Synthesis and structure of smooth, superhard TiN/SiN_x multilayer coatings with an equiaxed microstructure

TiN coatings have been widely used in various tribological applications. However, TiN coatings predominantly grow with a columnar grain structure, and these columnar grain boundaries become the sites for crack initiation, resulting in premature failure of TiN coatings. In this paper, we report the use of amorphous SiN_x to periodically interrupt the growth of TiN in order to suppress the columnar structure. The growth was performed in a dual-cathode unbalanced reactive magnetron sputtering system. The resulting TiN/SiN_x multilayer coatings, when deposited under optimum conditions (TiN layer thickness = 2.0 nm, SiN_x layer thickness = 0.5 nm and substrate bias = -80 to -90 V) are smooth and exhibit an equiaxed grain structure with no evidence of columnar growth. The highest hardness is in excess of 45 GPa.