Yttria-doped tetragonal zirconia containing 2 and 3 mol% Y20, (Y-TZP), and CeOz-doped Y-TZP containing 0 to 12 mol% CeOz were sintered at 1350°C in a tetragonal singlephase field for 2 h in air, and the degradation behavior at low temperature in air and in hot water was observed. X-ray photoelectron spectroscopy studies on the surface of hydrothermally treated samples show evidence for the formation of a YO(0H) species, along with the simultaneous formation of purely tetragonal zirconia nuclei that retain their coherence in the Y-TZP matrix. Above a critical size, the tetragonal nuclei spontaneously transform to a monoclinic structure, giving rise to macro-and microcracking. The strong tetragonal grains degrade to produce a spalling phenomenon that facilitates further degradation. Y-TZP ceramics alloyed with adequate amounts of CeOz show no tetragonal-to-monoclinic transformation after hydrothermal treatment. [

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