Abstract:
Porcelain-fused-to-metal (PFM) is a kind of restoration process used in pros- thetic dentistry, where a certain metal is cast as basement on which the dental ceramic is fused by firing. Due to the difference in the coefficients of thermal expansion of the two materials, all PFM restorations contain thermal stresses, which develop during the cooling phase after firing. These thermal stresses coupled with the stresses produced by mechanical loads may be the dominant reasons for failures in clinical situations. For an accurate calculation of these stresses, viscoelastic behavior of ceramic at high temperatures should not be ignored. In this study, the finite element technique is used to evaluate the effect of viscoelasticity on the bond strength of a three-point flexure test specimen, which is the current international standard, ISO 9693, to characterize the interfacial bond strength of metal-ceramic restorative systems. Although there are some studies which take into account the viscoelastic behavior of the ceramic in the determination of thermal stresses, none of them evaluates the effect of viscoelasticity on the bond strength, which is of great importance in assessing failure of PFM restorations. Evaluation of the results in the vicinity of the interface end, critical point for initial debonding, show that the load shear stress is offset by thermal residual stress, the normal stress is compressive for thermal load and tensile for mechanical load. This indicates that the probability of interfacial debonding due to normal tensile stress is higher than that due to shear stress.
