High damage-resistance and fracture toughness of transparent Nb-doped barium aluminoborate glass ceramics

Glass-ceramics offer the opportunity to address the main disadvantage of oxide glasses, namely low fracture toughness. The presence of crystals can improve the fracture toughness (crack growth resistance), but the crack initiation resistance will typically be deteriorated due to the induced residual stresses. Excessive crystal content and large crystal size can also lead to a loss of transparency. In this work, we report a Nb-doped barium aluminoborate glass-ceramic, for which both the crack initiation resistance (CR) and fracture toughness (K_Ic) get improved upon heat treatment, while still maintaining some transparency. Relying on combined structural and mechanical characterizations, we demonstrate that the coordination numbers of B and Al in the glass phase decrease upon heat treatment, which is beneficial to energy dissipation under mechanical stress to improve CR. Furthermore, the formed piezoelectric crystal phase BaNb₂O₆ also helps to improve both CR and K_Ic, likely by converting the stress-induced mechanical energy into electric energy. Meanwhile, the increase in crystallinity and crystal size after heat treatment also leads to higher K_Ic of the glass-ceramics. These findings thus help address the difficult problem of simultaneously improving the resistances to crack initiation and growth, which will facilitate the future design of transparent, yet highly damage-resistant and damage-tolerant glass ceramics.