The mechanical strength of a brittle material is controlled by three principal variables: the severity of preexisting flaws, the crack propagation resistance of the material, and the magnitude of residual and functional stresses. Thermal tempering is known to strengthen commercial glasses by inducing compressive stress in surface regions. However, the combined effect of tempering and incompatibility stress on the strength of dental ceramics as a function of design and flaw characteristics has not been analyzed previously. The long-range objective of this research program is to optimize the """"""""margin of safety"""""""" of metal-ceramic and all- ceramic structures by controlling the distribution of stresses which develop because of tempering, specimen design, contraction coefficient differentials, structural flaws, applied external forces, and thermal history.
The specific aims of this study are: 1) to test the hypothesis that thermal tempering by forced convective cooling in air progressively enhances the strength of castable ceramic, aluminous porcelain, conventional feldspathic porcelain, and highly-crystalline feldspathic porcelain as the thickness of the ceramic is increased because of larger temperature gradients which are produced during cooling, 20 to optimize stress-distribution profiles for layered ceramic structures as a function of tempering medium (air, silicone oil, and oil-water emulsions), contraction coefficient differentials, and externally applied forces based on the superposition principle for stresses, 3) to test the hypothesis that neither a positive contraction mismatch (alphaM>alphaC) nor a negative contraction coefficient (alphaM
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