Deformation and Failure in Metallic Nanowires Under Stress-relaxation, Cyclic and High Strain Rate Loading Conditions

Rajaprakash Ramachandramoorthy

doi:https://doi.org/10.21985/N2PB39

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Deformation and Failure in Metallic Nanowires Under Stress-relaxation, Cyclic and High Strain Rate Loading Conditions

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Metallic nanowires have become functional elements in many electronic devices such as the latest fin-FET (field effect transistor) from Intel, which has metallization layers with pitch as small as 90nm, flexible electronics, touch screens and nanoelectromechanical systems. This wide applicability of nanowires warrants extensive characterization of their fundamental properties such as mechanical, thermal and electrical properties, especially as a function of time. Unfortunately, the time-dependent mechanical behavior of metallic nanowires, especially at strain rates above ~0.1/s remain unexplored. Also, high strain rate experiments can help bridge the gap between experimental and computational efforts, as experimental characterization is conducted typically at strain rates of ~1e-4/s to 1e-1/s, while atomistic simulations are conducted at strain rates above ~1e6/s. Thus the aim of this thesis, is to explore the time-dependent mechanical properties of metallic nanowires under stress-relaxation, cyclic and high strain rate loading conditions. In order to understand the long-term reliability of silver nanowires in applications such as touch screen and flexible electronics, we used a combination of in situ experiments inside electron microscopes, along with atomistic and phase-field simulations for understanding the deformation mechanisms and failure of single crystal silver nanowires under stress-relaxation and cyclic loading conditions. We identified that when the single crystal silver nanowires are held at a constant strain, they initially undergo stress-relaxation due to the formation of twins and stacking faults. The stress reduction eventually saturates after some characteristic time. Surprisingly, after a few hours of holding at constant strain, nanowires fail abruptly. Similarly, when the nanowires were held at a mean strain and then cycled under a tension-tension loading with different strain amplitudes, we found that all the nanowires failed at approximately the same time irrespective of the strain amplitude used. We identified the reason for such nanowire failure, which depends only on the mean strain level they were held at, as stress-induced surface roughening via diffusion. Further, we explored the mechanical properties of metallic nanowires in the high strain rate regime, as it is critical to the design and manufacture of high performing and failure-tolerant nanoelectronics. Using a custom microelectromechanical system (MEMS) based device with a thermal actuator and capacitance based load sensor, we conducted the first tensile tests in bicrystalline silver nanowires at strain rates up to 2/s, by actuating at speeds of ~10 µm/s. We observed a remarkable rate dependent brittle-to-ductile failure mode transition in the nanowires at a strain rate of ~0.2/s. Further, using high resolution TEM we identified that as the strain rate increases the dislocation density and the spatial distribution of plastic regions in the nanowire increases, thus resulting in increased plastic strain. At lower strain rates, random imperfections on the nanowire surface results in localized necking, leading to a brittle-like failure. Thermal actuation method that uses Joule heating cannot be used for actuation at higher speeds, as it is limited by inertia and thermal transients. Thus, a hybrid experimental setup combining an external piezoactuator with displacement sensing and MEMS based load sensor was developed to increase the actuation speed to ~10mm/s. This setup was used to conduct the first high strain rate tests on single crystal silver nanowires at strain rates up to ~200/s. We found that the brittle-to-ductile transition also happens in single crystal silver nanowires beyond a strain rate of ~10/s. Also, the yield stress of silver nanowires was found to increase with increasing strain rate till ~2/s. At even higher strain rates above ~10/s, the yield stress saturates and almost becomes strain rate insensitive, as the nanowire undergoes a transition from surface dislocation nucleation to collective dislocation nucleation, as the nanowire stresses get close to the ideal tensile strength of the material. A basic validation for potentials used in nanowire MD simulations in the future, could be to check whether the yield stress obtained is similar to the rate insensitive yield stress obtained from high strain rate experiments

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