Viscoelastic stress analysis of thermally compatible and incompatible metal-ceramic systems

The purpose of this study was to analyze transient and residual midpoint deflections and stresses in metal-opaque porcelain-body porcelain systems with matched and mismatched thermal contraction coefficients.

Calculations and measurements were made for seven trimaterial strips that covered a wide range of thermal contraction mismatches among constituent materials. Midpoint deflections were measured in a beam-bending viscometer during slow cooling from an initial temperature of 700 degrees C. Linear regression analysis with a correlation coefficient of 0.950 was used to compare measured and calculated residual midpoint deflections. Stress relaxation data were fit to a three-term exponential series by nonlinear regression analyses with correlation ratios ranging from 0.9972 to 0.9999.

While finite element analyses correctly predicted the general shape of the deflection behavior as a function of temperature for all combinations, the best agreement between measured mean residual midpoint deflections and calculated values (+250 microns vs. +268 microns) was obtained for strips composed of a Au-Pd alloy (alpha m = 13.5 ppm/ degree C) with a medium expansion opaque porcelain (alpha o = 13.3 ppm/degree C) and a high expansion body porcelain (alpha B = 14.4 ppm/degree C). The highest calculated residual tensile stress of +26 MPa at the surface of body porcelain was associated with the 0.5-mm-thick Ni-Cr-Be alloy strip (alpha m = 15.1 ppm/degree C) with medium expansion porcelains (alpha o = 13.5 ppm/degree C and alpha B = 13.9 ppm/degree C). The smallest measured residual deflection (+10 microns) was also associated with this combination. The results of this study indicated that metal-ceramic strips are sensitive indicators of stress development caused by a thermal contraction mismatch; however, the magnitudes of the residual deflections do not necessarily correlate with the stress magnitudes in the ceramic.

Currently there are no U.S. or international standards that define the maximum difference in thermal contraction coefficients that can exist between a metal and its ceramic veneer without causing transient failures of ceramic during cooling or delayed failures in ceramic because of high residual tensile stresses. The present research represents a major step in understanding the various factors that influence the development of transient and residual stresses. A knowledge of the effects of process variables on stress development is necessary for selection of potentially successful metal-ceramic systems and for optimizing the design of dental prostheses.