Microstructural Degradation of Zirconia-Lithium Disilicate Bilayer Restorations Under Thermocyclic Fatigue: A Quantitative Fractographic Analysis in the Posterior Dentition

Abstract

Bilayer all-ceramic restorations combining zirconia frameworks with lithium disilicate veneering ceramics are increasingly prescribed for posterior load-bearing zones; however, their long-term cohesive fracture behavior under thermocyclic conditions remains insufficiently characterized. This study evaluated the fractographic profiles and crack propagation patterns in forty-eight bilayer specimens subjected to 10,000 thermocycles (5°C–55°C) combined with axial occlusal loading (200 N). Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and three-dimensional profilometry were employed to quantify delamination depth, radial crack density, and residual stress distribution. Results demonstrated statistically significant correlation between veneer thickness exceeding 1.8 mm and subsurface lateral crack initiation (p < 0.001). Phase transformation from tetragonal to monoclinic zirconia was confirmed at interfacial zones. These findings provide clinically actionable thresholds for veneer dimensioning protocols aimed at reducing catastrophic cohesive failure rates in molar-region fixed dental prostheses.