Achieving ultrahigh crack resistance in glass through humid aging

Oxide glasses continue to be among the most important materials for a sustainable society owing to their unique advantages, e.g., transparency, formability, low cost, and tailorable properties and functionalities. However, their high brittleness has severely been hindering the use of oxide glasses for many engineering and functional applications. Previous approaches to overcome this limitation by improving the fracture toughness have largely failed, but herein we report the use of a simple post-treatment, namely surface aging in a humid atmosphere, to improve not the resistance to crack growth, but rather the resistance to crack initiation. The effect of such aging on the mechanical performance is found to be highly composition dependent, and in stark contrast to most previous work, we thus find that water incorporation is not always a foe, but can also be a friend for glass mechanics. Specifically, we demonstrate crack-free ultrasharp cube corner indents in an aged caesium aluminoborate glass at loads above 25 N, while other aged oxide glasses studied in this work easily crack at loads below 1 N. The aging effect is found to be due to a combination of stress and hydrolysis assisted structural changes in the flexible aluminoborate glass network.