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Quantification of the degradation of microstructured coatings

Final Report Summary - MICROCOAT (Quantification of the degradation of microstructured coatings)

Executive Summary:
Even modern passenger airplanes require several tons of kerosene for every hour of operation. Therefore, it is a matter of top priority to introduce some savings, due to both the environment and economy. Due to the implementation of winglets on airplanes, it was possible to increase the fuel efficiency by 3%. Another method to reach better efficiency consists in the coating of the airplane surface with a friction-reducing microstructure. For this microstructure, riblets are considered to be the best option. Riblets are trapezoid, triangular or parabolic structures which are aligned in the direction of the flow to significantly reduce surface friction, and, consequently, the overall flow resistance of coated objects. The typical dimensions of these structures depend on the Reynolds number, therefore, on the density, velocity of the surrounding medium, etc., and vary from a few millimetres for watery media to 20 - 100 µm for gaseous media. In this project, we developed methods for a laboratory research on specimens coated with riblets. Therefore, impressions of riblet structures of airplane surfaces were taken and measured by means of confocal microscopy. Here, missing values in the measurement data were interpolated and the relevant values to characterize the structure were extracted using newly-developed methods. This way, it was possible to observe the structure at different wear states and draw conclusions regarding the long-term stability.
Since large airplanes have a surface of a few thousand square meters, it is obvious that an automated application of riblet structures is necessary. Thus, it is necessary to use a lacquering technique to coat airplanes. In order to develop a system is capable of a quality control of the applied structures, in this project we developed a prototype system which can measure large microstructured surfaces in a short period of time. The prototype consists of a high-speed 2D camera, a telecentric lens, a conoscopic sensor, a linear stage system and a laser scanning microscope. It is capable of measuring flat topographies at a reasonable speed for an inline quality control. In addition to collecting images, the system can analyze them to find flawed areas. All spots of the test specimen can be reached both with the cameras and with the microscope. To assure a fast inspection, the software tasks were parallelized using the Intel Thread Building Blocks template library for C++.
Project Context and Objectives:
See executive summary.
Project Results:
In this project, a white light confocal microscope was used to collect measurement data from the specimens. Using the 50x lens, the area of one single measurement is only 320 x 308,8 µm. Since it is necessary to extract statistically representative values from these data sets, the measurement area had to be enlarged. The used microscope is capable of stitching together 24 single measurements. The used applied stitching method is a closed source. Since riblets represent a recurrent structure, the method fails at this task. This project introduced a new method which cuts apart the single measurements from the flawed stitched measurement data and restitches the measurements using a Fast Fourier Transformation. By means of this three-dimensional data set, it was possible to extract values, e.g. the tip radius, the riblet width and riblet height. Using this technique, it was also possible to increase the number of usable measurements could to analyze a single specimen. Although the radius of the riblets lies in the range of 1µm and is, therefore, comparatively small, it was possible to determine it precisely. Furthermore, it was possible to see reasonable progressions for the wear of the riblet values in the analysis of specimens at different flight hours.
For a fast quality control of riblet microstructured areas, we developed a system capable of determining the state of riblets on a large area. We also developed a scanning of these structures at a speed in the range of the application speed. Although it is impossible to do a complete extraction of the values extracted in the laboratory tests, it was possible to design a system which can find hints for an insufficient application.
Potential Impact:
The ability to determine the state of riblets is necessary for the development of optimized riblet geometries in terms of long-term stability. It was possible to use the methods developed in this project for further flight tests. Once riblets have been established in the commercial aviation, the developed methods can be used for a quality inspection of airplanes to determine the best point of time to renew the riblets.
It is also possible to use the prototype system within the production of riblet structures to monitor the process of applying the riblets. This way, it is possible to change the parameters of the process, if necessary. However, an adaption process for the system would be necessary. It is also possible to use the prototype initially developed for scanning riblet-structured surfaces for the analysis of other microstructured surfaces.
During this project two papers were released:
Scheuer R., Mueller T., Reithmeier E.(2013): Development of a Fast Measurement System for Microstructured Surfaces, OSA Imaging and Applied Optics, Arlington (VA)
Renke Scheuer, Eduard Reithmeier, Niklas B. Windeler(2013): Stitching and Memory-Optimized Processing of Three-Dimensional Riblet Surface Datasets for Oil Channel Tests, International Journal of Modeling and Optimization Vol. 3, No. 5, October 2013, p.453-457
List of Websites:
Institute of Measurement and Automatic Control
Leibniz Universitaet Hannover
Prof. Dr.-Ing. E. Reithmeier

Nienburger Straße 17
30167 Hannover
Phone: +49 511 762-3334
Telefax: +49 511 762-3234