Effect of Surface Adjustment and Hydrothermal Aging on the Mechanical Behavior and Microstructure of Zirconia With Different Yttria Contents.

OBJECTIVES: This in vitro study evaluated the effects of restorative surface adjustment and simulated hydrothermal aging on the microstructure and biaxial flexural strength of computer-aided design (CAD)/CAM zirconia with different yttria contents, including conventional 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and high-translucency zirconia (5Y-PSZ and 6Y-PSZ). METHODS: Disc-shaped specimens (12 mm diameter × 1.2 mm thickness; n = 40 per material) were fabricated from zirconia blocks containing 3, 5, or 6 mol% yttria. Specimens were allocated to two surface conditions: control (no adjustment) or grinding with 100 µm diamond bur followed by a three-step zirconia polishing system. Each condition was divided into aged and nonaged subgroups (n = 10). Hydrothermal aging was performed in an autoclave at 134°C and 0.2 MPa for 5 h as an accelerated low-temperature degradation (LTD) process. Surface roughness and hardness were evaluated using a profilometer and nanoindentation tester, respectively. Surface morphology, grain size, and fracture characteristics were analyzed using scanning electron microscopy (SEM). Crystalline phase composition was evaluated using qualitative X-ray diffraction (XRD). Biaxial flexural strength was measured via piston-on-three-ball test (ISO 6872). Data were analyzed using three-way analysis of variance (ANOVA) and Tukey's test (α = 0.05). RESULTS: Surface adjustment significantly reduced surface roughness. Biaxial flexural strength increased significantly in 3Y-TZP and showed a less pronounced response in 5Y-PSZ, whereas no significant differences were observed in 6Y-PSZ. Hydrothermal aging did not significantly affect surface roughness, hardness, grain size, or biaxial flexural strength under the applied conditions. No monoclinic reflections were detected by qualitative XRD. However, SEM revealed localized grain-boundary microcracking and surface degradation in aged 3Y-TZP. CONCLUSION: Finishing and polishing (FP) improved the biaxial flexural strength of 3Y-TZP and moderately enhanced that of 5Y-PSZ, whereas 6Y-PSZ showed limited sensitivity to surface adjustment. Simulated hydrothermal aging did not significantly compromise the mechanical performance or phase stability of the evaluated zirconia.