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

Abstract

ObjectiveTo 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. MethodsIn-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. ResultsIt 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. SignificanceIt 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.