Degradation During Aging of Transformation‐Toughened ZrO ₂ ‐Y ₂ O ₃ Materials at 250°C

Abstract

The detrimental aging phenomenon observed in ZrO 2 ‐Y 2 O 3 materials, which causes tetragonal ZrO 2 to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y 2 O 3 and SiO 2 content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter‐granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO 2 to produce clusters of small (20 to 50 nm) crystallites of α‐Y(OH) 3 . It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y 2 O 3 ) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.