Extended hydrothermal aging and glass infiltration on minimally processed recycled 3Y-TZP: microstructural and optical properties

This study evaluated the effects of glass infiltration and hydrothermal aging on the microstructural and optical properties of minimally processed recycled 3Y-TZP. Unprocessed 3Y-TZP remnants were milled into powder, compacted into discs, and sintered. Specimens were divided into six groups according to glass infiltration and hydrothermal aging (immediate, 50 h, and 100 h at 134 °C/2.2 bar). Particle size was measured by laser diffraction. Density and optical properties (contrast ratio and translucency) were longitudinally evaluated on the same specimens using Archimedes' principle and reflectance measurements (n = 10), whereas crystalline phases (Raman spectroscopy, n = 3), surface and cross-sectional morphology (n = 3), and elemental composition (n = 3) were analyzed on independent specimens allocated to each experimental condition. Color stability and translucency variation were compared with perceptibility (PT = 0.81; TPT = 0.62) and acceptability (AT = 1.77; TAT = 2.62) thresholds (n = 10). Density data were analyzed using one-way repeated-measures ANOVA, while optical properties were analyzed using two-way repeated-measures ANOVA, followed by Tukey's post hoc test. Mean particle size of recycled powder was 1.61 μm. Relative densities for immediate, 50 h, and 100 h groups were 99.68%, 99.24%, and 99.04%. Raman spectra revealed predominance of tetragonal-cubic phases in immediate non-infiltrated group, while monoclinic phase prevailed in others. SEM showed homogeneous surfaces. EDS confirmed zirconium, yttrium, and hafnium in non-infiltrated groups, and silicon, aluminum, sodium, potassium, and calcium in infiltrated. Non-infiltrated groups showed lower contrast ratio and greater translucency than infiltrated, independent of aging. Aging minimally affected optical properties, except infiltrated specimens, where translucency was higher immediately than after 50-100 h. All groups exhibited color and translucency variation below perceptibility and acceptability thresholds. Prolonged hydrothermal degradation influenced the microstructural and optical features of recycled 3Y-TZP, while glass infiltration further modified these characteristics, reducing translucency while increasing opacity and color stability.