Mechanical properties of 2.0-3.5 mol% Y₂O₃-stabilized zirconia polycrystals fabricated by the solid phase mixing and sintering method

The crystal phase and mechanical properties of 2.0 to 3.5 mol% Y ₂O₃-stabilized zirconia polycrystals sintered from agitator milled powders (solid phase mixing, SPM) were investigated. Powders containing 2.5 and 2.8 mol% Y₂O₃ showed higher sinterability than those containing 2.0 and 3.5 mol% Y₂O₃. The thermal expansion curve of the 2.5 and 2.8 mol% Y₂O₃ samples exhibited a monoclinic to tetragonal phase transformation during the heating stage, with a linear relationship observed during cooling. However, a hysteresis was not observed during the heating and cooling stages when the Y₂O₃ content and sintering temperature were increased. The 2.0 mol% Y₂O₃ sample exhibited the transformation and hysteresis, even though the sintering temperature was increased. The relationship between the flexural strength and grain size for the sample with 2.0 mol% Y₂O₃ was not linear, but exhibited a peak. The flexural strength of the 2.5 and 2.8 mol% Y₂O₃ samples increased as the grain size increased, and then over a certain grain size the flexural strength remained constant and independent of the grain size. The flexural strength and fracture toughness of the 2.0 mol% Y₂O ₃ sample might be dominated by microcrack-toughening. On the other hand, those for the 2.5 and 2.8 mol% Y₂O₃ samples were dependent not only on the stress-induced transformation, but also on the compressive residual stress and microcrack-toughening caused by the transformation. The relationship between the fracture toughness and V _f √h, where V_f is the tetragonal phase fraction and h is the transformation depth, for the 2.0, 2.5 and 3.5 mol% Y₂O ₃ samples was non-linear and exhibited a peak; however, that for the 2.8 mol% Y₂O₃ sample was linear.