Downloaded from orbit.dtu.dk on: Dec 18, 2017 Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects Gudla, Visweswara Chakravarthy; Ambat, Rajan Publication date: 2015 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Gudla, V. C., & Ambat, R. (2015). Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects. Kgs. Lyngby: Technical University of Denmark (DTU). 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Optically Designed Anodised Aluminium Surfaces Microstructural and Electrochemical Aspects PhD Thesis Visweswara Chakravarthy Gudla July 2015 Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects Visweswara Chakravarthy Gudla E-mail: [email protected] E-mail: [email protected], [email protected] Prof. Rajan Ambat E-mail: [email protected] Department of Mechanical Engineering, Materials and Surface Technology Technical University of Denmark Produktionstorvet, Building 425 2800 Kgs. Lyngby, Denmark Copyright: Reproduction of this publication in whole or in part must include the customary bibliographic citation, including author attribution, Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects, V. C. Gudla, 2015. Cover Image: Bright field transmission electron micrograph of anodised layer on Aluminium-TiO2 composite and the proposed light interaction mechanism with the anodic layer. Published by: Department of Mechanical Engineering, Section for Materials and Surface Engineering, Produktionstorvet, Building 425, 2800 Kgs. Lyngby, Denmark ii iii This thesis is submitted in partial fulfilment of the requirements for obtaining the degree of Ph.D. in Mechanical Engineering at Technical University of Denmark (DTU). The Ph.D. project was funded by the Danish National Advanced Technology Foundation (HTF, Højteknologi-fonden) through the project ‘Optically Designed Anodised Aluminium Surfaces (ODAAS)’. It was carried out at the Department of Mechanical Engineering, Section of Materials and Surface Engineering (MTU) in DTU during the period 1st April, 2012 – 14th July, 2015 under the supervision of Professor Dr. Rajan Ambat. Visweswara Chakravarthy Gudla Kongens Lyngby, 15th July 2015 iv 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. v Denne ph.d. afhandling beskriver resultater og overvejelser fra et forskningsarbejde, der har haft til formål at udvikle anodiserede aluminiumsoverflader, der fremstår med en blank og dekorativ hvid farve. Sådanne overflader har store potentielle anvendelser inden for f.eks. transport-, fly- og rumfarts-, bygnings- og designindustrierne og har i lang tid været efterspurgt i brancherne. Konventionelle teknikker til at farve anodiseret aluminium kan ikke fremstille skinnende hvide overflader, idet den fysiske størrelse af porerne i det anodiserede lag ikke er store nok til at rumme pigment med hvid farve. En overflade fremstår perfekt hvid, hvis den spreder alt indkommende lys i det synlige spektrum, uden at der absorberes noget lys i materialet. Derfor er det et krav til overfladens optiske egenskaber, at al synligt lys spredes uden signifikant absorption. I dette projekt er anvendt forskellige teknikker til at skræddersy aluminiumsoverfladen, f.eks. ved at ændre mikrostrukturen af og ved anvendelse af alternative anodiserings processer. Formålet med dette var at øge spredningen af synligt lys i det anodiserede lag og i grænselaget mellem basismaterialet og det anodiserede lag. Belægninger fremstillet ved magnetronforstøvning blev optimeret, så lys vil spredes effektivt ved en efterfølgende anodisering. Egenskaberne ved anodisering af belægninger baseret på Al-Zr og Al-Ti binære systemer blev undersøgt, både med og uden efterfølgende varmebehandling. Udviklingen af mikrostrukturen af Al-Zr belagte overflader samt effekten af Si i grundmaterialet blev undersøgt in-situ ved transmission elektron mikroskopi og ex-situ ved røntgen spektroskopi. Overflader af Al-metaloxid kompositter, baseret på TiO2, Y2O3, og CeO2, blev fremstillet ved friction stir processer og er anvendt til at skabe anodiserede overflader med integrerede partikler af metaloxider, der effektivt spreder synligt lys. De optiske egenskaber af anodiseret Al med inkorporerede TiO2 partikler er modelleret ud fra teoretiske beregninger, og eftervist med fysiske modeller med Polyuretan-TiO2. Konventionel DC anodisering samt anodisering med højfrekvente puls mønstre er anvendt til at fremstille lyse, anodiserede overflader. Effekten af puls frekvensen og potentialerne af de anodiske og katodiske pulser, er undersøgt systematisk. De frembragte overflader og substrater er karakteriseret med scanningselektronmikroskopi (SEM), transmissionselektronmikroskopi (TEM), røntgendiffraktion (XRD), optisk emission spektroskopi (GDOES),