Fraunhofer Institute for Surface Engineering and Thin Films IST
CFRP microwave reflector with a PVD + PACVD functional coating for MetOp-SG satellites.
Customized mirror coatings for your application
Versatile applications and requirements for mirror coatings
Mirror coatings play an important role in various fields such as laser or solar technology, telecommunications, astronomy, and medical or automotive engineering. The specific requirements for the coating vary depending on the application and can be related to aspects such as reflectivity, resistance to mechanical and chemical stress, special spectral requirements and temperature resistance. As a rule, either metal-based or dielectric mirrors are utilized. Metal-based mirrors are thereby characterized by high reflection capability in the entire visible spectrum, while dielectric mirrors can fulfill customized spectral requirements in that they can precisely reflect light in different wavelength ranges.
Application-specific development of mirror coatings
Based on your specific requirements and the substrate used, we develop suitable layer systems for your mirror coating and perform optical simulations. From our extensive portfolio of technologies (PVD, CVD, ALD), we select the appropriate deposition processes and optimize them in interplay with simulation processes, experiments and analytics. Our extensive expertise in the field of metallic depositions, their integration into layer systems, and their bonding to various substrates thereby comes into play. This is based, amongst other things, on the development of low-e and sun-control coatings. In our Optical Systems and Applications department, we use state-of-the-art system technology such as the EOSS® coating platform in combination with the MOCCA+® software for process control in the customized production of dielectric mirrors. In addition, we contribute our extensive expertise in surface technology, particularly in coating design and high-precision, automatically monitored and optimized deposition.
Our team is at your disposal to discuss your requirements with regard to mirror coatings or special issues, e.g. relating to the development and simulation of metal-based or dielectric mirrors. We are also happy to advise you on the optimization of existing coating processes.
We look forward to receiving your inquiry.
CFRP microwave reflector with PVD and PACVD functional layer for application in MetOp-SG satellites. The metallic microwave reflector made from aluminum was deposited on a lightweight CFRP base body (Ø = 600 mm) by means of magnetron sputtering. As a result of the longer wavelength of the microwaves, the requirements regarding the surface quality of the Al coating are less stringent than, for example, for typical optical applications in the visible wavelength range.
AFM image of the surface of the astronomy mirror to evaluate the roughness of the mirror coating. The roughness values exhibit only a low level with Ra =1.63 nm and Rq = 2.02 nm. The grain size of the silver crystallites reaches a maximum of 100 nm. The grain/structure size thereby lies below the wavelengths observed with the mirror and, as a result, there is no undesired light scattering during the reflectance.
SEM image of the surface of the astronomy mirror to evaluate the quality of the mirror coating.
Reflectance of a metal-based astronomy mirror, manufactured in compliance with the specifications of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO). The mirror coating also includes a 15.8 nm thin surface-layer structure to protect it from mechanical and chemical stress. This enables the mirror to withstand brush cleaning with deionized water. For comparison, the reflectance of a non-covered silver mirror is shown. This has a higher reflectance level due to the lack of a partially absorbing surface-layer structure. Both mirrors were manufactured at the Fraunhofer IST.
Reflectance of a silver-backed mirror for the near-infrared range, λ > 1 235 nm. The variants include differing adhesion layers, type a and b, as well as different adhesion-layer thicknesses. The type and thickness of the adhesion layer affect the reflectance properties. However, a minimum adhesion-layer thickness is necessary in order to ensure adequate adhesion.
Development of the layer resistance of silver-based low-e coating systems in the corrosion test with gaseous ammonium sulfide. The conventional low-e reference system degrades strongly due to the fact that the silver layer, despite being embedded in further layers, corrodes after some time. Different variants of low-e systems, Setup I and II, with a structure adapted with respect to corrosion resistance, show little to no degradation despite the highly corrosive atmosphere. Such measures can also be applied to particularly resistant metal-based mirror coatings.
Comparison of the reflectance spectra of dielectric conventional (left) and dielectric rugate mirrors (right). The continuous refractive-index profile within the rugate mirror leads to a suppression of the rippling of the spectrum. Furthermore, due to the lack of interfaces, it provides the desired increased temperature stability for utilization in solar concentrators for solar-thermal applications.