Microstructural evolution and physical behavior of a lithium disilicate glass-ceramic

Background Elucidating the microstructural responses of the lithium disilicate system like the popular IPS e.max® CAD (LS₂), made specifically for computer-aided design and computer-aided manufacturing (CAD-CAM), as a temperature-dependent system unravels new ways to enhance material properties and performance. Objective To study the effect of various thermal processing on the crystallization kinetics, crystallite microstructure, and strength of LS₂. Methods The control group of the LS₂ samples was heated using the standard manufacturer heating-schedule. Two experimental groups were tested: (1) an extended temperature range (750-840°C vs. 820-840°C) at the segment of 30°C/min heating rate, and (2) a protracted holding time (14 min vs. 7 min) at the isothermal temperature of 840°C. Five other groups of different heating schedules with lower-targeted temperatures were evaluated to investigate the microstructural changes. For each group, the crystalline phases and morphologies were measured by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Differential scanning calorimeter (DSC) was used to determine the activation energy of LS₂ under non-isothermal conditions. A universal testing machine was used to measure 3-point flexural strength and fracture toughness, and elastic modulus and hardness were measured by a nanoindenter. A one-way ANOVA/Tukey was performed per property (alpha = 0.05). Results DSC, XRD, and SEM revealed three distinct microstructures during LS₂ crystallization. Significant differences were found between the control group, the two aforementioned experimental groups, and the five lower-targeted-temperature groups per property (p < 0.05). The activation energy for lithium disilicate growth was 667 (±29.0) kJ/mol. Conclusions Groups with the extended temperature range (750-840°C) and protracted holding time (820-840°C H14) produced significantly higher elastic-modulus and hardness properties than the control group but showed similar flexural-strength and fracture-toughness properties with the control group. In general, rapid growth of lithium disilicates occurred only when maximum formation of lithium metasilicates had ended.