Growth and characterisation of multifunctional nanocomposite coatings

Thirumalai, Sundararajan 

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Novel nanocomposite coatings of the type Fe-Ti-N and Fe-Ti-O-N, having in-situ formed periodic multiphase nanocolumnar microstructures, have been developed by a two-target reactive magnetron sputter deposition and nanoparticle-assisted electro-co-deposition processes. Advanced thin-film characterisation techniques, such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, surface wettability, nanoindentation and nanowear testing, have been utilised effectively to study the coating structure and evaluate their performance. The origin of such a unique microstructure is attributed to the thermodynamic immiscibility of the phase constituents. The columnar width and tilt angle could be precisely tuned by modifying various deposition parameters, such as sputter gas pressure, target power, substrate tilt and rotation. A vector summation approach has been developed to derive the effective angle of incidence from the two sputter targets, and validate the classical tangent relationship between the said effective angle of incidence and the columnar tilt angle of the coatings.

Moreover, a cosine relationship has been established to correlate the coating composition and angle of incidences of the sputtered species. Furthermore, a detailed structure-property correlation has been developed with due emphasis on understanding the mechanical and functional properties of the coatings. Preliminary finite-element modelling of the coating contact deformation have been performed, to explore the stress field distribution and crack healing potential of the innovative microstructures developed in this work. Understanding of the formation mechanism and microstructure control of such unique periodic nanostructures have been exploited, to develop dense biphase helical metal-nitride interpenetrating nanocomposite coating microstructures with an exceptional nanohardness of up to 17.1 GPa, despite having a high iron content of 59 %. Finally, nanoporous nanocomposite coating forms have also been fabricated using a high-pressure/power sputter deposition process in the classical Thornton zone-1 deposition regime, for potential niche applications such as photocatalyst/catalyst-support for dye-degradation and anti-bacterial surfaces. A modified-Wenzel equation has been proposed to effectively correlate the surface wettability of the porous films with their surface roughness.

Greer, Alan Lindsay
Antimicrobial surfaces, Cermets, Electro-co-deposition, Nanocomposites, PVD/PE-CVD hard coatings, Surface engineering, Thermal stability, Thin film photocatalysis, Tribology
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (642642)