Adaptive NbN/Ag coatings for high temperature tribological applications

Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbalanced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000 °C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T > 700 °C. Post-wear testing structural and chemical characterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750 °C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates.