In situ tensile testing of silica glass membranes in the TEM

Abstract

Increasing research on strength of glasses, which was greatly influenced by Griffith [1], has spawn strengthening strategies such as topological engineering [2]. Pioneering works by Takamori and Tomozawa [3], and Brückner [4] on cooling of glass melts under load and introducing structural anisotropy into the glass structure have been followed by strengthening of glasses by targeted mechanically‐induced structural anisotropy [5]. Moderate electron beam (e‐beam) irradiation has been exploited to induce enormous ductility and superplasticity into nanoscale silica spheres and wires, and was shown to affect their mechanical response [6‐8]. It is, however, not yet known whether e‐beam irradiation in combination with tensile loading can lead to anisotropic glasses, and how this affects their mechanical properties. Recently we have reported that e‐beam‐assisted quenching under load inside the transmission electron microscope (TEM) alters the mechanical properties of nanoscale silica spheres and attributed this to compression‐induced structural anisotropy [9]. Here we transfer this approach to tensile loading of nanoscale silica membranes. Tensile specimens are prepared with the focused ion beam (FIB) from commercially available silica membranes (Plano GmbH) on push‐to‐pull (PTP) devices (Fig. 1). Raman spectroscopy was performed to investigate the structure of silica membranes and damage induced by FIB (Fig. 2). Raman spectra show that as‐received membranes exhibit a structure of vitreous silica [10,11]. After Ga‐irradiation in the FIB densification of the membranes occurs, while the membranes still maintain the character of vitreous silica. In situ tensile experiments are carried out with the Hysitron PI95 TEM Picoindenter TM inside of a Titan 3 Themis 300. To achieve mechanical quenching inside the TEM moderate e‐beam irradiation is used to mimic temperature, while the e‐beam is switched off during elongation of the silica membrane. While the deformation of silica under e‐beam irradiation is superplastic [6], the sudden absence of the e‐beam during tension (quenching point) translates the deformation from superplastic to elastic (see Fig. 3a)), and finally leads to fracture. The Young's modulus E = 73 GPa of the membrane drawn at beam‐off conditions (Fig. 3b)) almost matches the value known for bulk fused silica [12], while the value of the membrane quenched under load ( E = 78 GPa) is slightly increased. The tensile strength is in the range of values known from silica glass fibers with comparable dimensions [13], but clearly exceeds values known for microscale silica glass fibers [5]. Finally, we demonstrate how to directly track structural changes in silica glass during in situ tensile experiments in TEM by in situ electron diffraction. The unique combination of in situ electron diffraction with tensile experiments in TEM enables direct relation of structural changes in silica glass to quantitative nanomechanical data.