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Low temperature degradation (LTD) occurs by tetragonal-to-monoclinic phase transformation of yttria-stabilized tetragonal zirconia polycrystal dental ceramics (Y-TZP). In this study, the influence of the hydrothermal aging on phase transformation and flexural strength of Y-TZP ceramics was investigated. Bar-shaped specimens (22.0mmx1.5mmx4.0 mm) of two Y-TZP (Ceramill ZI and Ceramill zolid FX) were subjected to hydrothermal aging at 134˚C under 0.2 MPa for 0, 1 and 2 hours (n=6). The phase transformation (tetragonal-to-monoclinic) was evaluated by x-ray diffraction (XRD). The flexural strength was determined using four-point bending test. The amount of monoclinic phase conversion and flexural strength data were statistically analysed by two-way ANOVA at α=.05. The results from the XRD analysis showed that the monoclinic phase increased from 0.1 to 4.8 % for Ceramill ZI with an increase in the autoclaving time from 0 to 2 hrs, respectively. For Ceramill zolid FX, an increase in monoclinic phase fraction was minor, ranging between 0.8 (control) to 1.7 % after autoclaving for 1 and 2 hrs. The flexural strengths of these materials were not significant different after aging for 2 hours. In conclusion, the hydrothermal aging induced monoclinic-phase transformation in Y-TZP ceramics. However, an increase in monoclinic phase caused from hydrothermal aging in this study did not significantly affect the flexural strength of Ceramill ZI and Ceramill zolid FX.
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