Effects of Viscoelastic Parameters on Residual Stresses in Zirconia/Glass Ceramics

Objectives: The aim of this study was to test the hypothesis that the residual stresses in a zirconia based bilayer dental composite system can be tailored through heat treatment above and below the glass transition temperature of glass veneers. Methods: Ceramic bilayer disc specimens were prepared from a zirconia core (Lava, 3M ESPE, Seefeld, Germany) and a glass veneer(Lava Ceram veneer, Lava, 3M ESPE, Seefeld, Germany). Each bilayer ceramic group was heat treated 40°C below, 20°C and 40°C above and at the glass transition temperature of the glass veneer, and cooled using a fast or a slow cooling rate. Specimens were tested for flexure strength using a biaxial bending fixture. Monolithic annealed glass veneer disc specimens served as the control group. Residual stresses were calculated using a fracture mechanics approach. One-way ANOVA was performed to compare each of the eight bilayer heat treated groups against the annealed monolithic glass veneer control. In addition, two-way ANOVA, with factors for heat level and cooling rate, was used to compare the eight bilayer heat treated groups against each other. Results: Heat treatments produced significant differences (p £ 0.05) between the mean flexural strengths of the heat treatment groups when the specimens were cooled using a fast cooling rate. However, there was not a significant difference (p > 0.05) between the mean flexural strengths of the heat treatment groups when a slow cooling rate was used. Conclusions: Heat treatment above and below the glass transition temperature of the veneer layer, and the cooling rate have a significant effect on the flexural strength and the residual stresses of the bilayer ceramic laminates. Residual stresses can be modified using the elastic-viscoelastic relaxation behavior of a glass veneer. This study was supported by NIH/NIDCR grant DE06672.