Microstructure and properties of transition metal nitride and medium entropy alloy coatings

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Copyright: Cao, Fuyang
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Abstract
Hard thin film coatings play an important role in many industrial applications, such as cutting tools and biomedical devices. Magnetron sputtering, as one important deposition method, has been widely applied to a wide range of coating types. In this thesis, the microstructure and mechanical properties of a number of different families of coatings, deposited by magnetron sputtering, were investigated in detail. First, the influence of reactive gas ratio on the microstructure and mechanical properties of Ni-Ti-O-N nanocomposite coatings was studied. It was shown that an increase in reactive gas flow ratio resulted in a transformation from a dense structure with columnar grains into a porous structure with equiaxed grains, accompanied by a degradation in mechanical properties. It was shown that intergranular porosity had a detrimental effect on both hardness and elastic modulus. Further, the effect of substrate bias on the TiAlSiN coatings deposited at ~ 500°C was investigated. Variations in substrate bias had little impact on the chemical concentration of the coatings, but significantly influenced the microstructure, phase composition and mechanical properties. An evolution of microstructure from coarse to fine columnar grains with increasing bias voltage was identified, together with enhanced hardness and elastic modulus. Further, a transformation from a mixed fcc TiN + fcc AlN phases into a single fcc TiAlN phase was observed with increasing substrate bias. Finally, a series of equiatomic medium entropy alloy CoCrNi coatings, including both monolayer coatings and a tri-layered CoCrNi/Ti coating, was deposited onto M2 substrates. It was found that the monolayer CoCrNi coatings were composed of mostly a fcc CoCrNi phase, together with a small amount of a hcp Co-based phase. Interestingly, for the tri-layered coating, the outmost layer exhibited a fcc structure, while a bcc structure was observed in both the bottom and middle layers. The transition from fcc to bcc phase may be explained by Shockley partial dislocation motion under compressive stress. A remarkable hardness of ~10 GPa was determined for these coatings. The presence of nanotwins in these coating may account for the high hardness and toughness.
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Author(s)
Cao, Fuyang
Supervisor(s)
Munroe, Paul
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Publication Year
2019
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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