Effect of Ag contents on the microstructure and tribological behaviors of NbN–Ag coatings at elevated temperatures

NbN/Ag composite coatings with variable silver contents were prepared by magnetron sputtering technology. The effects of Ag content on the microstructure, mechanical properties and tribological properties were investigated. Results show that the crystal structure of coatings varied from Nb–Ag–N solid solution to NbN–Ag nanoclusters with the increase in Ag content. The NbN/Ag coating prepared with 0.2 A sputtering current of Ag target (Nb–Ag–N solid solution coatings) exhibited the highest hardness. The friction coefficients of all the coatings gradually decreased while the wear rate tended to increase with the increase in temperature from 25 °C to 550 °C. The decrease in friction coefficient was mainly attributed to the formation of lubricating oxide mixtures of AgNbO3 and Nb2O5 by tribo-chemical reaction. The wear rate of NbN–Ag-0.2A coating showed a 68.9% reduction in comparison with the NbN coating at 25 °C, while the NbN–Ag-0.4A coating exhibits a better wear resistance than other samples at elevated temperatures. This is mainly due to the different lubrication mechanism triggered by different temperatures.