Size Dependent Brittle-to-Ductile Transition of Silicon Nanopillars under Uniaxial Compression

Silicon’s mechanical response at the nanoscale is governed by a competition between cleavage and dislocation-mediated plasticity. Dislocation-free silicon crystals exhibit a sharp Brittle-to-Ductile Transition (BDT), but also exhibit a soft transition when pre-deformed to introduce dislocations and dislocation sources. There are several factors affecting the BDT temperature, e.g., size, crystallographic orientation, and strain rate, etc. A large number of experimental studies have investigated the mechanical properties of silicon nanostructures in the past decade, such as tension, compression, and bending tests, but the exact atomic-scale mechanisms causing this BDT phenomenon still remain unknown. We investigated the size-dependent brittle-to-ductile transition (BDT) of single-crystal silicon nanopillars subjected to uniaxial compression through large-scale molecular dynamics (MD) simulations at room temperature. Cylindrical pillars, with varying diameters and heights (diameter: 40 – 400 nanometers; aspect ratio ≈ 2–3) with 〈001〉 loading and {111} slip systems (they are the most studied system in the past, both experimentally and MD Simulations) are simulated using ReaxFF interatomic potentials under constant strain rate. We used ductility as a function of diameter, and results reveal a critical diameter below which the pillars shows transition from crack propagation to slip deformation (BDT), with the threshold shifting to larger sizes at lower temperatures, which stem the size–mechanism that governs failure in silicon at small scales and offers a better designing perspective for damage-tolerant silicon nano-architectures in MEMS/NEMS.