High-Temperature Deformation Mechanism of Y-TZP-Al2O3 Composites
Shun ENOMOTO, Yasunori OKAMOTO and Takeshi SHIONO
Abstract: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 Γ vs true stress Π 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 Π0 for superplastic flow. From the results of Γ vs Π0 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 Π0 decreases with grain size. Key Words:Y-TZP -Al2O3 composite, Diffusional creep, Superplasticity, Diffusional coefficient, Threshold stress