Browsing by Subject "Strain hardening"

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    Deformation mechanisms in nanoscale brittle materials.
    (2011-05) Stauffer, Douglas Dean
    In the past ten years nanotechnology has developed from a buzzword to an integral part of our modern life. The promise of bottom up devices has turned into better, faster, and stronger products utilizing nanoscale materials. Tires designed with carbon nanotubes, touchscreens, reformulated steel, self-cleaning fabrics, drug delivery systems, and semiconductor devices all rely on nanoscale materials. However, the mechanical property relationships are not fully understood, and the cross-roads of mechanical performance and electrical properties is still being explored. For example, the role of electrical contact in mechanical systems is important for reliability in systems that contain interconnect, switches, or relays. MEMS switches in particular can have reliability issues if the conducting area is decreased, or the switch fails due to plasticity. In this thesis, an attempt is made to characterize failure modes of several fundamental nanoscale materials using nanoindentation. In this thesis, ostensibly brittle materials such as alumina, chromia, and silicon are chosen as being archetypal examples of brittle materials. The use of conductive probe indentation is used here as a measure of plasticity under the indenter in constrained metal films with native oxide layers, as well as to determine the point of oxide fracture. In situ transmission electron microscope indentation is used to explore dislocation velocities and strain hardening in compressed silicon pillars. Dislocation velocities, in compression at room temperature, are found that approach that of those at 600°C in bulk tensile specimens. The dislocations, of unknown type, also contribute to strain hardening exponents of approximately 0.4 in pillars, and approach unity in silicon spheres.