The morphological stability of polycrystalline fibers

Abstract

Zirconium acetate (yttrium nitrate) precursor fibers, produced by dry spinning, were placed upon a single crystal Al 2O 3 substrate. Upon heat treatment, the precursor fibers pryrolyzed and densified, producing polycrystalline ZrO 2 (8 mol% Y 2O 3) fibers which were sintered to the substrate. Further heat treatment to induce grain growth produced the following sequential microstructural changes: 1. (a) grain growth proceeded until single grains spanned the fiber to produce a “bamboo” configuration, 2. (b) grain boundary grooves deepened and the surfaces between the boundaries became curved, and 3. (c) if the distance between grain boundaries was greater than 1.4 times the fiber diameter, the fiber broke into a row of separate grains. Thermodynamic calculations show that when the grain size to fiber diameter ratio exceeds a critical value, the break up of the fiber into individual grains lowers the free energy. These observations and calculations can be used to predict the conditions required to retain fiber morphology within composites during high temperature processing and use.