EFFECT OF CUBIC LEUCITE STABILIZATION ON THE FLEXURAL STRENGTH OF FELDSPATHIC DENTAL PORCELAIN

Previous studies (Mackert and Evans, 1993) have shown that, when felds pathic dental porcelain is cooled, leucite undergoes a transformation from cubic to tetragonal, associated with a 1.2% volume contraction. T his contraction leads to the formation of microcracks in and around th e crystals and the development of tangential compressive stresses arou nd the crystals. Our aim was to stabilize increasing amounts of the cu bic form of leucite in a leucite-reinforced dental porcelain, evaluate its effect on the flexural strength, and characterize its microstruct ure. The hypothesis was that in the absence of crystallographic transf ormation, the contraction of the leucite crystals would be lower, ther eby limiting the formation of microcracks and minimizing the developme nt of tangential compressive stresses around the leucite particles. We prepared 8 porcelain compositions by mixing increasing amounts of eit her leucite (KAlSi2O6) or pollucite (CsAlSi2O6) with Optec HSP porcela in (Jeneric/Pentron Inc., Wallingford, CT). Porcelain disks were made from each composition (n = 10 per group). X-ray diffraction analyses s howed that the amount of stabilized leucite increased with the amount of pollucite added. The microstructure of the specimens containing tet ragonal leucite was characterized by twinned leucite crystals, whereas no twinning was observed in the specimens containing cubic leucite. T he evaluation of crack deflection showed that significantly less defle ction occurred in the specimens containing cubic leucite. The mean bia xial flexural strength for the group corresponding to 22.2 wt% added p ollucite, fired at 1038 degrees C, was significantly lower than that f or the control group. The group corresponding to 22.2 wt% added leucit e fired at 1150 degrees C exhibited a mean biaxial flexural strength s ignificantly higher than that of all other groups that were not signif icantly different from the control group. Overall, the stabilization o f cubic leucite reduced the flexural strength and the number of crack deflections in leucite-reinforced porcelain. Apparently, the developme nt of tangential compressive stresses around the leucite crystals when cooled is responsible for a significant amount of strengthening of fe ldspathic dental porcelain.