3Y-TZP hydrothermal stability under steam (ISO 6872:2024) vs water immersion.

OBJECTIVE

This study investigated the effect of the two hydrothermal aging methods on the microstructure, optical and mechanical properties of 3Y-TZP zirconia.

METHODS

150 disc-shaped specimens of 3Y-TZP were fabricated by uniaxial pressing and allocated into three groups: immediate (Zpex-I), aged at 134 °C and 2.2 bar for 10 h in a hydrothermal reactor either under steam (Zpex-St, as per updated ISO 6872:2024) or immersed in water (Zpex-Iw). Density, crystalline content, microstructure and grain size were assessed. Translucency parameter (TP) and contrast ratio (CR) were also evaluated. Mechanical performance was assessed by biaxial flexural strength testing. Weibull statistics was applied to determine characteristic strength, Weibull modulus. TP, CR, and grain size data were analyzed by one-way ANOVA and Tukey's tests.

RESULTS

The synthesized 3Y-TZP exhibited density above 99%. XRD revealed predominantly tetragonal phase in Zpex-I, whereas monoclinic phase was observed in both aged groups. The Zpex-Iw group exhibited higher monoclinic content (37%) compared with Zpex-St (24%). SEM demonstrated significantly larger grain size in Zpex-St relative to Zpex-I, while Zpex-Iw showed no statistical difference from the other groups. Cross-sectional micrographs identified a deeper degraded layer in Zpex-Iw than in Zpex-St. No significant differences were observed in CR; however, TP was significantly reduced in Zpex-Iw. Zpex-Iw presented significantly increased characteristic strength compared to the other groups.

SIGNIFICANCE: When immersed in water, hydrothermal aging produced greater t→m transformation than steam, leading to a deeper degraded layer, reduced translucency, and higher characteristic strength.