Zirconia ceramics, widely used in engineering and biomedical fields, exhibit exceptional mechanical properties as compared to other oxide ceramics, thanks to a transformation toughening driven by a stress-induced tetragonal-to-monoclinic (t–m) phase transformation.
However, the strength and toughness of certain zirconia compositions may be compromised by low-temperature degradation (LTD), known to occur in the presence of moisture. LTD may occur spontaneously (i.e. without the need of applied stresses) in the presence of water species, typically between room temperature and 400°C. The challenge for zirconia ceramics is thus to achieve an optimal trade-off between toughness, strength and aging resistance.
The present work deals with the aging behavior of a 1.5Y-TZP zirconia developed by Tosoh (Japan) under the tradename ‘Zgaia 1.5Y-HT’, entirely retained at the tetragonal phase after sintering at low temperature (i.e. 1350°C), with a low concentration of stabilizer, i.e. 1.5 mol% Y2O3 and addition of 0.25 mass% Al2O3. It offers a very attractive combination of strength (1 GPa) and toughness (8.5 MPa ), near the optimum (transition) between a brittle and ductile behavior (which occurs by transformation-induced plasticity).
Aging tests were conducted at humid environments (water or water vapor) at temperatures between 100 to 140°C and the rate of t-m transformation was determined at different aging times by X-Ray Diffraction (XRD) analysis. Moreover, isothermal transformation was investigated at various temperatures between 120 and 400°C in a dry environment with in situ XRD analysis.
Interestingly, the aging kinetics of the present material at 134°C in humid environment is slower than for standard 3Y-TZP. For all the investigated conditions, the amount of t-m transformation increased with a sigmoidal type of curve and the results were well described by the Mehl-Avrami-Johnson (MAJ) law relative to nucleation and growth processes.
The results showed that the exponent n of the MAJ law varied with temperature, which can be attributed to the change in the ratio of nucleation and growth rates with temperature. Combining the data of isothermal transformations at dry environment allowed to determine Time-Temperature-Transformation (T-T-T) curves that showed a typical C-shape with a nose at a temperature of 275°C.  This also can be interpreted in terms of the competition between the driving forces for nucleation and the growth rates. Finally, The T-T-T diagram was used to discuss the impact of the cooling rate on the phase transformation in the studied material.
Overall this study shows that this 1.5Y-TZP material presents an appealing trade-off between strength, toughness and aging resistance.