Resistance to Low‐Temperature Degradation of Equimolar YO 1.5 –TaO 2.5 Stabilized Tetragonal ZrO 2 Ceramics in Air

A narrow range of composition exists along the ZrO 2 –YTaO 4 quasi‐binary in which the tetragonal phase can be retained on cooling. Ceramics within this region, corresponding to equimolar YO 1.5 and TaO 2.5 stabilizer concentrations, have been subject to accelerated testing for their susceptibility to moisture‐induced low‐temperature degradation (LTD) by annealing in air at 200°C. No low‐temperature transformation from tetragonal to monoclinic phase was evident even after 400 h in the tetragonal YO 1.5 –TaO 2.5 –ZrO 2 ceramics, while 50% of tetragonal phase in a sintered 3 mol% Y 2 O 3 ‐doped ZrO 2 transformed into monoclinic after the same long‐term annealing. The results not only demonstrate the superiority in LTD of equimolar yttria–tantala‐doped zirconia (ZrO 2 ) but have important implications for the proposed mechanisms of LTD in polycrystalline ZrO 2 .