Effect of Hydrothermal Aging on Mechanical and Microstructural Properties of Zirconia Ceramics

The mechanical and microstructural properties of monolithic zirconia ceramics are significant factors for their long-term clinical performance. This study aims to investigate the effects of hydrothermal aging on these properties for the 3Y-TZP, 4Y-TZP, and 5Y-TZP formulations. Specimens were prepared from 3 different zirconia blocks: 3Y-TZP (HT), 4Y-TZP (ST), and 5Y-TZP (XT). Half of the specimens were aged in an autoclave (134 °C, 2 bar, 5 h) while the others remained as controls. Three-point flexural strength, Vickers hardness, and surface roughness tests, as well as XRD, AFM, and SEM/EDS analysis, were performed. The material type significantly affected the flexural strength, Vickers hardness, and surface roughness. Aging did not significantly affect the flexural strength or surface roughness but reduced the Vickers hardness in the 3Y-TZP sample. The 3Y-TZP and 5Y-TZP samples displayed the highest and lowest flexural strength, respectively. In the non-aged groups, 3Y-TZP and 5Y-TZP exhibited higher hardness than 4Y-TZP, and after aging, 3Y-TZP displayed the lowest hardness. Further, 5Y-TZP showed the highest surface roughness before and after aging. XRD revealed an increased monoclinic phase in the aged 3Y-TZP and 4Y-TZP. No monoclinic phase was observed in 5Y-TZP. According to AFM measurements, aging led to a smoother surface in 3Y-TZP but increased roughness in 4Y-TZP and 5Y-TZP. SEM/EDS revealed changes in the elemental compositions following aging. According to the results of this study, different material formulations affect the mechanical behavior and microstructural properties of monolithic zirconia ceramics. Further, hydrothermal aging displayed effects on the Vickers hardness and phase transformations.