Investigation of the Mechanical Behavior of Freestanding Polycrystalline Gold Films Deposited by Evaporation
Type of DegreeDissertation
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The Membrane Deflection Experiment was employed to perform the micro-tensile testing on freestanding polycrystalline gold thin films. Films with varying thickness of 0.25 µm, 0.50 µm and 1.00 µm were deposited on Si substrates by both EBeam evaporation and sputtering techniques. High-resolution scanning electron microscope, including electron-backscattered diffraction, was employed to provide a morphology and crystallographic analysis. The Young’s modulus of gold deposited by EBeam evaporation was measured consistently in the range of 53-62 GPa while 68-72 GPa was measured for the sputtered films. An analysis of film texture is employed to explain this difference. Plastic yielding of the evaporated and sputtered films was compared due to the varying microstructure of each deposition technique. The differences in their microstructures appear to assert a measure of control on the deformation mechanics. Low applied strain rates have been one of the least studied conditions and considering that engineered components are often subjected to them, garnering a more fundamental understanding of their influence on deformation of nanostructured metals is important to their design. This work studied the strain rate dependence of freestanding, gold films systematically by the membrane deflection experiment technique with applied strain rates on the order of 10-4 s-1 to 10-6 s-1. The plastic properties were found to be particularly sensitive to strain rate, as well as film thickness and grain size, while the elastic property remained relatively unchanged. The thinner films exhibited significant strain rate sensitivity while the thicker films exhibited only marginal changes. Hall-Petch boundary hardening was observed and dominated plastic flow at larger strain rates while diffusion controlled deformation mechanisms appeared to be activated with increasing influence as strain rate decreased. Analysis of dislocation-based and grain boundary diffusion related creep suggested that the films were likely experiencing a mixture of grain boundary diffusion and sliding as the dominant deformation mechanisms at lower strain rates. Furthermore, the data from the evaporated nanocrystalline films suggested that the critical grain size for inverse Hall-Petch behavior was sensitive to strain rate. These results represent an important experimental confirmation of how nanostructured materials behave at very low strain rates.