Microstructural development during heat treatment of a commercially available dental-grade lithium disilicate glass-ceramic

Objective

To elucidate the microstructural evolution of a commercial dental-grade lithium disilicate glass-ceramic using a wide battery of in-situ and ex-situ characterization techniques.

Methods

In-situ X-ray thermo-diffractometry experiments were conducted on a commercially available dental-grade lithium disilicate glass-ceramic under both non-isothermal and isothermal heat treatments in air. These analyses were complemented by experiments of ex-situ X-ray diffractometry, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, and field-emission scanning electron thermo-microscopy.

Results

It was found that the non-fired blue block consists of ～40?vol?% crystals embedded in a glass matrix. The crystals are mainly lithium metasilicate (Li₂SiO₃) along with small amounts of lithium orthophosphate (Li₃PO₄) and lithium disilicate (Li₂Si₂O₅). Upon heating, the glassy matrix in the as-received block first crystallizes partially as SiO₂ (i.e., cristobalite) at ～660?°C. Then, the SiO₂ crystals react with the original Li₂SiO₃ crystals at ～735?°C, forming the desired Li₂Si₂O₅ crystals by a solid-state reaction in equimolar concentration (SiO₂?+?Li₂SiO₃?→?Li₂Si₂O₅). Precipitation of added colourant Ce ions in the form of CeO₂ appears at ～775?°C. These events result in a glass-ceramic material with the aesthetic quality and mechanical integrity required for dental restorations. It also has a microstructure consisting essentially of elongated Li₂Si₂O₅ grains in a glassy matrix plus small cubic CeO₂ grains at the outermost part of the surface.

Significance

It was found that by judiciously controlling the heat treatment parameters, it is possible to tailor the microstructure of the resulting glass-ceramics and thus optimizing their performance and lifespan as dental restorations.