|Ph.D Student||Kovalenko Oleg|
|Subject||Thermo-Mechanical Processing of Sub-Micrometer Sized|
Single Crystalline Metal Particles and their
|Department||Department of Materials Science and Engineering||Supervisor||Professor Eugen Rabkin|
|Full Thesis text|
The aim of this study was to investigate the size effect in mechanical strength of the single crystalline particles of the body centered cubic (BCC) metals. The samples for the mechanical testing were produced by the solid state dewetting of the thin films of respective metal deposited on sapphire. The problem with the preparation of the samples suitable for nanomechanical testing has led to the development of the novel thermo-mechanical treatment techniques at the submicrometer scale. These new techniques allowed equilibrating the sub-micron crystals, determining their equilibrium crystal shapes, and the corresponding surface and interface energies.
At the first stage, the feasibility of accelerating the equilibration of faceted Au particles of non-equilibrium shape on sapphire by plastic deformation with subsequent annealing was explored. During the next stage, the kinetics of the equilibration process was investigated. The self-healing and "shape memory" effects were observed after annealing of deformed particles. These effects were discussed in terms of short-circuit diffusion along the surface ledges formed during particles indentation .
The developed equilibration technique was then employed for the equilibration of Fe particles on sapphire, and for the first time the equilibrium crystal shape of α- Fe was obtained with the aid of Winterbottom construction. Afterwards, the uniaxial compression tests of submicron-sized iron particles were performed, and a strong size dependence of particles' strength was observed .
The same solid state dewetting technique was also utilized for producing of Mo particles on sapphire designated for compression testing. The kinetics of the initial stages of the Mo thin film solid state dewetting was found to occur by the nucleation of the holes at the grain boundaries and triple junctions. The holes expansion was modeled. The calculated holes expansion rates were in a good agreement to the experimental data .
Finally, Mo particles formed at the late stages of dewetting were equilibrated by a combination of scratching and subsequent sintering of the scratch-induced particle agglomerates. The morphology of the obtained single crystalline particles was strongly dependent on the oxygen partial pressure in the annealing atmosphere. The faceted crystals produced under high oxygen partial pressure environment exhibited strong size effect in compression, similar to the size effect observed in Fe particles. On the contrary, the rounded particles formed under low oxygen partial pressure demonstrated nearly constant strength for all investigated sizes, close to the theoretical yield strength of Mo .