Moisture-Ph Impact On Zirconia Bilayered Ceramic Flexural Strength

To evaluate the combined effects of moisture and pH variations on the biaxial flexural strength of zirconia/veneering porcelain bilayered ceramics under cyclic loading. Two protocols were implemented: (1) cyclic loading (10,000 cycles, 600 N) under dry and saliva-simulated wet conditions (pH 7); (2) cyclic loading followed by immersion in acidic (pH 4), neutral (pH 7), and alkaline (pH 8) environments. Post-loading assessments included biaxial flexural strength testing, fracture mode analysis via optical microscopy, and reliability evaluation using Weibull statistics. Moisture reduced flexural strength by 70% (dry: 942.8 MPa vs. wet: 551 MPa), with wet conditions inducing Hertzian cone cracks and circumferential cracks, leading to veneer delamination. Neutral pH (pH 7) exhibited the highest strength, surpassing acidic (pH 4) and alkaline (pH 8) groups by 14% and 45%, respectively. Alkaline environments showed reduced structural reliability (Weibull modulus: 8.9; R²: 0.94) due to heterogeneous crack propagation, while acidic conditions slowed crack growth (attributed to SiO₂/Al₂O₃ ≈4). Oral humidity and pH fluctuations synergistically degrade zirconia/porcelain bilayers via mechanical fatigue and chemical corrosion. Saliva amplifies stress and corrosion, necessitating dual-mechanism mitigation for durable all-ceramic restorations.