Effect of repeated firings on flexural strength of veneered zirconia

Methods: Blocks of pre-sintered zirconia (Aadva,GC,Tokio,Japan) were cut with a low speed diamond saw to obtain 30 bars. Dimensional changes occurring during sintering were taken into consideration in specimen cut. The specimen were sintered in the manufacturer facilities. After sintering, the bars had a final dimension of 15mm length, 4.0±0.2mm width and 0.8±0.1mm thickness. Each bar was measured with a digital caliper and values were recorded. The specimen were then veneered according to the following pattern. Group A : veneering ceramic was layered “in bulk” with a mold and 1 single firing was performed. Group B : veneering ceramic was layered in 5 layers. Each layer was controlled in thickness with a mold and individually fired. The veneering ceramic thickness for all the specimen was set at 1.2mm (total thickness of the specimen 2.0,mm). After final firing, each specimen was again measured with a digital caliper. Specimens were tested in a 3-point bending apparatus (span=13mm; cross-head speed=1mm/min) until failure. Flexural strength was calculated with a specific formula taking into account the bi-layered nature of the specimen. Data were statistically analyzed with One-way ANOVA.

Results: The calculated flexural resistance values and the statistical significance are reported in the following table:

Conclusion: PFZ showed to have a statistically significant higher flexural strength when the veneering ceramic was layered and fired in multiple layers rather than in one single layer.