Methods: Zirconia cores (e.max®ZirCAD) were veneered and cut to produce specimens with three different cross-sectional areas: 1.00 mm2; 1.44 mm2; and 2.25 mm2 using heat-pressed glass veneer (e.max®ZirPress). In addition, monolithic glass veneer (e.max®ZirPress) specimens with the same dimensions were also prepared (N=20 each). Bilayered and monolithic specimens were tested in tension until fracture and reported as interfacial bond strength and tensile strength respectively. Furthermore, fracture surface of all specimens were observed under optical and electron microscopes. The apparent toughness of both bonded bilayered and monolithic specimens was calculated from critical cracks and fracture strength. Mean interfacial bond strength, tensile strength, and apparent toughness were analyzed using one-way ANOVA and Tukey HSD (p<0.05).
Results: The results showed that cross-sectional area had effect on interfacial bond strength and tensile strength of bilayered dental ceramic and monolithic glass veneer respectively (Table 1). However, there was no significant difference in the apparent toughness of both bilayered dental ceramic and monolithic glass veneer groups.
Table 1: Tensile strength and apparent toughness (SD in parenthesis) of bilayered and monolithic specimens*
Materials |
Tensile strength (MPa) |
Apparent toughness (MPa.m1/2) |
||||
1 mm2 |
1.44 mm2 |
2.25 mm2 |
1 mm2 |
1.44 mm2 |
2.25 mm2 |
|
Bilayered |
17.6(6.1)A |
14.8(5.1)AB |
12.0(3.1)B |
0.23(0.10 |
0.21(0.07) |
0.19(0.08) |
Monolithic |
26.9(5.6)a |
23.5(5.3)ab |
20.4(6.1)b |
0.41(0.11) |
0.41(0.09) |
0.38(0.09) |
*The values with the same superscript are not significantly different.
Conclusions: This study demonstrated that cross-sectional area had no effect on apparent toughness of bilayered dental ceramics and monolithic glass veneer. Fracture mechanics approach, using critical crack size and strength, may be more reliable approach for testing bonding quality than bond strength test.