Mechanical properties of transparent sodium phosphosilicate glass-ceramics

Glass-ceramics feature excellent mechanical properties but tend to lack transparency due to the presence of large crystals with a different refractive index from the matrix glass. Here, we investigate the relationship between the heterogeneous microstructure, mechanical properties (hardness, crack resistance, and fracture toughness), and transparency in ${\mathrm{Na}}_{2}\mathrm{O}\text{--}{\mathrm{P}}_{2}{\mathrm{O}}_{5}\text{--}\mathrm{Si}{\mathrm{O}}_{2}$ glass-ceramics. The mechanical properties are determined by the combination of crystal type, content, and size, as well as the remaining glass-matrix structure. The crystal fraction and size increase upon heat treatment, whereas the network connectivity of the residual glass-matrix phase decreases. These observed changes have opposite effects on crack resistance and fracture toughness. Changes in crystallization behavior have a more significant effect on crack resistance relative to that on fracture toughness, while changes in crystal size have a more pronounced effect on fracture toughness. The glass-ceramic samples feature excellent transmittance and reach a maximum fracture toughness of $1.1\phantom{\rule{0.16em}{0ex}}\mathrm{MPa}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{0.5}$.