Effect of thermal expansion mismatch on the Y-TZP/veneer interfacial adhesion determined by strain energy release rate

Abstract

Purposes: The aim of this study was to assess the effect of differences in the thermal expansion behaviour of veneering ceramics on the adhesion to Y-TZP, using a fracture mechanics approach.
Methods: Seven veneering ceramics (VM7, VM9, VM13, Lava Ceram, Zirox, Triceram, Allux) and one Y-TZP ceramic were investigated. Thermal expansion coefficients and glass transition temperatures were determined to calculate residual stresses (σ_R, MPa) between core and veneer. Subsequently, the veneering ceramics were fired onto rectangular shaped zirconia specimens, ground flat and notched on the veneering porcelain side. Then specimens were loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the adhesion interface was evaluated to calculate the strain energy release rate (G, J/m²) for each system.
Results: Residual stresses ranged from −48.3 ± 1.5 MPa (VM7) to 36.1 ± 4.8 MPa (VM13) with significant differences between all groups (p < 0.05). The strain energy release rate of the Y-TZP/veneer specimens ranged from 8.2 ± 1.7 J/m² (Lava Ceram) to 17.1 ± 2.8 J/m² (VM9). Values for G could not be obtained with the VM7, Allux and VM13 specimens, due to spontaneous debonding or unstable crack growth. Except for Triceram and Zirox specimens, strain energy release rate was significantly different between all groups (p < 0.05).
Conclusion: Thermal residual stresses and strain energy release rates were correlated. Slight compressive stresses in the region of −20 MPa were beneficial for the Y-TZP/veneer interfacial adhesion. Stresses higher or lower than this value exhibited decreased adhesion.