The effect of hydrothermal aging on ultra-translucent 4Y-PSZ Zirconia

Resumo

© 2025 Elsevier Ltd and Techna Group S.r.l.This study investigated the effects of hydrothermal aging on the microstructural, optical, and mechanical properties of commercial and experimental 4Y-PSZ zirconia (4 mol% yttria-stabilized tetragonal zirconia polycrystal). Samples of commercial and experimental 4Y-PSZ were produced by milling CAD/CAM blocks and uniaxial pressing, respectively. Each group was characterized in three conditions: control, aged 20-h, and aged 50-h in a hydrothermal reactor. Density, microstructure, crystalline phase composition, optical, and mechanical properties were evaluated using Archimedes' principle, scanning-electron-microscopy (SEM), X-ray diffraction (XRD) with Rietveld refinement, reflectance tests, and biaxial-flexural-strength (BFS) tests, respectively. Data were analyzed using Weibull statistics and two-way ANOVA with Tukey's tests. Both groups achieved densification above 99%. Commercial zirconia displayed a homogeneous microstructure with smaller grains compared to experimental 4Y-PSZ. XRD patterns and Rietveld refinement revealed significant differences in phase composition between the groups in function of aging times. Initially, both groups presented high fractions of tetragonal and cubic phase, with no detectable monoclinic content. After 20 h of aging, a substantial increase in the monoclinic phase was detected in the experimental (39%) and commercial groups (29%). After 50 h, both groups reached approximately 40 % of monoclinic content, indicating transformation saturation. Aging affected optical properties in the experimental group but not in the commercial one. While the commercial group presented higher strength compared to the experimental group, a high survival probability was observed in both groups for missions up to 800 MPa. Extended hydrothermal aging led to strength reduction in the commercial group, while the experimental group remained stable. Significant differences in Weibull modulus were observed only in the commercial group between immediate and 50-h aged conditions. The study confirms the susceptibility of 4Y-PSZ zirconias to phase transformation but supports their suitability for long-span fixed dental prostheses, given their strength and reliability.

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