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Low temperature degradation (LTD) of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is of concern.The purpose of this in vitro study was to assess the effect of accelerated aging on the Vickers hardness and fracture toughness of a newly developed Y-TZP and 2 primary Y-TZPs.Two primary 3mol% Y-TZP, Lava (LA), Everest Zirconium Soft (EV), and a new 3mol% Y-TZP, ZirTough (NZ) were assessed. Specimens (n=30 each brand) of 10×10×3 mm were hydrothermally treated for accelerated aging to examine LTD. Five conditions were used (n=5 per condition) as follows: control group (no aging); 5 hours at 134°C/0.2 MPa (5h-134°C); 100 hours at 134°C/0.2 MPa (100h-134°C); 5 hours at 180°C/1.0 MPa (5h-180°C); and 20 hours at 180°C/1.0 MPa (20h-180°C). Fracture toughness was measured by using the indentation fracture (IF) method under a loading of 294 N and calculated from the obtained measurements. To observe differences in particle composition and fracture patterns, mirror-polished test specimens (n=5 each brand) were re-sintered at 1200°C for 1hour as a thermal etching process, and a Vickers indenter was pressed into the test specimens according to the IF method. Test piece surfaces and cracks were observed with scanning electron microscopy (SEM). One-way ANOVA and the post- hoc (Scheffé test were used to examine) interlevel significant differences (α=.05).The Vickers hardness and fracture toughness were as follows: 1319 HV and 7.36 MPa·m1/2 for LA, and 1371 HV and 6.76 MPa·m1/2 for EV in no aging; 1334 HV and 7.02 MPa·m1/2 for LA, and 1346 HV and 6.07 MPa·m1/2 for EV in 5h-134°C. No significant differences were found between no aging and 5h-134°C for LA and EV for Vickers hardness and fracture toughness. Measurements could not be made for LA and EV for 100h-134°C, 5h-180°C, or 20h-180°C because of fractures in the surface layer. For NZ, Vickers hardness and fracture toughness were as follows: 1261 HV and 15.60 MPa·m1/2 in no aging; 1217 HV and 14.98 MPa·m1/2 in 5h-134°C; 1231 HV and 15.13 MPa·m1/2 in 100h-134°C; 1252 HV and 15.51 MPa·m1/2 in 5h-180°C; 1224 HV and 15.01 MPa·m1/2 in 20h-180°C. No significant differences were shown in the Vickers hardness and fracture toughness. SEM observations after the thermal etching processing of NZ showed zirconia particles and scattered alumina particles.Measurements with LA and EV could only be made for no aging and 5h-134°C, and no significant differences were found in Vickers hardness and fracture toughness. Measurements were made with NZ under all conditions and no significant differences were found in Vickers hardness and fracture toughness.