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

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_x, Ti₃CNT_x, or V₂CT_x, leaving multimetal MXenes largely unexplored. Here, we present a comprehensive tribological and mechanochemical evaluation of ordered double-transition metal Mo₂TiC₂T_x and Mo₂Ti₂C₃T_x coatings under dry sliding in ambient conditions. Using nanoindentation mapping, X-ray photoelectron spectroscopy, Raman spectroscopy, and electron microscopy, we demonstrate that Mo₂Ti₂C₃T_x 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_x 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_x exhibited the best tribological and mechanical performance under comparable conditions, clearly outperforming Ti₃C₂T_x, Ti₃CNT_x, and Mo₂TiC₂T_x. 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.