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Inspection of microstructured surfaces in industrial conditions

Special microstructured surfaces can reduce the drag of components. The shape of this structure is motivated from skin of sharks and is called riblets. The field of application of this structure is found mainly on the outer skin of aircraft and compressor blades of aircraft engines. The structure consists of periodic elevations of the component surface. The period length of this structure corresponds to the prevailing flow conditions and is in the range of approx. 50-500 microns and the structure height is in the range of approx. 10-200 microns. The flow reducing effect of riblets has been known for some time, however, efficient production method of riblets are still in development.

In this project, new measurement systems are developed in order to make the manufacturing process of riblets efficiently. This includes in particular the development of measurement systems that allow microscopic inspections not only under laboratory conditions, but directly during the manufacturing process. These systems must firstly have a very high resolution, so that defects in the range of <10 microns can be found. On the other hand, the newly developed measurement system must be very robust against all manufacturing conditions. These include in particular impurities such as coolant during a grinding process or vibration.

The high resolution of the measuring systems for the inspection of riblets also leads to very low area ranges for a single measurement. In this project, possibilities are investigated to measure large, microstructured surfaces efficiently despite the small areas rates.

Surface metrology based on Scanning Electron Microscopy

Sketch of the structure to reduce SE3 yield with possible electron trajectories

Surface metrology based on Scanning Electron Microscopy (SEM) makes it possible to study materials up to the atomic level. To do so, a sample is irradiated point by point and the resulting radiant intensities are recorded. Classic studies limit themselves to various intensities which are applied to image processing or visualization methods. For many years, the Institute of Measurement and Automatic Control (IMR) has been researching a method of high-precision 3-D electron microscopy which, in addition to intensity values, also reconstructs the surface profile of the sample. This 3-D SEM measurement technique does not only represent a unique new measurement method, but also provides new insights into the microstructure of material surfaces, based on which it is possible to develop new analysis techniques in various areas of application. For the time being, these research methods are still being developed, but in several years they could be market-ready.
The measurement technique developed at our institute uses several detectors for secondary electrons (SE), so that it is possible to work with lower radiation energies and to obtain high precision of the surface reconstruction. To do so, imaging procedures apply directional characteristic, from which it is possible to calculate the surface slope using sophisticated material models and, therefore, create a surface profile. At one of the first stages of development two detectors placed at a 180° angle were used. The results were promising, so that the next stage of development is currently being expanded by four Everhart-Thornley detectors and an elaborate electron "lens". Simulations and the first results obtained at this stage of development also allow to expect that the measurement results have a novel quality in comparison to the existing imaging procedures. The only obstacle in this approach is created by backscattered electrons (BSE) which can distort the measurement results, because these high-energy electrons release further SE on the surfaces of the sample chamber.
To significantly reduce this effect, the IMR plans to develop an electron-trap system based on nanostructures. This way, it could be possible to intercept the unwanted electrons. Therefore, the IMR expects to further improve the triggering of measurements. It makes most sense to place the electron-trap system on the bottom of the electron gun, because the majority of BSE occurs here. The structure should collect high-energy electrons and intercept the so-called SE3 emission that occurs in it. In order for the electron-trap system to perform this way, it is necessary, in addition to finding a suitable material, to find an optimal surface structure for the electron-trap system. It is most probably a square wave shape, however, the measurements of squares or the depth of holes must be first determined by means of several series of experiments.

Contact person: Renke Scheuer

Active Microoptics

This project works on the development of a novel concept for tunable micro-optic systems that are based on fluidic principles.

A magnetic liquid is moved by means of miniaturized coils in an adaptive opto-fluidic system. This mechanism can influence the shape of the fluidic lens indirectly. By moving the electro-magnetic liquid body (the ferro-fluid), the other optically active liquid is displaced within the fine canal structures. In order to guarantee the optimal adjustment to technical features like focal length and geometrical size, the project team works on the further development and improvement of the ferro-fluidic and optically active part of the system. To begin with, the researchers want to show that such a system permits the adaptive tuning of a fluidic lens. In a second step, they will inject a liquid into the especially molded cavities of the channels. The liquid’s refractive index is adjusted accordingly. Selected optical elements that are tailored to demand can be  placed in the light path. Due to the chosen micro-fluidic principle, it will be possible to realize different components of geometrical and diffractive optics. This modular concept is suitable for a variety of micro-optic applications. The actual optical component and the tuning mechanism are built in a compact way. This compactness reduces the size of these systems, which facilitates their integration into more complex devices. Such systems can be applied in adaptive lenses or tunable optical filters. The chosen approach, however, is most suitable for compact low energy systems.

