PHASE-EQUILIBRIA OF ZIRCONIA-DISPERSED CERAMIC IN NIO-AL2O3-ZRO2 SYSTEM

Powders of NiO, Al2O3 and ZrO2 mixed in various ratios were sintered between 1200 and 1600-degrees-C and their phase equilibria studied by X-ray diffraction and electron microscopy. Partially stabilized ZrO2 (PSZ) consisting of tetragonal ZrO2 (t-ZrO2) precipitated in the cubic ZrO2 (c-ZrO2) matrix was not formed because the solubility of NiO and/or Al2O3 in ZrO2 was not enough to stabilize the c-ZrO2 phase. Instead, t-ZrO2 particles were constrained in the matrix phase of NiO and/or spinel which was formed by reacting NiO with Al2O3. At 1600-degrees-C, t-ZrO2 equilibrated with the stoichiometric NiAl2O4 spinel when free NiO was present, with spinel of varied nickel-to-aluminium stoichiometry when Al2O3 content exceeded NiO, and with A12O3-rich spinel and free Al2O3 (corundum) when Al2O3 was saturated in spinel. The implications of these results to TBCs are discussed.