On the trail of a brand-new type of durable colour

If DTU Mechanical Engineering succeeds with its ODAAS project, we may experience a brand new type of coloured metal in just a few years' time. ODAAS is a consortium that stands for ”Optically Designed Anodised Aluminium Surfaces” and its aim is to create the colour white in a durable form that looks like milk on the surface of a mirror. A kind of luminous colour that is inherent in the metal and not just added with a layer of paint. Many researchers around the world are currently involved in similar projects, but the DTU researchers are taking a unique approach. And that gives them excellent chances of success and being first to market with their invention. It could add a whole new dimension to our traditional colour perception,and open up unique design possibilities for Danish industry.  

 

The idea of challenging our colour perception came to light at B&O, whose ambition is to add a completely unique element to the future design of sound systems. B&O has worked on the concept – known as aluminium anodising – for several years. The process electrochemically oxidises the surface of the aluminium, which develops a protective oxide film. This film is filled with microscopic pores/holes that can be subsequently embedded with colour, thus making the colour an integrated part of the film and not an added layer. And when the metal surface is mirror-polished prior to anodising, the result is the very special ’porcelain glaze’ finish that characterises B&O products.

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Figure 1: Two aluminium objects with different colours. The one with the shiny red colour is made using a standard anodizing and colour dyeing process, while the white object is created by painting as it cannot be produced by the standard dyeing process because it is based on the absorption of unwanted colours, and white is composed of all colours. The desired white appearance therefore requires a different type of optical light interaction with the anodized metal surface (described in Figure 2). The aim in this project is to create a white surface with the same visual appearance as the red one, thus giving it a higher aesthetic value.

 

Seeking even more beautiful finishes and colours

Anodised metal finishes are beautiful. But Associate Professor Rajan Ambat, Professor Ole Sigmund, PhD student Chakravarthy Gudla and PhD student Villads Egede Johansen aim to create something even more unique. They are seeking a special white colour that can best be described as what you see when you pour milk on the surface of a mirror. A colour that isn’t just a surface coating, but which comes from within – a luminous layer within the metal itself. A glassy finish, but one that is very strong and far more durable than traditional finishes.

 

Special particles create the colour

The DTU researchers represent their respective sections – Materials and Surface Engineering and the TopOpt group (Topology Optimization) under the Solid Mechanics section. Using their different perspectives, the researchers are pursuing several potential strategies. All of them deal with providing the metal surface with particles that can reflect and diffuse light in a way that causes the eye and the brain to perceive the sought-after fairy-tale white colour. The question is, which particles can do the job – and how can they be deposited in the surface of the metal in the correct quantities, and in the right positions.  

One research track pursues advanced work with the principle of anodising the metal surface. The other track deals with coating the surface of the metal with what is known as PVD treatment (Physical Vapour Deposition). In this process, condensation of a vaporised form of the desired film material is deposited onto the surface of the metal or alloy. In connection with this coating process Rajan Ambat is working very closely with researchers at the Tribology Centre at the Danish Technological Institute to create the embedded particles that can reflect the light in exactly the right way. This work requires cooperation across a number of disciplines, which is made possible by the ODAAS partnership.

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Figure 2: Schematic illustration of the light interaction with the anodized metal surface showing specular reflection and sub-surface reflection due to scattering inside of the anodized layer. Combination of specular and diffused scattered reflection could generate diffused to white appearance together with other optical parameters related to the anodized layer and embedded particles.

More complex than expected

In their pursuit of the right particles, the researchers use methods that include optical computer modelling. The aim is to find both the right size and shape of the particles intended for the metal surface. The work entails a continual interplay between creating and testing the surfaces in the physical laboratory, and then using optical computer modelling to change the design of the particles to move closer to achieving the right shade of white. And it seems that there are several parameters in play during the creation of this brand new colour effect. But the researchers are hopeful – many obstacles have already been overcome and the remaining ones will probably be solved soon. So when the project concludes in 2015, it is very likely that the colour white will have been given a brand new dimension. And also expect some spin-off colours to be invented along the way – shades of grey or blue – are sure to become beautiful new shades of those colours as well.

 

Huge potential within design and architecture

The ODAAS research project deals primarily with creating the exact shade of white for the B&O aluminium cabinets. But over the long term the project’s potential is even greater. Metal surfaces are used for a wide range of design products as well as within architecture. Therefore it is not impossible that future homes and cars may feature luminous colours that shimmer within the building facade or the body of the car itself. 

 

Facts:

The project is entitled: ”Optically Designed Anodised Aluminium Surfaces (ODAAS)”. The project duration is 4 years until the end of 2015, and the project has a total budget of DKK 27 million, of which DKK 13.5 million is provided by The Danish National Advanced Technology Foundation. The project is carried out in conjunction with: DTU Mechanical Engineering, DTU Fotonik (Department of Photonics Engineering), Risø, Bang & Olufsen A/S and the Tribology Centre at the Danish Technological Institute.

 

http://www.mek.dtu.dk/english/Research/Feature_Articles/On_the_trail_of_a_brand-new_type_of_durable_colour
26 SEPTEMBER 2018