Y-TZP/Al₂O₃複合体の高温変形機構

抄録

Yttria-stabilized tetragonal zirconia (Y-TZP) and fine-grained alumina deform plastically at high temperatures and the mechanisms have been studied. Y-TZP shows superplastic behavior with extensive grain boundary sliding. On the other hand, Al₂O₃ possibly deforms by diffusional creep which is limited by interface reaction. Thus, a number of studies about deformation of ceramic materials have been reported, however, its mechanism has not yet been fully clarified. In this study, sintered Y-TZP-Al₂O₃ composite samples were prepared, and in order to examine its behaviors in a wide range of stress (∼2 to 350MPa), the specimens were used for three kinds of deformation tests : constant cross head speed compressive test, constant load compressive test and bending creep test. The results of strain rate έ vs true stress σ on a logarithmic scale, which generally determines stress exponent n, revealed four stages of flow (region I∼IV) exhibiting a sigmoidal curve. This indicates that the behaviors change from diffusional creep with n∼1 in region I at low stresses through a transition state with n >3 in region II at intermediate stresses and to superplasticity with n∼2 in region III at high stresses. The decrease in n with increasing stress from ∼2 in region III to ∼1.5 in region IV implies that diffusional creep tends to be dominant at higher stress. Furthermore, the increase in n from ∼2 in region III to >3 in region II suggests the occurrence of a threshold stress σ₀ for superplastic flow. From the results of έ vs σ₀ with different grain sizes, as the grain size is larger, the stress exponent n in region II seems to decrease gradually with increasing grain size, suggesting that threshold stress σ₀ decreases with grain size.