Mechanical behavior of LIGA nickel MEMS materials

In this study, various kinds of experimental techniques were used to investigate the basic mechanical properties, plasticity and fatigue behavior of LIGA (the German acronym for lithography, electroplating, and molding) Ni MEMS (Microelectro-mechanical Systems) materials. Yield strength, ductility and fracture behavior were first studied in LIGA Ni dog-bone samples with different thicknesses using micro-tensile technique. It was found that the measured properties deviate largely from those of bulk Ni. However, no significant effects of thickness dependence on mechanical properties were found. This absence of size effects was attributed to the absence of applied global strain gradients in uni-axial tensile tests.; Micro-bend experiments and a newly-developed cyclic micro-bend technique were then used to study the plastic behavior of LIGA Ni thin films under both monotonic and repeated loading. The observed length scale effects were analyzed within the frame work of the phenomenological Fleck-Hutchinson strain gradient plasticity (SGP) theory. However, the observed length scale effects diminish, as the number of bending cycles increase. This was attributed to the evolution of a fatigue substructure with increasing number of fatigue cycles.; Nano-indentation experiments were used to extract the stretch gradient length scale parameter. Special efforts were given to correct for the effects of the tip rounding, material pile-up and tip geometry. The plasticity length scale parameters were then determined within the context of the MSG and Fleck-Hutchinson theories. Thus, a new constitutive relationship was obtained for the modeling of plasticity in LIGA Ni MEMS materials at the micron and sub-micron scales. Multi-scale indentation experiments were also performed on both LIGA Ni MEMS materials and single crystal Ni thin films. A bi-linear indentation behavior was observed for both materials indented between the micron and nano-scales. The measured indentation size effects were also discussed in light of insights from direct discrete dislocation simulations.; Finally, the results of micro-fatigue tests were presented for LIGA Ni thin films with two different thicknesses. The measured stress-life data for the thinner samples is comparable to that of hardened bulk Ni. The stress-life behavior of the thicker samples is close to that of annealed bulk Ni. Also, low cycle strain-life fatigue behavior in LIGA Ni MEMS thin films studied by cyclic micro-bend technique is well characterized by the Coffin-Manson relationship.