Structure, mechanical properties, and high temperature stability of nanolayered titanium nitride/titanium diboride and zirconium nitride/zirconium diboride thin films

The focus of this research is the deposition, structure, mechanical properties, and high temperature stability of polycrystalline nanolayered thin films. TiN/TiB₂ and ZrN/ZrB₂ nanolayers were magnetron sputtered with periods (Λ) ranging from 2 to 28 nm. The films were chemically and structurally characterized using Auger electron spectroscopy (AES), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties were characterized using nanoindentation, scratch testing, Rockwell indentation, and substrate curvature measurements. High temperature annealing, 750–1100°C, was used to investigate the thermal stability of the layered structures.; As-deposited TiN/TiB₂ and ZrN/ZrB₂ nanolayers displayed an amorphous structure for the boride layers and a (200) and (111) preferred orientation for TiN and ZrN respectively. Annealing of the coatings was required to crystallize the diboride layers. XRD showed the presence of a mixed orientation (001) and (101) TiB₂ and (001) ZrB₂. The improved crystallinity raised the nanolayer hardness from a maximum of 38 GPa to 49 GPa for both material systems, a hardness enhancement of ~25% over the rule-of-mixtures value for the constituent materials. Analysis of the annealed TiN/TiB ₂ microstructure with TEM revealed a randomly oriented, rotated nanolayer domain structure.; The deposition process used allowed for variation of the substrate bias voltage, thereby allowing the production of nanolayered coatings with different amounts of residual stress. Increasing the amount of compressive stress was shown to have a positive effect on hardness, but was detrimental to film/substrate adhesion. The compressive stress of the nanolayered coatings was observed to be as much as 7 times less than the monolithic component materials. The interface structure was assumed to allow for stress relaxation through point defect annihilation and stress compensation. Deposition of a series of nanolayers with differing interface densities proved this assumption to be correct.; To prove the industrial relevance of nanolayered nitride/boride coatings, ZrN/ZrB₂ coated cutting inserts were tested in a continuous turning mode. Whether cutting aluminum or medium carbon steel, the nanolayer exhibited a comparable lifetime to industrially available TiN or TiAlN coated tools, but with less wear at the cutting edge.