Phase Transformation of Yttria-Stabilized Tetragonal Zirconia Particles in Mullite/Zirconia Composites.

Abstract

The tetragonal to monoclinic (t-to-m) phase transformation of 2 mol% yttria-stabilized zirconia appeared during cooling when it was incorporated in mullite matrix phase. This transformation was significantly enhanced with increase in zirconia volume fraction and length of exposure at the high temperatures (>1600°C). High internal stresses of a tensile nature which develop in the zirconia grain owing to the large difference in thermal expansion coefficients of mullite and zirconia, the lack of yttria in solid solution in zirconia due possibly to some yttria that reacted with alumina and/or silica from mullite during sintering, and the zirconia grain coarsening were believed to be responsible for the enhanced phase transformation. An analytical model showing thermal stress configurations and empirical correlations pertaining to the existence of grain boundary glass phase were presented. The t-to-m phase transformation of zirconia in composites occurred at temperatures between 610°and 700°C during cooling which could be identified in the dilatation curves. The temperature for start of the martensitic phase transformation (M5) gradually shifted to higher temperatures with increase in ZrO2 volume fraction and, for a given composition, increase in number of cycles.