The focus of this research is the development of high strength, high conductivity
copper films. Pure copper is soft and traditional strengthening mechanisms cause
substantial decrease in conductivity. To address the challenge, epitaxial nanotwinned
copper films are synthesized on HF etched Si (110) substrates. These films show high
hardness (~ 2.8 GPa) due to high density of coherent twin boundaries (CTBs) which
effectively block the motion of dislocations similar to grain boundaries (GBs).
Resistivity of CTBs is calculated to be an order of magnitude lower than that of GBs.
Hence, conductivity of nanotwinned copper is still comparable to that of pure copper. In
addition, it is shown that average twin spacing can be controlled by adjusting deposition
rate. Analytical studies together with experimental evidence show that nanotwins can
improve the strength-to-resistivity ratio significantly in copper.
In general, nanocrystalline metals suffer from low ductility. To study plastic
deformation via rolling, thick polycrystalline nanotwinned copper foils are sputtered on
SiO2 and then peeled off the substrate. Despite the high strength, room temperature
rolling experiments show that nanotwinned copper films exhibit stable plastic flow with no shear localization or fracture even at thickness reduction of over 50%. Postdeformation
studies of microstructure reveals that the plastic deformation is facilitated
by the migration of CTBs normal to the twin boundary plane due to the glide of twinning
dislocations in the twin plane. X-ray pole figure measurements show insignificant out of
plane rotation as a result of 50% rolling thickness reduction.
Thermal stability of nanocrystalline metals is also a concern. Free standing
nanotwinned polycrystalline copper films show remarkable thermal stability after
annealing at 800 degrees C. The driving force for twin growth is much lower than that for grain
coarsening because the energy stored in CTBs is an order of magnitude lower than that
of GBs. As a result, the average twin spacing stays below 20 nm after annealing. Such
high thermal stability of nanotwins leads to the retention of hardness of 2.2 GPa. Low
energy twin boundary may provide a unique way to achieve both high strength and high
temperature thermal stability in certain metallic materials.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-05-7724 |
| Date | 2010 May 1900 |
| Creators | Anderoglu, Osman |
| Contributors | Zhang, Xinghang |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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