Abstract
Computer calculations were used to simulate the effects of thermal shock on the stresses developed in porcelain-fused-to-metal (PFM) systems. A three-dimensional-finite-element model was employed to first calculate time-dependent temperature distribution in a beam shaped specimen 1.25-2.5×8×20mm. These were then used in a two-dimensional analysis of stresses in a beam consisting of 50 elements. Transient stresses in a beam consisting of 50 elements were calculated for porcelain, porcelain with an opaque layer, and porcelain-opaque-alloy system; the effects of porcelain thickness, thermal expansion coefficients, and quenching temperatures were evaluated. Experimental tests were also conducted for correlation with the theoretical calculations.
Results show that for αm(metal)<αp(porcelain), the highest stresses occur on the surface, and the interface stress is lower than αm=αp. When αm>αp, the surface stress is lower, and the interface stress is higher than αm=αp. When αp-αm>2×10-6°C-1, cracks are more easily initiated in the porcelain surface rather than the interface. Theory and experiment are in general agreement.
