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 (R) 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 degrees C vs. 820-840 degrees C) at the segment of 30 degrees C/min heating rate, and (2) a protracted holding time (14 min vs. 7 min) at the isothermal temperature of 840 degrees 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 degrees C) and protracted holding time (820-840 degrees 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. Published by Elsevier Ltd on behalf of Academy of Dental Materials.