Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects

This thesis presents the research work aimed at generating anodised aluminium surfaces with bright, white, and glossy decorative appearance. The aluminium surface finishing industry has been on the lookout for such surfaces due to their potential applications in aerospace, architecture, and design industry. Conventional colouring techniques applied to anodised aluminium surfaces cannot generate glossy white appearing surfaces due to the fundamental differences in the interaction with visible light that is required. Surfaces appearing as perfect white are due to the scattering of visible light providing high level of diffused reflection, which is similar to a mirror, but without the specular reflection component. Therefore the nature of the white surface should provide high scattering of light without significant absorption. Achieving white anodised surfaces using conventional dyeing techniques is not possible as the anodic pore sizes are an order of magnitude smaller than the traditional white pigments.
The approaches presented in this thesis focus on different techniques like modification of the aluminium microstructure, engineering of the aluminium surface, and application on non-conventional anodising processes. The idea behind the mentioned approaches is to enhance the scattering of visible light from the anodised aluminium surface and aluminium substrate interface for achieving high reflectance.
Magnetron sputtered coatings were employed to modify the Aluminium surface and tailor the microstructure in order to impart light scattering ability to the anodised layer. Coatings based on Al-Zr and Al-Ti binary system were studied for their anodising behaviour with and without heat treatment. The structure evolution of the Al-Zr sputtered coatings and the effect of Si during heat treatment was studied in-situ in a transmission electron microscope and also ex-situ using grazing incidence X-ray diffraction. The Al-Metal oxide surface composites based on TiO₂, Y₂O₃, and CeO₂ prepared by friction stir processing were employed to generate light scattering anodised surfaces by embedding oxide particles. Theoretical modelling and physical modelling of the anodised layer with TiO₂ particles were simulated using Polyurthenane-TiO₂ coatings and investigating the optical appearance.
The conventional DC anodising and high frequency pulse and pulse reverse pulse anodising were employed to generate bright-anodised surfaces. Effect of pulse frequency, anodic and cathodic cycle potential values was systematically investigated. Characterization of surfaces and substrates was performed using Scanning and Transmission Electron Microscopy, Grazing-Incidence X-ray Diffraction, Glow Discharge Optical Emission Spectroscopy, and Scanning Kelvin Probe Force Microscopy. Optical characterization was performed using integrating sphere measurements.
Combining the results and understanding obtained from anodising of magnetron sputtered coatings, Al-TiO₂ surface composites and their electrochemical behaviour under high frequency anodising, highly reflecting, bright and glossy white anodised aluminium surfaces were generated for the first time. Further, these white anodised surfaces were conventionally dyed to generate new appearances based on pastel colours.
The work presented in this thesis is mainly divided into two parts. The first part is based on magnetron sputter deposition, their heat treatment, characterization, and anodising behaviour (Chapter 5-8, 14). The second part of the thesis is based on Al-TiO2 composites, their preparation characterization, and anodising behaviour (Chapter 9-13). The outcome of the work is presented in the last chapter (Chapter 15). The results from white anodising of the Al surfaces, which is the primary goal of this thesis and also the generation of a new set of pastel coloured anodised surfaces is presented. The optical characterization of these white anodised surfaces along with other reference white surfaces is presented and discussed.