Transient and residual stresses in dental porcelains as affected by cooling rates

The development of either transient or residual stress in a slab of dental porcelain during cooling was simulated by use of a super-computer. The temperature dependences of the elastic modulus, the thermal expansion coefficient, and the shear viscosity, and the cooling rate dependence of the glass transition temperature, Tg, were considered in this calculation. Internal stress and viscoelastic creep were computed for several cooling rates. Calculated results display stress profiles which agree reasonably well with reported measured profiles in quenched, tempered glasses. The calculated residual surface stress, sigma, could be fit by the following empirical formula, sigma = kl2(q/q0)n, q is the cooling rate, q0 is a reference cooling rate and l is the half-thickness of the porcelain. The method by which residual stress develops is also discussed. This discussion suggests a method for strengthening of the porcelain by the development of high-compressive residual stress on the surface.