Effect of hydrothermal aging on fracture properties of yttria-stabilized zirconia with different yttria contents

This study systematically investigated the effect of hydrothermal aging on the fracture properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) with different yttria contents (3 mol%: 3Y; 4 mol%: 4Y; 5 mol%: 5Y) by both four-point bending and biaxial flexural tests based on ISO standards. Flexural strength significantly decreased with increasing yttria content, regardless of aging treatment and flexural test method. Hydrothermal aging did not significantly affect the four-point flexural strength in all tested Y-TZP, whereas it affected the biaxial flexural strength, depending on yttria content. Relatively higher Weibull modulus values after hydrothermal aging were observed in 3Y-TZP and 4Y-TZP, while the variation in 5Y-TZP was comparatively limited. 3Y-TZP and 4Y-TZP exhibited mainly intergranular fracture pattern, whereas 5Y-TZP showed a mixed intergranular/transgranular fracture pattern. After hydrothermal aging, 3Y- and 4Y-TZP showed higher tetragonal-to-monoclinic phase transformation. Overall, the biaxial flexural test appeared more sensitive to aging-induced strength variations, whereas the four-point bending test exhibited comparatively more stable strength trends.