Electric field-enhanced superplastic flow and microstructure development in TZP
In polycrystalline ceramics, high temperature plastic deformation often occurs by diffusional flow mechanism such as diffusional creep and grain boundary sliding. In fine-grained ceramics with the grain size of typically less than 1 μm can exhibit superplastic flow, in which large elongation to failure is achieved in tensile manner by grain boundary sliding mechanism at high temperature. Typical experimental conditions for superplastic flow in fine-grained Y2O3-stabilized tetragonal ZrO2 polycrystal (TZP), for instance, are temperatures greater than 1400°C and strain rates slower than 1×10−4 1/s. Such high temperature is required in order to accelerate diffusional mass transport as an accommodation process for grain boundary sliding.
It has been recently pointed out that diffusional mass transport is highly accelerated by the application of a strong electric field. For instance, sintering densification of TZP is enhanced by the application of DC field of 40 V/cm, and highly enhanced by DC field beyond 60 V/cm; the densification finishes within 10 seconds at 850°C under the field of 120 V/cm, while the densification is completed by conventional sintering at 1400°C for several hours without electric field (M. Cologna et al., J. Am. Ceram. Soc. (2010)). The abrupt densification under a strong electric field is called flash sintering. We have demonstrated that by applying a strong DC field, dense TZP body with the grain size of about 0.4 μm can show the flash event, and that the TZP under the occurrence of the flash event may exhibit superplastic deformation with an elongation to failure of >150%, at a lower furnace temperature of 800°C and a higher strain rate of 2×10−3 1/s compared to previous methods (H. Yoshida and Y. Sasaki, Scripta Mater. (2018)). The microstructure development during superplastic flow under the field has been investigated in detail. High temperature three-point flexural test in the TZP at lower field has also been performed to elucidate the role of the field and current on the plastic flow. It would seem that the electric field and/or current may accelerate the grain boundary sliding of the TZP.