Size-Dependent Brittle-to-Ductile Transition in Silica Glass Nanofibers

Abstract

Silica (SiO2) glass, an essential material in human civilization, possesses excellent formability near its glass-transition temperature (Tg > 1100 °C). However, bulk SiO2 glass is very brittle at room temperature. Here we show a surprising brittle-to-ductile transition of SiO2 glass nanofibers at room temperature as its diameter reduces below 18 nm, accompanied by ultrahigh fracture strength. Large tensile plastic elongation up to 18% can be achieved at low strain rate. The unexpected ductility is due to a free surface affected zone in the nanofibers, with enhanced ionic mobility compared to the bulk that improves ductility by producing more bond-switching events per irreversible bond loss under tensile stress. Our discovery is fundamentally important for understanding the damage tolerance of small-scale amorphous structures.

A Novel Approach for Preparation and In Situ Tensile Testing of Silica Glass Membranes in the Transmission Electron Microscope

“…used during irradiation, compared to beam current densities of ~10 −3 A cm −2 used for irradiation of the glass membranes in the present study. In this context, Luo et al (2016) irradiated silica glass fibers with electrons inside the TEM by using a beam current density of 2.5 × 10 −3 A cm −2 for 1 min, and subsequently performed EELS measurements on the irradiated material. No differences in the chemical composition and electronic structure (or bonding characteristics) are noticed upon irradiation with the e-beam, in comparison to the non-irradiated silica glass fibers.…”

“…Silva et al (2006) performed bending experiments on silica nanowires with diameters ranging from 280 to 1950 nm and observed values of E similar to ours (measured at beam-off conditions). Just recently, Luo et al (2016) performed in situ tensile experiments with nanoscale silica glass fibers (with diameters well below 50 nm) at beam-off conditions in the TEM and performed complementary molecular dynamics simulations. The authors noticed plasticity at length scales of 10 nm and a significant brittle-to-ductile transition upon reducing the fiber diameter below 18 nm (Luo et al, 2016).…”

“…Hence, a broader knowledge and classification of its mechanical behavior occurring on smaller scales is of particular interest and importance. Despite its well-known brittleness on macroscopic scale, several approaches have proven that silica glass can exhibit a certain amount of plasticity when scaled down to the micro-and/or nanometer regime (Zheng et al, 2010;Nomura et al, 2011;Lacroix et al, 2012a,b;Kermouche et al, 2016;Luo et al, 2016). In addition to volume conservative plastic flow (Kermouche et al, 2016), plastic deformation in silica glass relies on densification, a mechanism which is related to permanent volumetric changes of the glassy lattice (Ernsberger, 1968;Grimsditch, 1984;Lambropoulos et al, 1996;Perriot et al, 2006;Wondraczek et al, 2007;Wakabayashi et al, 2011;Lacroix et al, 2012b).…”

“…The authors reported that silica fibers with diameters ranging from 280 to 1950 nm exhibited stiffness values of bulk silica (Silva et al, 2006). Just recently, Luo et al (2016) used a two-step in situ process (modulated taper-drawing, followed by drawing inside the TEM) to prepare silica glass fibers with diameters below 50 nm. In situ tensile experiments performed in the absence of e-beam irradiation inside the TEM showed a brittle-to-ductile transition of silica glass nanofibers at room temperature when the fiber diameter is reduced below 18 nm.…”

“…In situ tensile experiments performed in the absence of e-beam irradiation inside the TEM showed a brittle-to-ductile transition of silica glass nanofibers at room temperature when the fiber diameter is reduced below 18 nm. Moreover, in case of fibers with diameters <20 nm, a strong scatter in their fracture stress (σf) values ranging from 1 to 13 GPa is observed, depending on the used strain rate during the in situ tensile experiments (Luo et al, 2016).…”