Influence of Topological Anisotropy on the Mechanical Properties of Silicate Glasses

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Although glasses are generally viewed as isotropic material, freezing-in the flow structure of a glass under load can easily be used to produce anisotropic glass components. One example of such a process is the drawing of oxide glass fibers. Macroscopically, the anisotropic nature of the glass manifests itself in the phenomenon of optical birefringence. Moreover, mechanical properties of glass may be strongly affected by anisotropy. Indeed, it was recently shown that topological anisotropy, i.e. a direction dependence in the way the silica tetrahedra are connected with each other, is the main cause for the one order of magnitude higher strength of glass fibers compared to bulk glasses of the same composition. Given the technical relevance of these findings, relatively little is known about the nature of the topological changes which lead to anisotropic properties, and on how topological anisotropy influences the various mechanical properties.The aim of this proposed research project is to study how anisotropy develops in silicate glasses, how it can be characterized on a topological level, and how topological anisotropy affects the stress-strain response and toughness of silicate glasses. For this purpose, we will combine experimental investigations on the macro scale with in-situ nanomechanical testing in the transmission electron microscope (TEM) and atomistic computer simulations.In detail, the key objectives for the experimental work are the production of bulk anisotropic oxide glasses, the detailed characterization of their structure by scattering techniques and fluctuation electron microscopy and the determination of their (direction-dependent) mechanical properties by macroscopic and microscopic tensile tests, fracture experiments and indentation studies. In addition, silica nanostructures (nanospheres, nanofibers) will be rendered anisotropic by in-situ mechanical quenching in the TEM exploiting the recently discovered phenomenon of superplasticiy that can be controlled via electron irradiation.The atomistic simulations will focus on characterizing the topological anisotropy and studying the mechanisms which lead to anisotropy, as well as on determining the direction dependent mechanical properties as function of anisotropy.Throughout the project, the experimental and simulations efforts are closely linked, e.g. by the simulation of electron diffraction patterns and fluctuation electron microscopy images of MD generated samples and comparison to experimental results, or by the comparison of MD simulations of nanomechanical tests with corresponding in-situ experiments.Such knowledge will be used to (a) specifically engineer anisotropic crack propagation by generating dedicated topological anisotropy and (b) understand, on a topological basis, crack propagation in a more complex (multiaxial) field of topological anisotropy and stress. The proposed research will strongly profit from collaborations within the priority programme “Topological Engineering of Ultra-Strong Glasses”, e.g. on the influence of topology on mechanical properties of bulk metallic glasses, where recently similar effects of anisotropy on elastic properties were reported, or on micromechanical testing of oxide glasses.