Microstructural Evolution and Physical Behavior of a Lithium Disilicate Glass-Ceramic

The partially crystallized IPS e.max^® CAD block is formed from the melt of specially formulated glass then transformed into its crystallized state via controlled nucleation and growth of lithium disilicate (LS₂) crystals.  Hence, the properties of LS₂ glass-ceramic restoration not only depend on the glass composition but also its phase assemblage and microstructural evolution induced by heat treatment, which can ultimately impact clinical performance.  Objective:   to characterize the phase transformation and microstructural evolution of IPS e.max^® CAD under the effect of different annealing conditions and to relate the various evolutionary stages to its physical properties.  Methods:   Five groups of IPS e.max^® CAD samples (n=5/group) were prepared and treated with a unique annealing condition per group.  For each group, the crystalline phase, crystalline content, flexural strength & modulus, microhardness, and fracture toughness were measured by X-ray diffraction (XRD), quantitative XRD analysis, a universal testing machine, a Vickers tester, and Hysitron nano-indenter respectively.  For flexural strength & modulus, beam specimens (2x2x17mm³) were created and tested using 3-point bending.  For microhardness testing, specimens were indented using a 1-kg load and a 10-second dwell time.  The microstructures of amorphous glasses and LS₂ crystals were investigated using scanning electron microscopy (SEM).  A mean and standard deviation (SD) were determined per group (see table).  A 1-way ANOVA/Tukey was performed per property (alpha=0.01).  Results:   XRD and SEM revealed two distinct phases during LS₂ crystallization.  Significant differences were found between groups per property (p<0.001).  Conclusion:   the phase assemblage, microstructure, and the amount of LS₂ crystals, governing the physical properties of LS₂ glass-ceramic restoration, can be tailored through annealing.

 

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