Flexural strength of high yttrium oxide-doped monochrome and multilayered fully stabilized zirconia upon various sintered cooling rates.

Purpose: Firing protocols influence the mechanical properties of dental ceramics. This study examined the impact of altering the cooling rate of mono- and multilayered 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) on their strength.

Materials And Methods: Ninety specimens (width × length × thickness = 10 × 20 × 2 mm) were prepared using 5Y-PSZ monolayer (Mo: Cercon-xt) and 5Y-PSZ multilayered (Mu: Cercon-xt ML) blocks. Randomly distributed specimens were sintered at the recommended firing schedule for three different categories of cooling rates (n = 15/group): slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). A universal testing machine with four-point bending test was used to measure the flexural strength (σ). The microstructure, fracture characteristics, and chemical composition were evaluated by scanning electron microscope and energy-dispersive spectroscopy. The monoclinic, tetragonal, and cubic phases were investigated using X-ray diffraction. Two-way ANOVA and post hoc Bonferroni comparisons were applied to determine the σ, ( ), and Weibull analysis was performed to determine the Weibull modulus (m) and characteristic strength (σ ).

Results: The highest σ and σ (MPa) were seen for MuN (454.2 ± 62.0, 480.8 ± 62.9) followed by MuS (453.5 ± 52.6, 476.4 ± 54.3) and MoS (451.5 ± 44.5, 471.2 ± 46.6), whereas MuF had the lowest σ and σ (379.8 ± 50.2, 401.6 ± 51.3). The σ value of S-cooling (452.5 ± 47.9) was higher than those for N-cooling (443.4 ± 61.3) and F-cooling (382.3 ± 58.0). The m-value for MoS was the highest (11.4 ± 3.6), whereas that for MoF was the lowest (6.1 ± 1.6). Different cooling rates resulted in a significant difference in σ values (p < 0.05).

Conclusions: S- and N-cooling resulted in significantly higher flexural strength than that obtained by F-cooling. Increasing the cooling rate of 5Y-PSZ resulted in smaller grain size, less grain boundary integration, and higher t- to m-transformation, leading to lower strength. Therefore, a slow and normal cooling rate was recommended to achieve the optimum strength for 5Y-PSZ.