Thermal tempering of layered ceramic structures

This study was designed to test the hypothesis that thermal tempering by compressed air is the most effective strengthening method for layered feldspathic porcelain structures and to determine the influence of cooling method, initial tempering temperature, thermal contraction mismatch, and specimen thickness on transient and residual stresses produced within dental ceramic structures.; Bilayered porcelain disks with a thermal contraction mismatch between {dollar}-{dollar}1.5 and +3.2 ppm/{dollar}spcirc{dollar}C and a thickness of 2 or 6 mm were prepared and thermally tempered in air, helium, and silicone oil. The transient temperature distributions were calculated by analytical and finite element methods and were incorporated in finite element and viscoelastic thermal stress models to determine the transient and residual stresses produced in each disk. The finite element method was also employed to determine the influence of ceramic thickness, loading orientation, thermal contraction mismatch, initial tempering temperature and cooling rate on the stress distribution within a dental ceramic molar crown. Statistical analysis revealed that forced convective cooling of disk specimens in air resulted in a significantly smaller induced crack size and increased the mean flexural strength on specimens by a factor of two compared with the specimens which were conventionally-cooled by free convection. However, the thermal stress models predicted transient tensile stresses far above the failure level of porcelain for the 6-mm thick bilayered disks subjected to free convective cooling and for the 2-mm thick bilayered disks subjected to quenching at high initial temperatures in silicone oil. These predictions were supported by experimental findings. The results of this study support the hypothesis that thermal tempering by compressed air is the most effective strengthening method for layered feldspathic porcelain structures. However, variables such as specimen dimensions, initial tempering temperature, heat transfer coefficient, and thermal contraction mismatch have a significant influence on the effectiveness of tempering. The most effective tempering treatment, which results in maximum flexure strength and minimum structural distortion by viscous flow and air blasting, was achieved by forced convective cooling in air using initial tempering temperatures between 750{dollar}spcirc{dollar}C and 850{dollar}spcirc{dollar}C.