Optimizing the Crystalline Kinetics, Thermal-Processing, and Strength of Lithium-Disilicate Glass-Ceramic

Elucidating 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 function of temperature 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 phase, flexural strength and fracture toughness, and crystalline morphology were measured by X-ray diffraction (XRD), universal testing machine, and scanning electron microscope (SEM) respectively.  Differential scanning calorimeter (DSC) was used to determine the activation energy of LS₂ under non-isothermal conditions.  A one-way ANOVA/Tukey was performed per property (alpha = 0.05).  Results:  DSC, XRD, and SEM revealed two distinct phases during LS₂ crystallization.  Significant differences were found between groups per property (p<0.05).  The activation energy for lithium disilicate growth was 377.4 (± 20.9) KJ/mole.  Conclusions:  Group with the extended heating range (750-840 °C) provided similar significant strength properties with the control group.  In general, explosive growth of lithium disilicates occurred only when maximum formation of lithium metasilicates had ended.

Group	Flexural Strength						Fracture Toughness
	σFS (MPa)			EFS (GPa)			KIC (MPa m0.5)
	Average		SD	Average		SD	Average		SD
Not-Fired	140.7	a	12.1	53.2	ab	2.0	1.8	a	0.3
530-590 °C	131.1	a	23.7	52.6	ab	4.0	1.8	a	0.2
590-750 °C	273.6	bc	31.0	57.9	b	1.5	2.1	ab	0.5
590-750 °C (H14)	215.8	b	16.2	50.8	a	2.9	2.3	abc	0.5
750-780 °C	243.6	b	44.0	53.7	ab	2.9	2.8	bcd	1.3
750-840 °C	332.4	cd	39.6	67.0	c	1.7	3.3	bcd	0.7
820-840 °C	345.9	d	31.4	66.3	c	5.0	3.5	cd	0.5
820-840 °C (H14)	320.6	cd	65.0	56.4	ab	2.3	3.8	d	0.9
Groups with the same lower case letter per column are not significantly different (p > 0.05)