Decomposition of 8.5 mol.% Y2O3-doped zirconia and its contribution to the degradation of ionic conductivity

Transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy was applied to analyze the microstructure and the local chemical composition of as-sintered and aged Y2O3-doped ZrO2 (YDZ) thick-film substrates with yttria contents of 8.5 mol.% (8YDZ) and 10 mol.% (10YDZ). Selected-area electron diffraction and dark-field TEM imaging show nanoscale precipitates of the tetragonal YDZ phase, which are coherently embedded in grains consisting of the cubic phase. A distinct coarsening of the tetragonal regions is observed after 2500 h annealing of 8YDZ at 950 °C in an oxidizing atmosphere. This coarsening is accompanied by decomposition on the cationic sublattice on the nanoscale, leading to the depletion of yttrium in the tetragonal regions. The decomposition is discussed in terms of spinodal decomposition which takes place even at a high doping concentration of 8.5 mol.% Y2O3.