In situ mechanical quenching of nanoscale silica spheres in the TEM

Abstract

The physical and mechanical properties of glasses strongly depend on their bonding configuration and topology, which includes near, intermediate and long range order [1]. It is well‐known that controlled application of mechanical load during cooling of glass melts can lead to topologically modified network structures [2,3]. Also uniaxial compression experiments can be used to introduce structural anisotropy into various glasses [4,5]. Moreover, moderate electron beam (e‐beam) irradiation in the transmission electron microscope (TEM) [6,7] and scanning electron microscope (SEM) [8] can be exploited to induce enormous ductility in nanoscale silica spheres under mechanical load. However, still the question remains whether e‐beam irradiation in combination with compression can lead to anisotropic glasses and how this affects their mechanical properties. Here we present a novel approach to perform athermal mechanical quenching experiments in the TEM and evidence its impact on mechanical properties of nanoscale silica spheres [9]. Nanoscale silica spheres are compressed in the TEM under different e‐beam conditions and loading scenarios by using the Hysitron PI95 TEM Picoindenter TM (Fig. 1). Prior to compression the silica spheres are irradiated with an e‐beam current density of 0.09 A/cm 2 , leading to a shrinkage of 15–18% [7]. In experiment 1 the silica sphere is compressed at beam‐off conditions and exhibits an elastic‐plastic deformation behavior without fracture [7]. In experiment 2 the silica sphere is quenched under load. To achieve this the compression is started under e‐beam irradiation (which we use to mimic temperature) and the e‐beam is switched off during compression. The sudden absence of the e‐beam quenches‐in the modified silica network structure. Surprisingly, starting from the quenching point the slope of the force‐displacement curve increases drastically, while a completely elastic loading‐unloading behavior is obtained. In case of experiment 3 directly after the beam‐on compression a holding segment is used, allowing for relaxation of stresses. During the following deformation at beam‐off conditions the silica sphere shows a completely elastic loading‐unloading behavior. Interestingly, complementary finite element method simulations reveal that the Young's moduli ( E ) of silica spheres are altered: E values of 45 GPa, 38 GPa and 29 GPa are obtained for silica spheres from experiments 1, 2 and 3, respectively. As a direct reason for this observation structural anisotropy is proposed (Fig. 2) [9]. Quenching of silica spheres under load leads to a partially anisotropic silica network, while quenching after relaxation generates an even more anisotropic structure. During the relaxation period the silica sphere is in a compressed and confined state, during which structural re‐organization is restricted along the compression direction [9]. This mechanism is further favored by residual tensile stresses acting perpendicular to the loading direction [10,11], which maintains the development of structural anisotropy reported here [9].