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.