Resistance to Low-Temperature Degradation of Equimolar YO1.5-TaO2.5 Stabilized Tetragonal ZrO2 Ceramics in Air

A narrow range of composition exists along the ZrO2-YTaO4 quasi-binary in which the tetragonal phase can be retained on cooling. Ceramics within this region, corresponding to equimolar YO1.5 and TaO2.5 stabilizer concentrations, have been subject to accelerated testing for their susceptibility to moisture-induced low-temperature degradation (LTD) by annealing in air at 200 degrees C. No low-temperature transformation from tetragonal to monoclinic phase was evident even after 400 h in the tetragonal YO1.5-TaO2.5-ZrO2 ceramics, while 50% of tetragonal phase in a sintered 3 mol% Y2O3-doped ZrO2 transformed into monoclinic after the same long-term annealing. The results not only demonstrate the superiority in LTD of equimolar yttria-tantala-doped zirconia (ZrO2) but have important implications for the proposed mechanisms of LTD in polycrystalline ZrO2.