Microstructure and crystallization behavior of Na2O-Al2O3-SiO2 glass-ceramics with MgO additions

Transparent Na2O–MgO–Al2O3–SiO2 glass-ceramics containing spinel crystals were prepared by melting and heat treatment method, and the effects of substitution of 2–8 mol% MgO for Na2O on the crystallization behavior, microstructure, and properties were investigated. As Na2O is replaced by MgO, there is higher concentration of 5-coordinated Al units in the glass structure, which played an important role in the crystallization behavior of the glass. The increasing substitution of MgO for Na2O promotes the formation of spinel but inhibits the nepheline and carnegieite. When containing 2 mol% MgO, the large-sized nepheline and carnegieite are the main crystalline phases, making the glass ceramics opaque. When containing 6 mol% MgO, there is no NaAlSiO4 crystal formed, but only spinel crystals with small size and uniform distribution are detected in glass-ceramics. The glass-ceramics with spinel as the main crystalline phase show high transparency above 82% in the visible wavelength range. The Vicker's hardness of transparent glass-ceramics can be further improved by ion-exchanged with KNO3 molten salt, which shows significant potential in the applications for the protective covers of mobile displays.