Molecular structure-property relationships of network glasses under mechanical loading

• Molekulare Struktur-Eigenschafts-Beziehungen von Netzwerkgläsern unter mechanischer Belastung

Ebrahem, Firaz; Markert, Bernd (Thesis advisor); Rolfes, Raimund (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Book, Dissertation / PhD Thesis

In: Report. IAM, Institute of General Mechanics IAM-09
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Generally, glasses are hard and transparent solids that are very resistant to corrosion, and show superb electrical and thermal insulating properties. Therefore, glasses are used across a broad range of industrial applications. Despite extensive research into their structure and properties, the highly complex mechanical behaviour remains, to date, poorly understood. It is widely accepted that the interrelation between the structure and the way a material behaves is of central significance for materials engineering. For crystals, finding structure-property relationships has been carried out successfully over the past decades. The periodicity of crystalline materials allows for a clear picture of the molecular structure, for instance via observation of dislocations by transmission electron microscopy. This led to the development of material models, such as crystal plasticity that is based on the dislocation slip mechanisms in a crystal lattice. On the contrary, finding such relationships for glassy solids has been limited by the difficulty in imaging their structure due to the lack of long-range order. Yet, it is quite natural that the properties of glasses are sensitive to the molecular structure. Therefore, the aim of this thesis is to understand how this structure dictates the deformation behaviour of a glass. Using molecular simulations, the structure-property relationships are investigated for silica glass, forming a network of covalently-bonded silicon and oxygen atoms. Thereby, the main focus lies on numerical methods and approaches that enable quantitative engineering of network glasses to achieve desired mechanical properties. First, bulk silica glass is realised by quenching the melt. Although lacking long-range order, the glass network can be evaluated statistically by means of the ring topology, i.e. the ring size distribution, where a ring is composed of a number of covalent bonds within it. By the use of different quenching rates, it is shown that the thermal history strongly influences the network topology. Subsequent deformation of the glass samples reveals that a change in the network topology results in stress-strain relations which vary to a significant degree. Based on this, in particular, the plastic deformation can directly be linked to the network topology. Furthermore, a 2D glass model is introduced based on statistical data extracted from recently discovered 2D silica. The two-dimensionality of this model allows for the direct observation of the molecular structure during deformation. In this way, the imaging limitations of the complex 3D network of bulk silica glass can be overcome. The 2D silica glass is investigated under both tensile and shear deformation. The athermal quasi-static deformation method is applied in order to study how the pure structural disorder correlates with the stress response. Here, the main objective is to identify and evaluate the elementary inelastic events which are localised rearrangements of a small number of atoms. In addition, the crystalline-to-vitreous transition is explored by controlling the network structure.