History matters for glass structure and mechanical properties

Oxide glasses are one of the most important material families, but their applications are severely restricted by the main drawback, namely high brittleness. Structure-property relations thus need to be established, but even glasses with the same chemistry and density can exhibit different mechanical properties. That is, the processing history (such as thermal and pressure history) matters. However, the structural origins are not yet fully understood, e.g., as structural variations cannot always be detected by typical techniques such as Raman and NMR spectroscopy. In this talk, we discuss how synchrotron-based X-ray scattering techniques can be used to reveal the structural differences in glasses with varying density, track the structural relaxation of densified structures, and couple these structural changes with relaxation behaviour of density and indentation hardness. In detail, glasses from four composition were initially hot compressed at 2 GPa pressure close to their respective glass transition temperatures to achieve permanent densification and thus changes in structure as well as mechanical properties. These glasses were then thermally annealed for different durations at ambient pressure to induce relaxation in volume, structure, and properties. We have found that vitreous silica mainly densifies by ring network deformation in the medium ranges, while soda lime and soda lime-boro silicate glasses deform in all structural length scales, and soda alumino borate glass by coordination changes in the short ranges during hot compression. Except soda lime silicate, for all other studied glasses hardness relaxes before density. Finally, we discuss how the typical features are controlled by the glass structure at different length scales, at and beyond the nearest neighbour. Essentially, our results help in establishing new processing-structure-mechanics relationships of oxide glasses.