Investigation of fatigue life characteristics of micropatterned freestanding NiTi thin films
NiTi shape memory alloys present themselves as high potential candidate in several applications. This fact arises from their superior attributes such as large stress output, recoverable strain and outstanding biocompatibility. Fatigue life prediction of bulk shape memory alloys have been extensively investigated in the literature. Yet so far, information about the fatigue life prediction on NiTi thin films is rarely discussed or missing which might limit the window of future applications. In this thesis, NiTi freestanding films with nearly equiatomic composition were fabricated by means of magnetron sputtering, UV lithography, and wet etching. The films were subsequently annealed in vacuum. The fatigue properties of the films were characterized by a self-developed fatigue testing device under tension-tension loading mode. Fatigue life diagrams were conducted and investigated at various testing variables such as different cyclic frequencies and different mean strains. In addition, the influences of the chemical composition, the film thickness, and the annealing conditions on the fatigue characteristics were investigated. The fatigue endurance limit (FEL) in this work was taken at 10 million cycles. A significantly improved FEL of more than 350 % was attained by using different surface finishing. The FEL in this work is the highest among all published literature data on NiTi films. Moreover, the biocompatibilities of NiTi films were examined by investigating the cell growth on films’ surface after different surface finishing. The result showed insignificant dependence of cell growth and adhesion on the film surface. The freestanding films showed a great biocompatibility levels. In addition, the measurements of the Ni ion release on NiTi films modified with the different surface finishing were performed in a balanced salt solution used to mimic human body fluids. The results show that the Ni concentration in the tested solution is below the maximum limit of the international biocombatibitly standards and that the Ni ion release can be modified through different surface finishings. Finally, the factors governing the fatigue characteristics and the biocompatibility aspects of NiTi films were identified and optimized to enhance the overall performance of the NiTi films and increase the design and the integration possibilities in different applications.
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