Structure–property relationships and densification-crystallization behaviours of simplified lithium disilicate glass compositions

The role of each oxide component on the structure of molten glass and on their densification and crystallization behaviours is expected to be more easily assessed for compositions comprising a small number of components. In the present work, binary (Li2O–SiO2), ternary (Li2O–Al2O3–SiO2), and quaternary (Li2O–K2O–Al2O3–SiO2) glass compositions were selected. The aim was to investigate the relationships between the compositions and structure of the glasses, namely, the effects of each component on the extent of liquid-in-liquid phase separation, their thermal behaviour and the phase assemblage in the disilicate glass–ceramics. The distribution of structural units in the experimental glasses was assessed using 29Si MAS-NMR spectroscopy. Adding Al2O3 played a dual role as network former and modifier when added to the binary system by enhancing Q2 and diminishing Q3 structural units. The processing window for sintering, the difference between the onset of crystallisation and glass-transition temperature (Tc−Tg), was too narrow for the less polymerised network structures (binary and ternary systems), hindering the densification of the corresponding glass powder compacts. Oppositly, compositions in the quaternary system featured excellent densification behaviour coupled with high mechanical strength. These features derive from a more rigid glass network comprising four coordinated (AlO4/2)− units and K+ cations in its vicinity.