Biaxial Flexural Strength and Fatigue Life of Two Y-TZP Ceramics

Objectives: The development of yttria partially-stabilized tetragonal zirconia polycrystalline (Y-TZP) has allowed the use of ceramics in load-bearing sites. The aim of this work was to examine the biaxial flexural strength, hardness and fatigue limit of coloured and uncoloured zirconia in the Lava™ system compared with uncoloured Cercon® system. Methods: Thirty standardised disc specimens of Lava™: uncoloured, FS1-7 (15x1.3 mm) and Cercon®: uncoloured (15 x 1.2 mm) were used to measure the biaxial flexural strength (ISO 6872 standard) using a Dartec HC10 Fatigue Tester (Zwick Ltd., UK). The uncoloured, FS4, FS7 of Lava™ and Cercon® group were also submitted to Vickers hardness and dynamic fatigue test. The reliability of strength was analysed with the Weibull distribution. Results: One-way analysis of variance (ANOVA) showed no significant difference in biaxial flexural strength among uncoloured and coloured Lava™ groups (p>0.05) whereas Cercon® group (820 MPa) had flexural strength significantly lower than Lava™ group (1100 MPa) (P<0.05). Two-way ANOVA analysis showed no significant differences in hardness value among four groups (uncoloured, FS4, and FS7 of Lava™ and Cercon®) (P>0.05). Furthermore, when comparing within the same specimen, hardness values for the polished side was not significantly different from the unpolished side (P>0.05) except FS7 (P<0.05). In addition, the Weibull modulus of Lava™ specimens were in the range of 8-15, the Cercon® specimens 7-11. Additionally, uncoloured, FS4, FS7, and Cercon® specimens survived 5x105 cycles when using a load 60-70% of mean biaxial flexural strength or lower . Conclusion: Y-TZP ceramic has high flexural strength, hardness and good strength reliability. In addition, specimen colouring did not have an effect on the flexural strength. The polishing process also did not affect the hardness of zirconia. The fatigue limit of Y-TZP ceramic may be defined as lying between 60-70% of the stress to failure.