Synthesis, mechanical and tribological properties, and thermal stability of sputter-deposited titanium nickel-based and titanium diboride/titanium carbide multilayer hard coatings

This thesis is focused on the synthesis of hard and smooth multilayer coatings with low internal stress and equiaxed structure using dual-cathode unbalanced reactive magnetron sputtering system. The studied materials include TiN-based and TiB₂/TiC multilayer coatings. These as-deposited coatings and after annealing at elevated temperature were characterized and compared in terms of microstructure, structural, mechanical, and tribological properties.; 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 research, we report the use of CN_x and SiN_x to periodically interrupt the growth of TiN in order to suppress the columnar structure. The effect of TiN buffer layer with (111) texture, substrate bias, CN_x layer thickness on mechanical properties of TiN/CN_x coatings are discussed. The columnar grain structure is partially suppressed in these TiN/CN_x coatings. TiN/SiN_x multilayer coatings, when deposited under optimum conditions, are smooth and exhibit an equiaxed grain structure with no evidence of columnar growth. These TiN/SiN_x multilayer coatings also show better mechanical and tribological properties than those of TiN coatings. Lubricated block-on-ring wear tests show that the wear rate of TiN/SiN_x coatings is three times better than that of TiN coatings. The fatigue life of the rod coated with 0.75μm thick TiN/SiN_x multilayer coating is more than 10 times better than that of uncoated ones and 5 times better than rods coated with TiN coatings. With proper control of the SiN_x thickness, TiN/SiN _x multilayer coatings achieved good thermal stability and high hardness ∼37 GPa after annealing at 1000C.; Both as-deposited and annealed TiB₂/TiC multilayer coatings are examined. These TiB₂/TiC coatings exhibit excellent wear resistance under dry block-on-ring tests at room temperature. These coatings still keep sharp layer interfaces and maintain high hardness ∼40 GPa after annealing at 1000C although the layer interfaces were shifted.