Mikro-Elektrostrukturieren planarer und zylindrischer Oberflächen mittels strukturierter, flexibler und mehrlagiger Gegenelektroden mit integriertem fluidischen Kanal

Abstract

Sensorial surfaces are a new generation of sensors for the use in condition monitoring. In contrast to conventional micro- or thin-film-sensors the sensor structures of sensorial surfaces are manufactured directly on the part to be monitored and are not mounted as separate sensor modules. By using microsystems technology, the sensor structures can be miniaturized and the measurement data can be gauged very closely to the occurance of the physical quantity. To enhance the mechanical durability of the sensor layers, the manufacturing process should result in a completely flat surface. This can be achieved by embedding the sensor layer into an insulation layer or directly into the surface of the part to be monitored. To do this, the surface needs to be structured in the shape of the sensor geometry. This thesis deals with a novel concept for Electrochemical Micro-Machining (EMM). EMM is a method of removing metal locally on the micrometer scale in a quick and cost efficient way. It is especially suited for hardened steels because it is run at room temperature. It does neither bring in thermal stress nor does it reduce the hardness of the steel. The thesis describes a Finite-Element-Method (FEM) model as well as an analytical model for a better understanding of the theoretical background of the process regarding the depth and resolution of the manufactured structures. Furthermore, the fabrication of a new type of structured counter- (work-) electrode is presented for different applications. The thesis concludes with the presentation of several results of manufactured structures and a comparison of the theoretical models with the practical results. Two models were developed to understand the influence of relevant parameters like current density, conductivity and velocity of the electrolyte and the distance between the tool and the workpiece on the resulting depth and resolution of the manufactured structures. The FEM model can predict the depth of the structure as well as its resolution. With the analytical model it is possible to investigate the influence of the electrolyte velocity on the structure depth. Both models are based on the assumption, that high current densities (above 30A/cm2) are used because the material removal rate is proportional to the amount of charge that is transported between the electrodes in this regime. In general it can be concluded that the lateral resolution decreases as the distance between the electrodes gets smaller. The depth of the structure increases over the length of the fluidic channel due to the metal ion concentration. This effect can be attenuated by using channels that decrease their cross section over their length and thus accelerate the electrolyte. The depth is also dependent on the area of the counter electrode. Larger areas yield in deeper structures. The new generation of tool electrodes are fabricated with the use of microsystem technologies. The fluidic channel is directly integrated into the electrodes. Rigid electrodes for planar work pieces as well as flexible electrodes for cylindrical work pieces are presented. With flexible, multi-layered electrodes it is possible to produce several different structures that are aligned with an accuracy of 1 μm. The practical results of the manufactured structures back the theoretical results of both models. The work distance between the electrodes is 20 μm and resolutions of 50 μm can be achieved. It is possible to use flexible counter electrodes with 500nm thick metallic layers at up to 100A/cm2. Three-dimensional structures can be fabricated on cylindrical surfaces in a few seconds. The typical process length for a 5 μm deep structure is about 20 s and does not depend on the structured area.