The investigation of residual stress effect on metal-ceramic bond

Porcelain-fused-to-metal (PFM) is playing a very important role in prosthetics dentistry. The bond strengths in metal-ceramic system have been focused on, since the method of PFM was used to prosthetics. In this paper, the thermal residual stress effects on metal-ceramic bond were considered during cooling of porcelain-fused-to-metal restoration to analysis the metal-ceramic bond stresses. The ISO crack initiation test specimen (three-point flexure bond test) was simulated by finite element method. The analysis was implemented in two steps. In the first step, the porcelain was assumed as viscoelastic material (720°C-550°C), while in the second step the porcelain was as elastic body (550°C-25°C). The results show that the compressive stress caused by difference of thermal expansion coefficients of two materials during cooling occurs in the ceramic. The shear stress induced by mechanical load is offset by thermal shear stress. The mechanical tensile stress and the thermal compressive stress normal to interface are concentrated at the end of the bond interface, but the tensile stress is much higher. It is clear that the thermal residual stresses are very important to metal-ceramic restorations, and it is greatly affected by the viscoelastic behavior of porcelain. This also indicates a higher probability of failure produced by the tensile stress rather than by shear stress.