Browsing by Author "Ronald O. Scattergood, Committee Chair"

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Characterizing the Ductile Response of Brittle Semiconductor Materials to Dynamic Contact Processes
(2004-06-22) Randall, Travis John; Ronald O. Scattergood, Committee Chair; Nadia El-Masry, Committee Member; John MacKenzie, Committee Member
It has been well documented that single crystal silicon and germanium exhibit plastic flow with the generation of high pressure on the surface by contact processes such as micro-indentation, scribing, and single point diamond turning. A high pressure phase transformation (HPPT) of the near surface region from the brittle, highly covalent diamond cubic (dc) structure to a metallic β-tin phase is thought be responsible for the anomalous plastic flow behavior during contact loading processes. The scope of this investigation is the study of the response of single crystal silicon and germanium to dynamic contact processes such as scratching and single point diamond turning. Plastic flow in silicon was noted for scribing experiments in SEM observation for various cutting directions on different crystal orientations. The both the ductile response and fracture behavior was shown to be greatly influence by the combination of cutting direction and tip geometry. Residual stress was measured by wafer deflection and quantified as a dipole force. The cutting direction and tip dependence of the fracture behavior was qualitatively explained using a stress model (modified from a model used for diamond turning of these materials) showing the directional propensity for fracture based on imparted tensile stresses on a certain set of fracture planes. Raman measurements to identify remnant phases indicative of HPPT in the scribe regions showing intense plastic behavior and the generated debris were inconclusive. Low RMS (1-9 nm,) optical quality surfaces were generated by diamond turning silicon and germanium at low feed rates (1-5 um/rev.) Symmetric damage patterns noted at high feed rates are partially explained using a damage orientation model based on elasticity theory developed from previous diamond turning work with these materials. Raman measurements of the machined areas showed signature of a near surface amorphous layer, perhaps remnant of the high pressure β-tin transformation, in both silicon and germanium samples. TEM observation of collected debris indicated dc structure perhaps to recrystallization of the amorphous material by heating. The implication of the ductile behavior is that using careful machining conditions, normally brittle materials such as single crystal silicon or germanium (and similar dc semiconductors) may be machined in the ductile regime to create high quality optics and substrates without intensive processing steps.
Molecular Dynamics Simulations of Plastic Deformation in Nanocrystalline Metal and Alloy
(2007-10-24) Jang, Seonhee; Ronald O. Scattergood, Committee Chair; Donald W. Brenner, Committee Member; Carl C. Koch, Committee Member; Korukonda L. Murty, Committee Member
Nanocrystalline metals have different mechanical properties from conventional grain sized metals. Hardness and yield strength have been found to increase with decreasing grain size in the nanocrystalline regime down to at least 15 nm on the basis of Hall-Petch mechanisms. Below grain sizes of ˜10 nm, the strength decreases with further grain refinement, leading to the inverse Hall-Petch effect. Although the experimental evidence has found these deformation responses in nanocrystalline materials, the underlying mechanisms are not well identified. Molecular dynamics simulations were carried out for uniaxial tensile straining of two-dimensional columnar microstructures of aluminum (Al) and aluminum-lead (Al-Pb) alloys. Pure Al has a critical grain size at dc ≈ 15 to 20 nm, the crossover from "normal" to "inverse" Hall-Petch effect, accompanied with intra-grain mechanisms by partial dislocations and twins as grain sizes increases. With increasing grain size there exists a transition in plastic deformation mechanism from inter-grain processes to one that consists of both inter-grain and intra-grain processes. For Al-Pb alloys with a 10 nm grain size, Pb segregates completely to the grain boundaries and the grain boundaries become wider and more disorganized as the Pb content increases. A softening effect was observed in agreement with, but less than that found experimentally. As the Pb content increases, partial dislocation nucleation at grain boundaries is completely suppressed and the plastic strain is accommodated by mechanisms other than dislocation slip. As the grain sizes increase up to 15 or 20 nm, dislocation generation at grain boundaries is also suppressed. However, dislocation generation is not entirely suppressed at 3 equivalent at% Pb, compared to the 10 nm grain size showing complete suppression.