Fracture Toughness of Glazed and Non-Glazed Fully Crystallized Lithium Disilicate

Objectives: This study aims to investigate the effect of glazing and thermocycling on the fracture toughness of a full crystallized lithium alumino disilicate ceramic.
Methods: Bending bars (16X4X3 mm) were cut from LisiCAD blocks (GC Corporation, Tokyo, Japan) according to ISO 23146 (n=40). The bars were randomly assorted into two groups: glazed and unglazed (n=20). The bars in the glazed group were coated with a layer of GC Initial IQ Lustre Pastes and subjected to a glazing firing cycle in a dental ceramic furnace according to the manufacturer’s parameters. The specimens in the unglazed group were left without glazing. A single edge v-notch beam method (SEVNB) was used to test the fracture toughness according to the ISO. Half of the specimens (n=10) were selected for the initial fracture toughness (K_IC) measurements. The remaining specimens underwent a thermocycling process for 30,000 cycles before testing.
K_IC was evaluated using a universal testing machine with a 5kN load cell. The machine was configured for three-point bending, with a 12mm span, and the loads were recorded at a crosshead speed of 0.5mm/min until fracture occurred. Shapiro-Wilk and were used to evaluate the normality of the data. The t-test was used for pairwise comparisons between groups. Significance level was set to (α=0.05).
Results: Glazed specimens had significantly higher fracture toughness than non-glazed ones (P<0.001). Glazed specimens had mean K_IC 3.05 ±0.18 MPa√m while non-glazed had K_IC 2.78 ±0.14 MPa√m after thermocycling. Thermocycling did not have any significant effect on the fracture toughness of both glazed and non-glazed specimens.
Conclusions: Despite the manufacturer's claims that glaizng of LiSiCAD is optional, our findings shows that glazing significantly enhances fracture toughness, therefore should be recommended.