Mechanical Properties of Multilayered Zirconia Materials Subjected to Repeated Firing

Objectives: To evaluate the flexural strength of multilayered zirconia materials that use varying chroma or yttria concentrations throughout the material.
Methods: Materials examined were ZirCAD Prime (ZP, Ivoclar), Origin Beyond+ Hybrid (OB, B&D Dental), and Katana HTML (Kuraray). 40 bars were produced from ZP (13.1 x 5.3 x 54 mm) and OB (11.1 x 3.6 x 32 mm) each and 80 bars from Katana (KT1, 11.5 x 3.6 x 32 mm) plus 45 smaller bars (KT2, 2.15 x 3 x 25 mm) from Katana. 12 specimens were made from inCoris TZI (monolayer, 11.5 x 3.6 x 32 mm) to act as a control. 10 specimens taken from each group were subjected to one, three, and five repeated firings. Specimens were obtained by sectioning the zirconia discs using IsoMet 5000 (Buehler), then sintered according to the manufacturer’s instructions. All large specimens were polished to a 0.5 μm level using EcoMet 250 (Buehler). All large multilayered specimens were annealed using Sirona inFire HTC at 1000 oC for 15 minutes before being subjected to repeated firing. None of the KT2 specimens were subjected to repeated firing. A three-point bend test was done using Instron Universal Testing Machine at a speed of 0.5 mm/min until specimen fracture. Specimens were placed with the enamel layer facing the compression side and dentin layer facing the tension side. One-way ANOVA and Least Squares Fit test was used to compare the difference at α = 0.05.
Results: Mean flexural strength values of multilayered zirconia materials and monolithic inCoris TZI are shown in Table 1. Table 2 shows flexural strength of Katana with varying dimensions and treatments.
Conclusions: Repeated firing showed no significant effects on the flexural strength of multilayered zirconia. However, specimen dimensions, annealing status, polishing status, and polishing methods had a significant effect on flexural strength.