Effect of Stoichiometry in Mo-Based Ordered Double Transition Metal Carbide MXenes on Solid Lubrication and Tribo-Film Formation

MXenes have emerged as promising solid lubricants due to their layered structure, tunable chemistry, and ability to form mechanically robust, wear-resistant tribo-films. However, most studies have focused on single-metal MXenes such as Ti₃C₂T_<i>x</i>, Ti₃CNT_<i>x</i>, or V₂CT_<i>x</i>, leaving multimetal MXenes largely unexplored. Here, we present a comprehensive tribological and mechanochemical evaluation of ordered double-transition metal Mo₂TiC₂T_<i>x</i> and Mo₂Ti₂C₃T_<i>x</i> coatings under dry sliding in ambient conditions. Using nanoindentation mapping, X-ray photoelectron spectroscopy, Raman spectroscopy, and electron microscopy, we demonstrate that Mo₂Ti₂C₃T_<i>x</i> tends to form dense, chemically stabilized, and mechanically robust tribo-layers thus maintaining a low and stable coefficient of friction (∼0.1) and wear rate (∼0.1 × 10^-3 mm³/N·m) under a contact pressure of 0.55 GPa. These tribolayers exhibit improved mechanical properties (hardness ∼ 4.2 GPa; Young's modulus ∼ 103 GPa), along with increased carbide retention and reduced surface oxidation. In contrast, Mo₂TiC₂T_<i>x</i> coatings display a less favorable behavior, resulting in a higher COF (∼0.5), greater wear rate (∼1.3 × 10^-3 mm³/N·m), and the formation of thinner, chemically degraded tribo-layers under comparable conditions. Mo₂Ti₂C₃T_<i>x</i> exhibited the best tribological and mechanical performance under comparable conditions, clearly outperforming Ti₃C₂T_<i>x</i>, Ti₃CNT_x, and Mo₂TiC₂T_<i>x</i>. Our study introduces Mo-based MXenes as an emerging frontier in solid lubrication and the importance of MXene structure and composition in their tribo-layer evolution and stress accommodation mechanisms.