Investigation of the Room Temperature Brittle-to-Ductile Transition of Single-Crystal Silicon at Sub-Micron Length Scale Using Accelerated Molecular Dynamics

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As is well-known, silicon is brittle so that it shatters on impact. Due to this brittle fracture, silicon-based structures are not machined, but fabricated in a variety of artful ways. Above a critical temperature though, at about 600 °C for bulk silicon, silicon becomes ductile, i.e., it can deform plastically. Several recent experiments have found that sub-micron silicon structures exhibit plastic deformation even at room temperature. While this size-dependent brittle-to-ductile transition has a strong potential to improve the reliability and manufacturability of silicon-based nanotechnology, our current understanding of the phenomenon remains incomplete. The objective of this grant is to achieve a fundamental atomic-level understanding of the size-dependent brittle-to-ductile transition of single-crystal silicon using computational modeling. This will be accomplished by reproducing the experimental observations with atomistic simulations and then analyzing the simulation results and constructing predictive multiscale models. The knowledge and understanding obtained in this research will improve the reliability of the ubiquitous micro- and nano-electro-mechanical systems through better designs as well as cost-efficient manufacturing processes, which will have significant impact on the national economy as the global nanotechnology market is estimated to reach $90.5 billion by 2021. This grant will also be used to engage undergraduates in research by leveraging the women in science and engineering summer research program and the co-op program, respectively, at the University of Cincinnati.In this study, accelerated molecular dynamics simulations will be performed to unveil the atomic-scale mechanisms responsible for the room-temperature plastic deformation of single-crystal silicon at sub-micrometer scale. Computational models of single-notched blocks and nanowires will be considered to analyze the effects of various key factors such as temperature, size, geometry, loading rate, and free surface structures and oxide layers. To carry out the simulations under near-identical experimental conditions, an accelerated molecular dynamics simulation method called hyperdynamics as well as a spatial multi-scale quasi-continuum method will be employed. The outcomes of this research will enable investigation of other brittle materials such as sapphire and zirconia whose machinability has also been an ongoing issue.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.