Effect of hydrothermal and mechanical aging on the fatigue performance of high-translucency zirconias

Objectives: This study aimed to evaluate the impact of mechanical fatigue cycling using the step-stress approach along with hydrothermaldegradation (134 degrees C with a constant pressure of 2 bars for 20 h), and a novel intercalated hydrothermal/fatigue aging protocol on different aspects of the aging resistance of three generations of dental zirconias. Methods: "Y"Z T (VITA), INCORIS "T"ZI (Dentsply Sirona) and "K"ATANA UTML (Noritake Kuraray) - 1st, 2nd and 3rd generation, respectively-, zirconia disks (N = 153), were divided into 6 groups (n = 3) for monotonic testing and 9 groups (n = 15) for mechanical fatigue testing, according to 3 proposed treatments for each zirconia: CF (control - only mechanical fatigue cycling); AF (aging in hydrothermal reactor at 134 degrees C for 20 h + mechanical fatigue cycling); AFA (Alternating protocol: 4 steps of 5 h of hydrothermal aging intercalated with mechanical fatigue cycling). Mechanical fatigue aging was performed according to the stepstress approach through biaxial flexural setup (piston-on-3-balls, initial strength: 100 MPa, step: 50 MPa/10,000, frequency: 20 Hz) until failure. Data were analyzed using Kaplan-Meier and Mantel-Cox test (alpha = 0.05), in addition to Weibull analysis. Fractured disks were analyzed in stereomicroscope, Scanning Electron Microscopy and X-Ray Diffraction. Results: Continuous hydrothermal and mechanical fatigue cycling decreased the fatigue strength of YAF group (516 +/- 38 MPa), while the alternating protocol increased it (730 +/- 58 MPa). KATANA UTML showed no differences for both treatments and did not undergo t-m phase transformation. The TAF group showed the highest fatigue strength (810 +/- 76 MPa) and cycles for failure (147,000.00 cycles). The fracture origin for all specimens was on the tensile side in pre-existing defects. Significance: INCORIS TZI zirconia had higher fatigue strength and survival rates after hydrothermal and mechanical fatigue aging. Although less resistant, KATANA UTML did not suffer chemical degradation. (C) 2022 Published by Elsevier Inc. on behalf of The Academy of Dental Materials.