Abstract
Temperature dependence of thermal coefficient of expansion was measured for six commercial body and opaque dental porcelains. The dependence of coefficient of thermal expansion on temperature in the glass transition range was over ten times higher than that of the glassy solid state of the porcelain during the heating process. The onset of the glass transition range and the temperature where the apparent coefficient of expansion reached the maximum value(determined from thermal deformation)were almost equal to the strain and the annealing points (defined by viscosity) . The surface compressive residual stress for the 2mm thick porcelain slabs was simulated to be 16〜25 and 63〜92MPa, respectively, for the porcelain-cooled in air at ambient temperature and for that cooled with forced convection of compressed air from a temperature higher than the porcelain sag point. These findings suggested that the coefficient of thermal expansion in the glass transition range and viscoelastic behaviour of the porcelain are the major factors determining the residual stress in the porcelain-fused-to-metal (PEM) restorations, because the difference of the coefficient of thermal expansion between porcelain and alloy at the temperature where incompatible stress builds up was calculated to be much higher than 10×10-6/℃.
