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.
|Number of pages||13|
|State||Published - Aug 1 2015|
Bibliographical noteFunding Information:
The authors would like to thank Dr. Angela Campbell and Dr. Gregory Ehlert at the United States Air Force Research Laboratory and Dr. Tao You and Dr. Todd Lincoln at the United States Army Institute of Surgical Research for their help and support. Also, special thanks goes to Dr. Shashikant Singhal at Ivoclar Vivadent (Schann, Liechtenstein) for supplying the IPS e.max® CAD blocs. This research was partially funded by the Dental Master's Thesis Award Grant from the Delta Dental Foundation .
© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
- Differential scanning calorimetry
- Heating schedule
- IPS e.max® CAD
- Lithium disilicate
- Lithium metasilicate
- Phase transformation
- Temperature threshold
ASJC Scopus subject areas
- Materials Science (all)
- Dentistry (all)
- Mechanics of Materials