Oxidation-mediated high-temperature lubricating and anti-wear properties in composition-graded Cr_5/2–5xTi_3/2+xMo_4xAlC₃ o-MAXs

Recently, a multi-elemental Cr_{5/2–5 x} Ti_{3/2+ x} Mo_{4 x} AlC₃ ( x = 1/3, 1/4, 1/6) MAX phase ceramics with an out-of-plane chemical ordering have open a new space for acquiring the robust mechanical properties. However, as a potential solid-lubricant of MAX phase family, the dependent of their self-lubrication performance on the elemental composition is still unrevealed. Herein, the tribological properties of these MAXs against the Si₃N₄ balls are carefully examined in air at temperatures as high as 800 °C. The high Cr content contributes to a high hardness/elastic modulus ( H / E ) ratio, which improves the low-temperature wear resistance. Meanwhile, high Mo content helps on building the oxide film and thus achieving low friction at 600 ºC and above. At 800 °C, a composite oxide film approximately 0.5 μm thick was formed on the surface of the Cr_0.83Ti_1.83Mo_1.33AlC₃. This oxide film significantly improved its tribological properties, stabilizing the average friction coefficient (COF) at 0.22 and reducing the wear rate to 1.51 × 10⁻⁷ mm³ N⁻¹ m⁻¹.