Johan F. S. Christensen

The practical strength of oxide glasses is greatly reduced by surface flaws that form during processing and use. Instrumented indentation can mimic such real‐life damage events and allows for systematic examination of the deformation and structural changes in the regions of the glass being indented. However, structural probing is nearly always performed after rather than during the sharp contact event, limiting our understanding of the indentation process. To overcome this, here we develop and apply an in situ nanofocus X‑ray scattering method for visualizing and quantifying indentation‐induced densification and cracking in glass materials. We demonstrate its feasibility in a proof‐of‐concept experiment by probing the local mechanical and structural response of vitreous silica during indentation. Two‐dimensional mapping of the scattering pattern in the zone below a sharp diamond wedge indenter reveals local changes in the atomic structure and density as well as cracking behavior. These in situ measurements during indentation reveal the formation and evolution of the densification zone and cracking, probing features down to the nanoscale. Although the developed experimental approach can be further improved (especially beam‐sample‐indenter alignment), we believe it will allow for a deeper understanding of the interplay between structural densification and cracking behavior in glasses.

Fracture toughness is a critical mechanical property of glasses, but a detailed understanding of its link to composition and structure is still missing. Here, focusing on the industrially important family of calcium aluminosilicate glasses, we measure the fracture toughness of two glass series using the single‐edge precracked beam method, one based on tectosilicate compositions with varying silica content and the other covering both percalcic and peraluminous compositions with varying Al/Ca ratio. To elucidate the structural origins of the variation in fracture toughness, we perform X‐ray total scattering measurements and molecular dynamics simulations. Our findings indicate that local coordination changes of especially Al atoms, so‐called bond switching, are positively correlated with fracture toughness within the investigated compositional range. We also compare this variation with that in other mechanical properties, including elastic moduli, hardness, and crack initiation resistance. We find that various structural aspects need to be considered to describe and understand the mechanical properties of calcium aluminosilicate glasses.

The brittleness of oxide glasses remains a critical problem, limiting their suitability for high‐performance and safety‐critical applications. In this study, we attempt to address this by synthesizing nanostructures in sodium borosilicate glasses through phase separation. While most previous work on the mechanical properties of phase‐separated glasses has focused on phase separation through nucleation and growth, we here create interconnected structures through spinodal decomposition. Interestingly, this leads to improvements in Vickers hardness (from 5.8 to 6.2 GPa), crack initiation resistance (from 4.9 to 8.1 N), and fracture toughness (from 0.85 to 1.09 MPa⋅m ^1/2 ). We show that the interconnected glassy phases deflect the propagating cracks, causing the required energy for cracks to cross phase boundaries to increase when subjected to external stress. This study deepens the understanding of how to address the brittleness problem of oxide glasses and provides a promising way to design high‐performance glass materials.

Methods to improve the fracture toughness of oxide glasses are needed since low fracture toughness is a major bottleneck for their applications. To overcome this, it is critically important to investigate the effect of both short- and medium-range structural features on fracture toughness. Recent work reported a record-high fracture toughness for a bulk lithium aluminoborate glass subjected to hot compression. Here, we further explore the structural origin of this high fracture toughness by subjecting different alkali aluminoborate glasses to hot compression. Through a combination of x-ray total scattering experiments and atomistic simulations, we find that hot compression causes significant changes to both the short- and medium-range order structure of the glasses, e.g., increased coordination numbers (CNs) of network forming species and decreased average size of ring-type structures. To this end, we reveal positive correlations between the pressure-induced increase in fracture toughness and (i) the increase in average CN of network forming species and (ii) the area of the first sharp diffraction peak in the structure factor. Our study thus improves the understanding of which structural features benefit intrinsic toughening of oxide glasses.

In this work, we study the structure–topology–property relations of a series of melt‐quenched lithium germanate glasses. These glasses exhibit the so‐called germanate anomaly, that is, the germanium atoms feature a distribution of four‐coordinated and higher coordinated germanium species, manifesting itself as anomalies in several material properties. Here, we couple variations in the number of atomic bond constraints with measured variations in thermal and mechanical properties, including thermal conductivity, Vickers hardness, and fracture toughness. For thermal conductivity, a strong correlation is found with sound velocity as well as with the volumetric constraint density. For hardness, a good correlation with volumetric constraint density is found, whereas, for fracture toughness, variations in network topology alone are insufficient to explain the composition–property relation. To account for this, we apply a recent model which incorporates knowledge of local structure, mechanical properties, and fracture patterns to predict the fracture toughness, showing a good qualitative agreement with the experimental data.

The low fracture toughness of oxide glasses is a key limitation for many of their applications. Inducing and controlling nanoscale phase separation in oxide glasses has been proposed as a potential toughening strategy, as, unlike many alternative extrinsic toughening approaches, it allows one to retain the optical transparency. Using molecular dynamics simulations, we here investigate the toughening mechanism in soda-lime-silica glasses with embedded glassy nanoscale silica droplets. This system is chosen as a model for the experimental structure of phase-separated soda-lime-silica glass, which is attractive considering its existing commercial use and the ease of inducing phase separation. We calculate the fracture toughness of glass structures containing nanodroplets of varying sizes and with different precrack positions, revealing that the glassy silica droplets toughen the material. The simulations show that crack propagation is impeded by crack arrest, crack deflection and diversion, and stress field alteration, ultimately increasing the fracture toughness. Our findings thus shed light on the toughening mechanism due to phase separation, with important implications for the experimental design of oxide glasses with controlled nanoscale phase separation.

Humans primarily interact with information technology through glass touch screens, and the world would indeed be unrecognizable without glass. However, the low toughness of oxide glasses continues to be their Achilles heel, limiting both future applications and the possibility to make thinner, more environmentally friendly glasses. Here, we show that with proper control of plasticity mechanisms, record-high values of fracture toughness for transparent bulk oxide glasses can be achieved. Through proper combination of gas-mediated permanent densification and rational composition design, we increase the glasses’ propensity for plastic deformation. Specifically, we demonstrate a fracture toughness of an aluminoborate glass (1.4 MPa m^0.5) that is twice as high as that of commercial glasses for mobile devices. Atomistic simulations reveal that the densification of the adaptive aluminoborate network increases coordination number changes and bond swapping, ultimately enhancing plasticity and toughness upon fracture. Our findings thus provide general insights into the intrinsic toughening mechanisms of oxide glasses.