Temperature-adaptive MoN-Ag coatings under operando tribology: multiscale Ag diffusion-oxidation coupling

The development of temperature-adaptive lubricant coatings for foil air bearings remains a major challenge. Here, dense MoN–Ag nanocomposite coatings with varying Ag contents (0–22.8 at.%) were deposited by HiPIMS–DCMS. The coatings exhibited low residual stress, strong adhesion, and high mechanical performance. Tribological tests showed that the coefficient of friction (COF) decreased with Ag addition at 350 °C and 500 °C, reaching 0.389 and 0.223, respectively, for the 22.8 at.% Ag coating. At 600 °C, however, the COF increased with Ag content, while the 13.4 at.% Ag coating displayed the lowest wear rate of 2.84 × 10^-5 mm³N^-1m⁻¹. Under simulated service conditions, the 13.4 at.% Ag coating demonstrated a reduction in COF from ∼ 0.4 to 0.26 across 25–600 °C, evidencing self-adaptive lubrication. This behavior is ascribed to the formation of a low-shear-strength surface layer, where Ag diffusion and the generation of MoO_x, Ag₂MoO₄, and Ag₂Mo₄O₁₃ promote facile slip along crystal planes, thereby enhancing lubricity.