Abstract: Objective: To investigate the effects of repeated firings on the adhesion of zirconia with different coloring methods. Methods: Two hundred and twenty-five bar-shaped zirconia specimens (10 mm×10 mm×1.5 mm) were prepared and randomly divided into 3 groups: stained by immersion (A2 shade), pre-shaded (A2 shade), and the control group (white). Each group was further divided into 5 subgroups, which were sintered for twice, triple, four times, five times, and six times, respectively. The self-adhesive resin cement tubes (Speed CEM), with a diameter of 1 mm, were bonded on the specimen surfaces. After 2 000 thermal cycles, an universal testing machine was used to measure the micro-shear bond strength of the resin cements to zirconia. A stereo microscope was used to observe the fracture model. Furthermore, SEM and EDS were applied to obverse the surface topography and composition of the zirconia. Results: Along with the multiple firings, the micro-shear bond strength of the resin cement to the zirconia was increased. The specimens fired for 6 times showed the best adhesion property. The micro-shear bond strength of the zirconia stained by immersion was significantly higher than other groups. Conclusion: Zirconia stained by immersion showed better adhesion property. Moreover, the firing times affected the zirconia adhesion property.

Key words: Stained by immersion, Preshaded, Zirconia, Repeated firing, Micro-shear bond strength