show that both hybrid coatings performed best under low relative humidities, thus notably improving the resulting friction and wear performance. The favorable tribological mechanism was mainly attributed to the low-shear stress between sulfides derived from MoS2. At higher relative humidities, the formation of oxides detrimentally affected lubrication, thus causing friction to increase and leading to more pronounced wear. This study sheds light on the underlying mechanisms governing friction and wear for hybrid MXene/MoS2 coatings tested at different humidity levels.