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New smooth self-cleaning coatings

Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. : 06 October, 2002  (Company News)
Only a few years ago, soil-repellant coatings for polished metal surfaces were unthinkable. Although it is possible to exploit the 'lotus effect' and create knobby microstructured surfaces, they are not very resistant to mechanical stress and the coated surface loses its sheen.
A more suitable solution are layers made of hard and anti-adhesive silicate-containing sol-gel systems. Thanks to their smoothness, they minimize the tendency of fingerprints and dirt particles to adher to the surface, allowing them to be rinsed off more easily. But a sophisticated coating process is called for because the adherence to the metal surface has to be as long-lasting as possible. Researchers at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA have managed to find an ingenious way to even coat chromium.

Dr. Siegfried Berg, IPA project manager, recalls when they were contacted by representatives of the metalworking firm HANSA Metallwerke: 'When they asked us to improve their coatings on chromium-plated bathroom fittings, it meant a great challenge to us. The biggest problem was that the commercial product did not permanently adhere.' In the case of chromium, it is the surface oxide layer formed with atmospheric oxygen which causes the problems. Together with FEW Chemicals, a producer of sol-gel materials, the three partners found out how to treat the surface of the electroplated parts prior to coating. This can be done in two ways, both sufficiently cheap: Chemical activation with the use of cleaning and reducing agents transforms the oxide to metallic chromium. Physical activation is done through sputter cleaning: Ions of the noble gas argon are electrically accelerated and shot as a beam onto the metal surface, like billiard balls. This bombardment removes the upmost oxygen and chromium atoms. The results of implementing these process steps were highly satifactory, as Berg reports: 'The subsequently applied sol-gel coating showed the desired mechanical properties. But more research and work has to be done in order to coat metal parts in a mass production line.'

Establishing such soil-repellant coatings on the market would be a breakthrough - not only for metal-processing companies. Commercial cleaning firms could benefit from reduced efforts and costs. But even antiadhesive surfaces can't be fully cleaned by the action of rain or bathwater alone. However, one day a soft rag and mild detergent will relegate scrubbing brushes, scouring powder and elbow grease to the history books.
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