Contact person: Dipl.-Ing. Thanin Schultheis

Stochastic Structures

multi sensor system to detect the stochastic structures
multi sensor system to detect the stochastic structures

The influence of stochastic shaft surface defects on the rotary shaft seal function is being investigated within the project “stochastic structures”. Two research centres are taking part in the project: Institute of Measurement and Automatic Control (IMR) and Institute of Machine Elements and Engineering Design (IMKT), Leibniz University in Hannover. The common purpose of the project is the investigation of stochastic surface defects with the view to the operation of rotary shaft seals. In this context, the influence of such defects on the leakage and wear is especially interesting. It is important to determine the limits for stochastic defects that are not critical and permissible for the further practical use. In other words, it is desirable to find the boundary between critical and non-critical shaft surface defects. An additional goal of IMR is the development of a production–related multi sensor system for the detecting critical surface defects.

To detect and to measure stochastic defects on the shaft contact surfaces the measuring methods, those are able to measure the lateral size of defects in the range of some millimeters (dents) as well as in the range of few tenths of millimeters (scratches), are needed. After the comparison of different methods, two measuring systems were chosen: light scattering sensor for the long-wave form deviations (dents) and chromatic sensor for the short-wave deviations (scratches). It was as well noted, that the light scattering sensor is able to detect the small scratches.

Contact person: Dipl.-Phys. Alexander Leis

Cutting Line Deviations in Stone Cutting

Schwingungsmode einer Trennschleifscheibe, mit dem IMR-Derotator gemessen
Deflection shape of a cut-off wheel

In cooperation with the Institut für Fertigungstechnik und Werkzeugmaschinen  (IFW) of the Leibniz Universität Hannover and the Forschungsgemeinschaft Werkzeuge und Werkstoffe e.V. Remscheid, the influence of various parameters on the cutting line in stone cutting was investigated. A major goal was to obtain a correlation between cutting parameters like cut-off wheel and flange diameter or cutting speed on the vibration of the cut-off wheel. The vibration conditions were measured under different rotational speeds of the wheel by using a Scanning Laser Doppler Vibrometer. To track a fix measuring point on the wheel - even under rotation - an optomechnical derotator was used to compensate for the rotation. Different series of measurements were carried out under lab condition with a free spinning wheel as well as under cutting condition using a stone cutting machine. The measurement data indicate a strong dependency of the eigenfrequencies and decay time on cutting wheel diameter, flange diameter and cutting speed. Under cutting conditions the frequencies of vibration were dominated by multiples of the rotational frequencies due to the teeth of the blade hitting the stone. Eigenfrequencies were suppressed but it is expected that Eigenfrequencies can be measured when the rotaional frequency matches a Eigenfrequency which was not the case in the experiments. The Project was supported by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen
"Otto von Guericke" e.V. (AiF)

Contact person: Dipl.-Phys. Maik Rahlves

Cleansky - Quantification of the Degradation of Microstructured Coatings

Experimental set-up consisting of linear stages, a high-speed camera and a projection lens with the sample.

Even modern passenger airplanes require several tons of kerosene for every operation hour. Therefore, it is a matter of top priority to introduce some savings, due to both the environment and economy. As it is clearly seen from the project „Riblets on Compressor Blades“, riblet-structured surfaces can drastically minimise friction losses on parts circulated by air. One of the possible approaches towards the reduction of friction losses on airplanes is the structuring of airplane surfaces with riblets. By means of laboratory tests developed by imr, it is possible to determine the riblet quality locally. To be able to apply it in the series production, however, it is necessary to guarantee a large-scale quality control close to the production premises. For this purpose the imr is engaged in the EU-funded project Cleansky, in order to develop a control unit based on a high-speed camera to be able to determine the quality of riblet-structured surfaces.

Contact Person: Dipl.-Ing. Renke Scheuer

Riblets on compressor blades

This project deals with the measurement of riblet structures on surfaces of compressor blades.

Riblets are trapezoid, triangular or parabolic structures which are aligned in the direction of 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, thus on the density, velocity of the surrounding medium, etc.,  and vary from a few millimeters for watery media to 20-50µm for gaseous media. Due to the typical width of riblets on compressor blades, the tip radii of the structures are in the range of 200 – 800nm.

The accurate production of structures with such measurements is a nontrivial problem, because slight changes in the geometry have a huge effect on efficiency. Therefore, the quality control plays an important role in the production of riblets structures. Because of the high possible scanning speed, an areal scanning method should be used to detect local fluctuations of the structure. Since a scanning electron microscope (SEM) fits the requirements, it may principally be used to evaluate the produced surfaces. However, there is a huge drawback to a SEM – it is only capable of producing two dimensional images. A logical consequence is a SEM which can be utilized as a three dimensional measurement device. Hence, the IMR is working on a design for a 3D-measurement device based on a conventional SEM.

Contact Person: Dipl.-Ing. Renke Scheuer