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 Ti 3 C 2 T x, Ti 3 CNT x, or V 2 CT x, leaving multimetal MXenes largely unexplored. Here, we present a comprehensive tribological and mechanochemical evaluation of ordered double-transition metal Mo 2 TiC 2 T x and Mo 2 Ti 2 C 3 T x coatings under dry sliding in ambient conditions. Using nanoindentation mapping, X-ray photoelectron spectroscopy, Raman spectroscopy, and electron microscopy, we demonstrate that Mo 2 Ti 2 C 3 T 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 mm 3 /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, Mo 2 TiC 2 T x coatings display a less favorable behavior, resulting in a higher COF (∼0.5), greater wear rate (∼1.3 × 10 –3 mm 3 /N·m), and the formation of thinner, chemically degraded tribo-layers under comparable conditions. Mo 2 Ti 2 C 3 T x exhibited the best tribological and mechanical performance under comparable conditions, clearly outperforming Ti 3 C 2 T x, Ti 3 CNT x, and Mo 2 TiC 2 T 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.