The effect of the leucite transformation on dental porcelain expansion

Nearly all commercial dental porcelains designed for fusing to metal rely upon the crystalline phase leucite to supply the needed high thermal expansion. Repeated firings and certain heat treatments are known to alter dental porcelain thermal expansion behavior, and such changes in expansion could induce cracking, checking, or spalling of the porcelain. Because of the likelihood of involvement of leucite in these expansion changes, this study sought to determine the role of leucite thermal expansion, and particularly the displacive (martensitic) transformation of leucite, in the expansion behavior of the leucite-containing Component No. 1 of the Weinstein patent. The thermal expansion of this frit was modelled as a weighted average of the leucite expansion (including the discontinuous volume change that accompanies the transformation) as measured by hot-state x-ray diffraction and the expansion of the glass matrix calculated from its composition. Close agreement was found between the predicted and measured expansion for Component No. 1. The non-linearity of the α curve for Component No. 1 is explained by the discontinuous volume change during the leucite transformation.