# Effects of Veneering Ceramic and Methods on Failure Load of Veneered Zirconia
## 4 Discussion  
Strength of dental ceramic can be influenced by many factors such as size, numbers and distribution of flaws, thickness of the core, presence of the veneering material in the system, loading conditions, etc., and is usually measured in flexure (bending) because this test is generally easier to perform than a pure tensile test. In bending test, tensile stress reaches a maximum on one surface and compressive stress reaches the maximum on the opposite side. In dental applications, ceramic copings are usually covered with an esthetic layer of feldspathic porcelain. Such a combination forms a layered structure with different elastic moduli and thermal expansion coefficients. It is thus important to evaluate the failure  
Figure 3: Failure load of bilayered veneer zirconia in control group (before the simulated aging) compared with after simulated aging. Figure 2: Failure load of bilayered veneer zirconia after 24 h in water. All groups under the same horizontal line are not significantly different to each other (\(p<0.05\)). stresses of such materials as layered composites. In the present experiment, bilayered bar-shaped specimens were used. This allowed the study to be reproducible and easier to standardize the specimens. Another advantage is that bar-shaped specimens required less materials compared with crown-shaped specimens. For example, one standard size (14 mm \(\times\) 14 mm) CAD/CAM block can be sectioned into 9 to 12 bars. In the present study, there was no statistically significant difference between the failure load of bilayered hand-layered veneered and pressed veneered zirconia. The flexural strength of both hand-layered and press-on porcelain, which are around 70-90 MPa, are relatively low compared to CAD-/CAM-milled ceramics [21, 22]. The hand layering technique is a very technique sensitive method and require many factors such as mixing quality of porcelain liquid, dental technician experience, cooling parameter, and ceramic shrinkage whereas the press-on technique minimize the firing shrinkage and can give good marginal adaptation [23, 24]. Others have reported that divesting procedure of pressed-on ceramics involves immersion in hydrofluoric acid solution and sandblasting, to remove the reaction layer, which significantly increases the surface roughness of the ceramics [25]. These imperfections on the outer surface of ceramics are prone to crack formation and propagation [25]. CAD-/CAM-milled veneer enables stronger veneering materials to be used and high-quality standards through prefabricated ceramic blocks manufactured by industrial pressing without any porosities to be achieved [26, 27]. Results from the present study showed that there are differences in the failure load between bilayered bars with different resin cement. These indicated that cement materials might play a significant role to determine the failure load of the bilayered system. The flexural strength of the interface materials and the bond strength between the interface materials and ceramic are the two properties that might be related to the failure load of the bilayered system. Low failure load of the bilayered bars cemented with Panavia 21 might be explained by the absence of zirconia primer of this system, thereby lowering the bond strength of the resin cement and zirconia. In the present study, the zirconia bars veneered with IPS e.max CAD have generally high failure load compared to other combination of zirconia and veneer materials. One of the reasons is the flexural strength of the veneer material itself, which is about 3-4 times higher compared with the others. IPS e.max CAD and IPS e.max ZirCAD fused by fusion glass ceramic possessed highest failure load. These can be explained as an effect of the high flexural strength of lithium disilicate veneer (360 MPa) combined with high flexural strength (160 MPa) and elastic modulus of the fusing porcelain. The crystallization and fusion at the same cycle may allow the fusion glass-ceramic to flow and adhere to the IPS e.max CAD better.