Overview
For a successful geometrical characterisation of the described micro structured coatings, the lateral resolution of the measurement device should be higher than 250 nm. Therefore, a confocal microscope or white light interferometer was used for the laboratory measurements. To obtain reliable and accurate geometrical information, the optical measurement results were verified by a secondary electron microscope (SEM). Its advantage is the higher lateral resolution and the ability to detect steep angles. Though a SEM only produces 2D images, it is possible to evaluate the 3D-geometry of local structures using the shape from shading method.
For a degradation analysis, the 3D measurement results were used for calculation of the task-relevant geometric parameters. For this purpose, different mathematical instruments such as Fourier analysis and statistical estimation can be used. The parameters are presented as histograms and correspondent distributions.
The solutions, which were developed for laboratory tests, cannot be directly used for quasi-real time measurements because the micro-coating production rates are usually distinctly higher. For this reason, the surface was examined using 2D cameras. Here not the real 3D topography, but secondary characteristics, such as the specific reflection of corners or the darker indications from tilted surfaces, are being controlled. The areal scanning will be achieved with the aid of highly precise linear axes. If a divergence from the desired course is found during the camera examination, this spot will be examined with confocal microscopes. These microscopes can be installed on positioning devices. The 3D measurement results are additionally tested for their precision and robustness. If necessary, improvements may be implemented which aim particularly at the elimination of oscillations in the system.
In conclusion, time optimisation was performed by distributing the control of the system on several synchronised workstations.
Funding
Results
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 micro structured 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 analyse 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++.