Microstructure Evolution of ZrO2-(Fe2O3, Al2O3) Materials Synthesized with Solution Precursors

Aqueous Zr‐nitrate solutions containing appropriate amounts of Fe(NO3)3·6H2O and Al(NO3)3·6H2O were used to synthesize ZrO2‐Fe2O3 compositions (up to 40 mol% Fe2O3) and one ZrO2‐Al2O3‐Fe2O3 composition. An amorphous phase was produced after pyrolysis, which subsequently crystallized to a single‐phase Zr(Fe)O2 solid solution (or Zr(Fe,Al)O2 ss) that appeared cubic by X‐ray diffraction, but tetragonal (c/a→ 1) by electron diffraction. The crystallization temperature increased with Fe2O3 content. At higher temperatures, the single phase partitioned to two phases, tetragonal‐ZrO2+γ‐Fe2O3. The γ‐ to α‐Fe2O3 transformation occurred at still higher temperatures to produce a two‐phase microstructure composed of two interpenetrating phases (ZrO2+α‐Fe2O3), each with equiaxed grains. This microstructure was relatively stable to grain coarsening, even to temperatures just below the apparent eutectic temperature. With the exception of the lower transition temperatures and the equiaxed grains observed for both phases, these observations are similar to that previously reported for the ZrO2‐Al2O3 system. The microstructure development reported here is another example for the synthesis via solution processing that involves diffusion‐limited crystallization. Namely, a two‐phase material can be produced where the grain size of each can be very small, and can be prevented from growing rapidly at high temperatures due to the constraint of each phase on one another.