Property degradation of tetragonal zirconia induced by low-temperature defect reaction with water molecules

Tetragonal ZrO2 exhibits good ionic conductivity, high strength, and fracture toughness. But while annealing at relatively low temperatures (63-400degreesC), tetragonal ZrO2 spontaneously transforms to a monoclinic one, and its electrical and mechanical properties degrade severely. The phenomenological observations of the low-temperature degradation of tetragonal ZrO2 are summarized, and major degradation mechanisms are critically reviewed. It is crucial to maintain sufficient oxygen vacancy concentration to stabilize the tetragonal structure; excess reduction of the oxygen vacancy concentration causes the tetragonal to monoclinic transformation. Water molecules can be incorporated into the ZrO2 lattice by filling oxygen vacancies, which leads to the formation of proton defects. Experimental and theoretical evidence support such a defect reaction between oxygen vacancies and water molecules. And a degradation mechanism based on this defect reaction satisfactorily explains all the phenomenological observations. The diffusion rate of oxygen vacancies at low temperatures is not high enough to cause the observed degradation depth; therefore, the relatively fast diffusion of proton defects most probably controls the degradation process.