Optimizing Ag content through grain boundary engineering in TiAlNbN-Ag films for balanced tribological performance and high-temperature oxidation resistance

The introduction of a lubricating phase into transition-metal nitride films such as TiAlN is a common strategy to reduce friction, yet the underlying mechanism by which lubricants regulate the coupled tribology-oxidation behavior remains poorly understood. To address this knowledge gap, we propose a novel strategy of lubricating phase-induced grain boundary engineering by incorporating Ag into a multicomponent nitride matrix. Here, we demonstrate that Ag, owing to its negligible solubility, segregates as discrete nanoparticles along columnar grain boundaries, refining the microstructure while reducing hardness. This grain-boundary decoration produces a dual effect. During sliding, grain-boundary Ag is mechanically activated to plastically smear into low-shear lubricious regions and concurrently helps stabilize an amorphous-encapsulated nanocrystalline tribolayer, thereby reducing the wear rate by up to 85.7%. During high-temperature oxidation, however, Ag addition increases the grain-boundary area fraction, while segregated Ag remains as a metallic grain-boundary phase with low oxygen affinity. As a result, the Ag-decorated grain boundaries are likely to act as short-circuit pathways for inward oxygen transport and to hinder the formation of a continuous Al 2 O 3 -rich protective scale. Combined with Ag coarsening during oxidation, this progressively degrades oxidation resistance. The oxidation trend further indicates that grain-boundary chemistry and Ag-network connectivity, rather than grain-boundary density alone, govern the degradation. Among the investigated compositions, the film containing 14.07 at.% Ag provides the best compromise between adaptive lubrication and acceptable oxidation resistance. These results highlight grain-boundary-network control as a practical route to balancing tribological and oxidation performance in adaptive nitride films.