Thermal cycling distortion of metal ceramics: Part II--Etiology

The three-dimensional geometry of conventional fixed prostheses complicates the study of the thermal cycling distortion in metal ceramic alloys. Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. The simplified experimental geometry developed in Part I was applied to elucidate the etiologic factors involved in metal ceramic deformation. Techniques to minimize the thermal cycling distortion were also studied. It was found that all of the significant distortion occurred during the first thermal cycling of the alloy (oxidation) and that no distortion resulted from the application of body porcelain. The specimens that were cold worked and then oxidized had significantly more distortion than any other group. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold worked. It was determined that the release of casting- and cold working-induced stresses had a synergistic effect.