The Effect of Multiple Firings on Microtensile Bond Strength of Core-Veneer Zirconia-Based All-Ceramic Restorations

Purpose: Differences in core and veneer coefficients of thermal expansion, firing shrinkage, and speed of increasing and decreasing the temperature may generate stress in veneered all-ceramic restorations. Given the necessity of performing multiple firing cycles to achieve improved contour, color, and esthetics, the purpose of this study was to determine the effect of multiple firing cycles on the microtensile bond strength (MTBS) of zirconia core to the porcelain veneer in zirconia-based all-ceramic restorations. Materials and Methods: Thirty blocks (12 x 12 x 4 mm(3)) of semi-sintered zirconia were machined and sintered according to manufacturer's instruction. Specimens were placed in three groups based on the number of firing cycles (4, 6, 8) for the veneering process. After veneering, the specimens were sectioned into microbars with 8 mm length and 1 mm cross-section. Twenty sound microbars in each group were stressed to failure in a microtensile tester machine at 1 mm/min. Fractured specimens were surveyed under a scanning electron microscope and classified as cohesive in core, cohesive in veneer, and mixed. MTBS data were analyzed using one-way ANOVA and Tukey test (p < 0.05). Results: The mean MTBS (MPa) after 4, 6, and 8 firing cycles were 30.33 +/- 2.13, 27.43 +/- 1.79, and 25.06 +/- 1.76, respectively. There was a statistically significant difference between the bond strengths of each of the three groups (p < 0.001). Conclusion: Increase in firing cycles decreased MTBS. Most of the failures (90-95%) in all three groups were cohesive in the veneering porcelain and did not change as the number of firing cycles increased.