Fatigue of LIGA Ni Micro-Electro-Mechanical System Thin Films
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IN recent years, LIGA (lithographic, galvanoformung, abformung) processing, which is the German acronym for electrodeposition into PMMA (polymethylmethacrylate) molds has been used to fabricate Ni microelectromechanical system (MEMS) thin films.[1–34] These have emerged as candidate materials for applications in larger and thicker MEMS devices (>20-lm thick) with high aspect ratios.[17] The potential applications include the following: microswitches, microgears, and linkage mechanisms, micromotors, and accelerometers for the deployment of airbags.[29] In many of these potential applications, fatigue failure can occur at stress ranges that are significantly below the measured strengths under monotonic loading.[30] However, until recently,[31] it has been difficult to diagnose fatigue failure in LIGA Ni MEMS structures, such as Ni MEMS accelerometers.[32,33] This was due largely to the limited number of fractographic studies of fatigue in LIGA Ni MEMS thin films/structures at the time of diagnoses. Recent efforts have been made to study the fatigue mechanisms in LIGA Ni MEMS thin films.[4,5,8,31,34] The initial work was done by Hemker et al.[8] and Cho Y. YANG, Graduate Student, and W.O. SOBOYEJO, Professor, are with the Princeton Institute of the Science and Technology of Materials (PRISM), Princeton, NY, USA, and the Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA. Contact e-mail: [email protected] S. ALLAMEH, Assistant Professor, is with the Department of Physics and Geology, Northern Kentucky University, Highland Heights, KY 41099, USA. J. LOU, Assistant Professor, is with the Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA. B. IMASOGIE, Associate Professor, is with the Department of Metallurgical Engineering and Materials Science, Obafemi Awolowo University, Ile-Ife 220005, Nigeria. B.L. BOYCE, Principal Member of the Technical Staff, is with the Materials and Process Sciences Center, Sandia National Laboratories, Albuquerque, NM 87185, USA. Manuscript submitted February 28, 2006. Article published online August 10, 2007. 2340—VOLUME 38A, SEPTEMBER 2007
et al.,[4,5] who measured the stress-life behavior of LIGA Ni MEMS thin films and compared their results with previously reported data for annealed and wrought bulk Ni. Most recently, Allameh et al.[31] have studied the effects of specimen thickness on the stress-life behavior and the fatigue fracture modes in LIGA Ni MEMS structures. Furthermore, Boyce et al.[34] have suggested that fatigue cracks nucleate from an oxide film that forms on the surfaces of slip bands that are induced on the surfaces of cyclically deformed LIGA Ni MEMS thin films. However, an integrated understanding of the mechanisms of fatigue crack nucleation and growth in LIGA Ni MEMS structures is yet to emerge. This article presents the results of an experimental study of the mechanisms of fatigue crack nucleation and growth in LIGA Ni MEMS thin films with columnar microstructures and a top lay
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