Mo-doped Cr-Ti-Mo ternary o-MAX with ultra-low wear at elevated temperatures

In this work, the Cr-Ti-Mo ternary o-MAX ceramics based on the Cr₂TiAlC₂ phase are synthesized, and their tribological performance at elevated temperatures up to 800 ºC in the air is evaluated. With Si₃N₄ as a tribocouple, an unexpected ultra-low wear rate reaching 6.1 × 10⁻⁸ mm³ N⁻¹ m⁻¹ is observed in Cr_1.9Ti_0.9Mo_0.2AlC₂ at 800 ºC, accompanied by a stable coefficient of friction (COF) around 0.33 in a wide temperature range. X-ray photoelectron spectroscopy (XPS) depth profiling confirms a tribofilm with gradient composition, which simultaneously offers fluid lubricating at elevated temperatures and self-healing after cooling. Particularly, confirmed by the theoretical simulations, the doping of Mo improves the interlayer binding as well as alters the oxidation behaviors of Cr₂TiAlC₂. With an optimal interlayer binding strength and oxidation rate, the Cr_1.9Ti_0.9Mo_0.2AlC₂ can generate a tribofilm possessing ideal composition, which simultaneously promotes lubrication and anti-wear performance at elevated temperatures.