Influence of Ag additions on the structure, mechanical properties and oxidation behaviour of CrAlNAg coatings deposited by sputtering

In this work, the influence of Ag additions into CrAlN coatings on the surface morphology, chemical composition, structure, hardness and oxidation behavior was studied. Increasing the Ag content decreased the size of the morphological features and produced a more compact coating. All coatings displayed a fcc structure typical of transition metal nitrides; the silver phase was indexed for the coatings with Ag concentration ≥ 8.6 at.%. XPS results confirmed that Ag existed as nanoclusters within the film coating structure. A maximum hardness of around 23 GPa was observed for the 8.6 at. % of Ag. The decrease of the hardness with further increase in the Ag content was ascribed to the much softer character of this phase. The reference CrAlN coating displayed the best oxidation resistance and the lowest onset point of oxidation. The good oxidation resistance of this coating was attributed to the formation of a continuous and protective thin Cr2O3 layer on the top of the oxide layer. Ag additions degraded the oxidation resistance of the coatings, since the diffusion of Ag to the top surface of the oxide scale created extra paths for ions diffusion in the protective Cr-O layer. The formation of AgCrO2 phase in CrAlNAg9 coating, for the particular temperature of at 800°C, was responsible for the low oxidation resistance of the coating as compared to the same coating oxidized at even when compared to the one achieved at 900°C.