Fatigue of Nickel MEMS Structures

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This grant is aimed at a fundamental understanding of the micromechanisms of fatigue crack initiation and growth in pure polycrystalline nickel MEMS structures produced by the LIGA method. The microstructures of the as-received and heat-treated nickel MEMS structures are studied using scanning and transmission electron microscopy techniques. The initial surface topologies are characterized with atomic force microscopy. Monotonic and cyclic deformation experiments are used to determine the constitutive behavior of the nickel MEMS structures. Micro-bending experiments measure the plasticity length scale for the subsequent modeling of deformation. Microstructural and substructural features associated with this length scale are elucidated via scanning and transmission electron microscopy. The fatigue lives of smooth (polished and unpolished) specimens are studied as functions of stress range and mean stress. The initial specimens are deformed continuously to failure to determine the number of cycles to failure. However, subsequent experiments will be stopped at different fractions of the measured number of cycles to failure to facilitate the incremental characterization of surface topology (atomic force microscopy), micro-texture (orientation imaging microscopy), and damage (scanning and transmission electron microscopy). At the senior high school level, the P.I. will interact with a senior class in a Princeton High School in a program designed to expose students to exciting MEMS technology. At the undergraduate level, two minority undergraduate students funded by the Princeton work/study program (per year) will work on the proposed program during the regular academic year,. Two additional undergraduate students (per year) from other institutions will be hired to work on the project during the summer, supported by NSF REU funds. %%%The fundamental understanding gained from the experiments will guide the development of mechanism-based models for the prediction of fatigue crack initiation and growth. The implications of the models will be explored for damage tolerant design of Ni MEMS structures. The P.I. will develop a new graduate course on MEMS structures, including the fundamentals of MEMS fabrication, microstructures, textures, physical/ mechanical properties and device considerations. Overheads and notes from the class will be made freely available to the global audience on the worldwide web. ***