Flash-sinterforging of nanograin zirconia: Field assisted sintering and superplasticity

Abstract

We report on the influence of a uniaxial applied stress on flash-sintering and field assisted superplastic behavior of cylindrical powder preforms of 3 mol% tetragonal-stabilized zirconia. The experiments use the sinterforging method, where, in addition to pressure, a dc electrical field is applied by metal electrodes sandwiched between the push-rods and the specimen. The axial and radial strains in the experiment provide simultaneous measurement of the time-dependent densification and shear strains. Large effects of the electric field on sintering and superplasticity are observed. We see flash-sintering which is characterized by a threshold level of temperature and electric field. With higher applied fields, the sample sinters at a lower furnace temperature. Surprisingly, the applied stress further lowers this critical temperature: a sample, which sinters at 915°C under a stress of 1.5 MPa, densifies at only 850°C when the stress is raised to 12 MPa. This stress induced reduction in sintering temperature maybe related to the additional electrical fields generated within the specimen by the electro-chemo-mechanical mechanism described by Pannikkat and Raj [Acta Mater., 47 (1999) 3423]. Remarkably, we also show that the sample deforms in pure shear to 30% strain in just a few seconds at anomalously low temperatures. The specimen temperature was measured with a pyrometer, during the flash sintering, as a check on Joule heating. A reading of 1000°C-1100°C was obtained, up to 200°above the furnace temperature. This temperature is still too low to explain the sintering in just a few seconds. It is suggested that the electric field can nucleate a defect avalanche that enhances diffusion kinetics not by changing the activation energy but by increasing the pre-exponential factor for the diffusion coefficient, noting that the pre-exponential factor depends on concentration of defects, and not upon their mobility. © 2011 The American Ceramic Society.