Microstructure, Mechanical and Tribological Properties of Ternary Transition Metal Nitride Coatings

Abstract

The evolution of transition metal nitride (TMN) coatings with excellent mechanical and tribological properties has been shown to be a successful strategy in protecting tool components. In this thesis, the microstructure, mechanical properties and tribological performances of a range of TMN-based coatings, synthesised by physical vapor deposition techniques, were explored. First, the influence of substrate bias, on the structure and properties of magnetron sputtered TiSiN coatings was investigated. Enhanced scratch resistance i.e., higher critical loads (Lc1 and Lc2) was found in the coating deposited at the lower bias voltage (-40 V), which was ascribed to higher H/Er and H3/Er 2 ratios arising from fine nanocomposite structure and the presence of a higher compressive residual stress. An approximately 21% increase in wear rate was obtained for the coating prepared at higher bias (-50 V), which was attributed to slightly higher Si concentrations (~9.4 at.%) and, in turn, lower hardness. Further, a notable increase in Lc1 (~54%) and Lc2 (~27%) values, was obtained for a thick TiSiN coating (magnetron sputtered at a different condition) in comparison to the binary TiN coating that were underlain by its superior mechanical properties and graded structure, promoting the capacity to resist crack formation and delamination. Furthermore, the influence of Ni content, regulated by cathode composition, on the structure and properties of cathodic arc evaporated TiNiN coatings was examined. A transition from a fine columnar structure, at low Ni contents (~2 at.%), to a much finer equiaxed structure at higher Ni concentrations (≥ 4 at.%) was noted. In addition, the density of macroparticles generated during arcing was shown to be inversely related to the melting temperature of the target material. Finally, the effect of Ni content, controlled by the NiCr target current (INiCr) on the structure mechanical properties and scratch and wear behaviour of magnetron sputtered CrNiN coatings was studied. Significant damage-tolerance, coupled with good hardness values (greater than ~12 GPa), was found in the CrNiN coatings deposited at INiCr ≥2 A. The presence of a metallic nickel-rich phase, together with nanoscale porosity, may contribute to stress dissipation and help maintain structural integrity.