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

Abstract

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, ((Figure presented.)), and Weibull analysis was performed to determine the Weibull modulus (m) and characteristic strength (σ0). Results: The highest σ and σ0 (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 σ0 (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.