Effects of Ag content on the microstructure, mechanical and tribological properties of NbN-Ag films

• NbN-Ag films with different Ag doping levels were prepared. • The microstructures and properties of the films were investigated. • The film doped with 2.1at. % Ag possessed the maximum hardness with a value of 17.6 GPa. • NbN-Ag with Ag content of 2.1at. % showed the best combined friction performance. • Positive correlation between wear resistance and H/E ratio in NbN-Ag films. The metal phase Ag, due to its low critical shear stress, has been widely applied in the research of wear reduction and toughening of transition metal nitride hard films. In this study, NbN-Ag nanostructured films with various Ag content were deposited via reactive magnetron sputtering to explore the intrinsic relationship between their microstructure, mechanical properties, and tribological behavior. At an Ag doping level of ∼ 2.1 at. %, the fcc-Ag phase was found to coexist with the fcc-NbN matrix phase, where Ag was embedded either as a two-dimensional overlayer or as nanocrystalline grains. The film hardness initially increased and then decreased with rising Ag content, peaking at ∼17.6 GPa for the ∼2.1 at. % Ag-doped film. This enhancement was primarily attributed to grain boundary strengthening induced by the second-phase Ag. Moreover, the average friction coefficient exhibited a decreasing trend with increasing Ag content. The film with ∼2.1 at. % Ag demonstrated the best wear resistance, exhibiting an average friction coefficient of ∼ 0.54.