Electric Field-Induced Grain Boundary Degradation Mechanism in Yttria Stabilized Zirconia

Abstract

The degradation mechanism of flash sintering 8 mol% yttria stabilized zirconia, which results in poor densification and crumbling of the samples under direct current, was investigated. Microstructure analysis revealed highly strained grain boundaries and nanopore formation at the grain boundaries after flash sintering with a direct current. A higher current density experiment gave a varied microstructure with dimpling similar to metallic microvoid coalescence near the negative electrode, a highly porous “swiss-cheese” like microstructure in the pellet interior, and melting and recrystallization near the positive electrode. These characteristics were not observed under an alternating current. Phase field simulations predicted an oxygen vacancy buildup and potential reduced phases along the grain boundaries, which lead to the depletion of free holes and dramatic increase of free electrons. This results in high tensile stresses at the grain boundaries, which explains the degradation along the grain boundaries after flash sintering under direct current.