Auswirkungen der Korngröße auf das Kontaktermüdungsverhalten von Nickel

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The fatigue behaviour of metals is strongly influenced by the grain size. It is known that the formation of cracks due to cyclic tensile loading occurs the later the smaller the grain size of the metal is. In this work the influence of the grain size of Nickel on the contact fatigue behaviour in cyclic indentation and cyclic sliding is investigated. A (111)-single crystal, microcrystalline, ultra-fine-crystalline and nanocrystalline Nickle where examined. For the cyclic indentation a failure criterion on the basis of the measured contact stiffness was defined. A decrease of 10% in respect of the maximum value of the contact stiffness was defined as failure. The time to failure was the larger the smaller the grain size was. The loss in contact stiffness was identified to be related to the formation of cracks. Microstructural changes have been examined by focused ion beam microscopy. Due to cyclic indentation grain growth occurred in the nanocrystalline Nickel. This was more pronounced with higher load amplitude and duration. In cyclic sliding, grain growth was observed for the nanocrystalline Nickel, a slight grain coarsening took place in ultra-fine-crystalline Nickel and a distinct grain fining took place in microcrystalline Nickel. The resulting grain structure of the examined samples in the area under the cyclic sliding paths exhibited a grain size of a view 100 nm. Measurement of the hardness of the sliding paths revealed a hardening of the microcrystalline samples, a slight softening of the ultra-fine-crystalline samples and a strong softening of the nanocrystalline samples, which became more pronounced with higher number of cycles. This is in agreement with the change in grain structure. During the microscopic examination, a material deposition around the cyclically loaded areas was observed. This deposition became stronger for longer duration and maximum load. The deposition has been identified as carbon, with the source of the carbon being presumably adsorbed carbon monoxide. A comparison between a finite element simulation and a real cyclic indentation experiment was conducted for nanocrystalline Nickel. With the number of cycles the load-displacement curves differed more and more. This has been related to microstructural changes in the material. In addition experiments and a FEM-simulation where conducted to exclude the possibility of grain growth in the nanocrystalline Nickel due to a rise in temperature because of the cyclic loading. The results of the simulation indicate that there is no relevant rise in temperature. So the change in the grain structure is related to the mechanical loading condition.