Fatigue of LIGA Ni Micro-Electro-Mechanical System Thin Films

摘要

This article presents the results of an experimental study of the mechanisms of fatigue in 270-μm-thick LIGA (lithographic, galvanoformung, abformung) Ni micro-electro-mechanical systems (MEMS) thin films with columnar microstructure. Stress-life behavior is compared with the previously reported data for LIGA Ni MEMS films and bulk Ni. The LIGA Ni thin films are shown to have comparable fatigue lives to bulk annealed Ni. The underlying mechanisms of fatigue crack growth are elucidated via scanning electron and focused ion beam (FIB) microscopy. Stress-driven recrystallization was revealed near the fatigue crack-tip using FIB microscopy. Microvoids were also found to form and coalesce in the recrystallized grains. This led to subsequent fatigue crack growth at the microscopic and macroscopic scales. The crack profiles revealed that fatigue crack growth was retarded by crack deflection and branching. The implications of the results are then discussed for the analyses of fatigue in nickel MEMS structures.