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Abstract
Hard coatings have been widely used for decades in a range of applications, such as cutting tools or medical devices. In this thesis, the microstructure and mechanical properties of a number of nitride, silicide and high entropy alloy coatings were investigated. First, for molybdenum nitride coatings, the influence of nitrogen content on microstructure and mechanical properties was studied. It was shown that an increase in nitrogen concentration resulted in a structural transformation from a bcc α-Mo phase to fcc γ-Mo2N and then to fcc B1 MoN, as well as a change in the nature of the Mo-N bonds from covalent bonds to weaker ionic bonds, which led to changes in mechanical and tribological behavior. In addition, the formation of lubricious MoOx layers on MoN contributed to a low coefficient of friction. Further, for silicide-based compounds, the effects of Ag on MoO3-SiO2 and Nb5Si3 coatings, together with the effects of Al on Ta5Si3 coatings were investigated. MoO3-SiO2 coatings consisted of hexagonal-structured MoO3 together with amorphous SiO2 and a discrete fcc Ag phase. The addition of Ag increased damage tolerance, but reduced hardness and modulus and degraded scratch adhesion. For Nb5Si3 coatings it was found that Ag dissolved in solution in tetragonal α-Nb5Si3 by locating on Nb sites in the α-Nb5Si3 lattice. The contact damage resistance was increased by the addition of Ag, while both hardness and elastic modulus were reduced and scratch resistance was degraded. For Ta5Si3 coatings, it was found Al atoms substituted for Si sites in the β-Ta5Si3 unit cell. Al additions to Ta5Si3 coatings generated improved damage tolerance, but also lowered hardness, elastic modulus and coating adhesion. Finally, the effect of deposition temperature on the structure and properties of high-entropy alloy TiMoWZrHf coatings was investigated. Coatings were found to be composed of nanoscale mixtures of fcc HfO2/ZrO2-based oxides together with a bcc Mo0.4W0.6 phase. The coating deposited at the higher temperature exhibited a lower density of voids resulting in higher hardness and elastic modulus, as well as reduced damage tolerance and higher critical loads during scratch testing.