An in situ and ex situ study of the microstructural evolution of a novel lithium silicate glass-ceramic during crystallization firing

Abstract

Objective: To elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle.

Methods: Blocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1mm thick plates and polished to 1μm finish. Some of them were crystallized prior to polishing. Firstly, ex situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy, and X-ray diffractometry. Secondly, the pre-crystallized samples were subjected to in situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy.

Results: The microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li₂SiO₃) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO₂; elemental oxides do not precipitate unless over-heated above 820°C; and the Li₂SiO₃ nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li₂Si₂O₅) crystals. Nonetheless, the Li₂SiO₃ nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li₃PO₄) nanocrystals precipitate from the glass matrix.

Significance: The identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.

Keywords: Compositional and microstructural developments; Crystallization firing; Ex situ characterization; In situ characterization; Lithia-based glass-ceramics.