Effects of post-deposition annealing on the mechanical and chemical properties of the Si3N4/NbN multilayer coatings

Multilayered thin films consisting of alternate layers of silicon nitride (Si3N4) and niobium nitride (NbN) have been prepared by a dc reactive sputtering technique in nitrogen and argon atmosphere using high purity Nb and Si targets for various flow ratios of N2/Ar. It has been found that the hardness of the multilayered system is higher than that of the constituent individual layers of equal thickness. Although a single layer of amorphous Si3N4 has higher hardness compared with a single layer of nanocrystalline NbN at all the deposition conditions used in this experiment, the hardness of the multilayer coatings consisting of consecutive Si3N4/NbN layers strongly follows the hardness variation of the polycrystalline NbN. When the multilayer coatings are subjected to post-deposition annealing at high temperatures, it has been found that both the hardness and the adhesion strength of the coating decrease with increasing annealing temperature. X-ray photoelectron spectroscopy (XPS) results reveal that oxidation of the coatings during annealing plays a crucial role behind such deterioration in mechanical properties. Further, it has been noted that NbN is a more oxidation resistant material than Si3N4. Therefore, it has been proposed that during preparation of multilayers with consecutive thin layers of NbN and Si3N4, the topmost layer should be made of NbN, instead of Si3N4, to prevent the oxygen diffusion from the top surface layer to the next layer underneath