SUMMARY

Nanocrystalline metallic thin films are routinely used in a large range of applications involving thin functional coatings, structural components in micro/nano electromechanical systems and conductive layers in microelectronics devices. In most of these applications the thin film is exposed to cyclic loading. Therefore, it is essential to have a fundamental understanding of crack initiation mechanisms along with quantitative characterization of fatigue properties. Early research shows clear evidence of improved fatigue endurance limit of pure FCC metals compared to coarser grain counterparts. However, these models are based on nucleation and annihilation of dislocations in persistent slip bands, too large for nano-sized grains. Although there has been significant improvement on understanding monotonic plasticity of nanocrystalline metals only a few studies have been conducted so far on fatigue of NC materials and irreversibility of deformation sources. In the proposed research, a novel MEMS-based experimental technique is presented to perform quantitative in situ transmission electron microscopy (TEM) fatigue experiments on nanocrystalline thin films. Specifically, the cyclic plastic deformation properties and fatigue crack initiation mechanisms of nanocrystalline thin films are studied using a MEMS tensile testing device. The MEMS device provides both actuation and sensing of the specimen via electrical signals while TEM imaging provides information on the microstructure evolution during cyclic deformation. It comprises two identical capacitive sensors on each side of the specimen that are used to measure both the specimen elongation and applied force with nano meter and micro newton precision, respectively. The outcome of the proposed research can significantly advance the fundamental understanding regarding the structural reliability of nanostructured thin film devices.