Various aspects of the tetragonal (t) to monoclinic (m) transformation during degradation have been studied experimentally and theoretically in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), i.e., polycrystalline t-ZrOz containing Y203 in solution. Transmission electron microscopy (TEM) revealed that protruding grains at the surface of Y-TZP specimens do not transform under corrosive conditions (250°C, humid atmosphere) even after an annealing time of 168 h. Eigenstresses due to anistropic thermal expansion in and around protruding and bulk grains have been calculated for Y-TZP containing 2 and 3 mol% YZO3. The prominent role of these stresses on subsequent transformation nucleation during degradation is shown to agree qualitatively with an established free energy concept. The lack of complete transformation of rn-ZrOz is attributed to characteristics of the nucleation-and growthcontrolled transformation process.
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