Journal of Dental Research, Vol 71, 1623-1627, Copyright © 1992 by International & American Associations for Dental Research Online Journals
Influence of tempering method on residual stress in dental porcelain
K. Asaoka, N. Kuwayama and J. A. Tesk
Department of Dental Engineering, School of Dentistry, Tokushima University, Japan.
The porcelain component of a porcelain-fused-to-metal restoration is
strengthened by residual (tempering) stresses which are induced by cooling
procedures followed in dental laboratories. The thermophysical properties
of materials and cooling rate are the main factors which determine the
residual stress. In this paper, the temperatures in the midplane of
body-porcelain disks were measured from a heat-soak temperature (1000
degrees C) to room temperature during two different cooling procedures:
slow cooling in air and forced-air cooling. Experimental results
approximated exponential cooling wherein the cooling rates could be
represented by a linear equation of temperature. Residual stresses, as
affected by the tempering method and thickness of a porcelain disk, were
calculated by computer simulation for regions away from the edges. The
following temperature-dependent factors were incorporated into the
simulation: elastic modulus, viscosity, and coefficient of thermal
expansion. The cooling rate dependencies of the glass transition
temperature and the temperature distribution during cooling were also
included. The cooling rates used in this simulation were derived from the
tempering data. The agreement between development of transient and residual
stresses--calculated by computer simulation for various cooling methods,
and the tendency toward failures of porcelain disks subjected to the
tempering processes--was examined. Simulated residual stresses were also in
good agreement with those measured by the indentation fracture method of
Marshall and Lawn (1977) and Anusavice et al. (1989).
