Dr.-Ing. Olaf Dambon, Fraunhofer IPT: As hard as diamond
The "Fine Machining and Optics" department at the Fraunhofer IPT develops technologies for manufacturing and machining precision components. These include optical systems, precision tools and wafers for semiconductor applications. Dr.-Ing. Olaf Dambon has been in charge of the "Fine Machining and Optics" department since 2006.
1. The Herschel and Planck satellites were launched into space in 2009 with a silicon carbide mirror measuring 3.5 meters in diameter. What makes the material particularly suitable for space applications?
Dr.-Ing. Olaf Dambon: Silicon carbide is very strong and hard, has a relatively low density and a low thermal expansion. Thanks to these properties, the material makes it possible to produce mirrors which are lighter than the ones normally used in space. Given the great forces exerted when a rocket is launched, even low masses play an important role. The silicon carbide is also able to withstand the great temperature differences in space – depending on whether objects are facing towards or away from the sun. With its low thermal expansion, it is particularly suitable for optical components and helps to ensure that they remain functional.
2. In which other areas can the material be used?
Dr.-Ing. Olaf Dambon: Silicon carbide is ideal wherever precision components in combination with low expansion or masses are required. Examples include structural components in measuring devices, in semiconductor systems or in expendable parts.
3. What advantages does silicon carbide offer over other materials such as metal or glass ceramic?
Dr.-Ing. Olaf Dambon: Silicon carbide offers above all a very high specific strength. Compared to Zerodur, even semi-finished parts can be produced with contours approaching their final shape.
4. What difficulties are encountered when working with the material?
Dr.-Ing. Olaf Dambon: Given its hardness – it is one of the hardest materials after diamond – silicon carbide can only be ground and polished. The machining times are therefore relatively long and the processes are very complex. In addition, a great deal of experience is needed in order to machine the material with precision.
5. What is special about your manufacturing procedure?
Dr.-Ing. Olaf Dambon: Essentially speaking, our methods do not differ much from today's standard procedures. We too use grinding and polishing processes. With our wealth of experience of machining brittle and hard materials, however, we are able to produce particularly high-quality components in terms of their shape and surface finish.
6. How does the quality of the silicon carbide mirror compare to the best mirrors made of glass or glass ceramic? What is the micro-roughness of the mirrors?
Dr.-Ing. Olaf Dambon: Silicon carbide comes in a wide variety of forms. For space applications, a fiber-reinforced SiC ceramic from ECM Engineered Ceramic Materials GmbH has proven particularly suitable. The dimensional accuracy that can be achieved is comparable to that of glass or glass ceramic. However, the surface quality is much more moderate owing to the material characteristics. Usually, Ra values of just under 5 nm can be achieved. With glass or glass ceramic, a Ra value of less than one nanometer is possible. In order to achieve higher surface qualities, the material is coated with silicon after grinding. The silicon is then polished to produce the required surface quality.
7. With what diameters can silicon carbide mirrors be produced economically?
Dr.-Ing. Olaf Dambon: There is no general answer to this question because cost-effectiveness always depends on the individual application. According to ECM, sizes of up to 2.5 m x 1.8 m are possible from a technological point of view.
Thank you for the interview