Diameter-dependent toughness and strength of diamond nanowires

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University of Delaware
Nanowires(NWs) belong to an important class of nanostructured materials, which have tremendous potential for a number of applications including gas sensors, transistors, fiber-based composites, piezoelectric nanogenerators, and light-emitting diodes. It is commonly understood that the diameter affects the mechanical proper- ties of NWs substantially and the surface plays the most important role in regulating the diameter-dependent behavior. While stiffness has been well-explored for NWs, an understanding of their toughness and strength is yet to emerge – particularly for brittle nanowires – due to inadequate theoretical understanding and limitations in experimental techniques. To address the knowledge gap, this thesis research aims to explore the fundamental mechanisms that govern toughness and strength in brittle nanowires under pristine or defective conditions. ☐ Toughness is a well-defined quantity for bulk materials and defined as the maximum elastic energy density required for a crack to nucleate in a defect-free solid. For a domain containing an initial crack, it is defined as the critical energy release rate required for its propagation. However, for NWs it is a nontrivial task to develop a notion of toughness. This is primarily because of the difficulty in determining the roles of surface and core at finite deformation that are needed to construct a foundation for toughness. To overcome the difficulty, this thesis introduces an energy-based scheme that calculates stress directly from the elastic energy density and enables determining the influence of the surface and core on effective toughness and strength of NWs. Applying the energy-based approach, we find three characteristic regimes (surface, core, and surface-core intersection regimes) in defect-free NWs that control their toughness and strength. The intersecting regime possesses the highest elastic energy density and gradient in stress. It localizes both energy and stress in a few locations, and makes them heterogeneous across the cross-section of the NW. Consequently it controls the sites where crack(s) are nucleated and propagated from. ☐ Furthermore, for defective NWs, two additional characteristic regimes appear. They are the defective regime and the defect-core intersecting regime. The competition between the two intersecting regimes changes the nucleation site and the propagation path of cracks in the NW. As a result defects dramatically affect toughness and strength of the NWs, even under dilute concentrations. For example, single vacancy-defect reduces strength and toughness by around 5% and 12%, respectively. Moreover, results show monotonic correlation between the defect size and stiffness, whereas non-monotonicity appears in the correlation between the defect size and toughness, or strength. An atomistic analysis reveals surface-softening and stress-localization to form the basis for the observed anomalous behavior. These findings are expected to make an important contribution to our ability to engineer the extreme mechanical behavior of NWs and construct a pathway for realizing large-scale productions of NWs.
Diamond nanowires, Fracture mechanism, Molecular dynamics simulation, Surface-softening