Microstructure analysis for zirconium oxynitrides formed in 8 mol% yttria-doped zirconia by a flash event under a direct current electric field in an ambient atmosphere

Yttria-doped zirconia has been shown to undergo nitridation in air by flash sintering using direct current (DC) electric fields, resulting in the formation of zirconium oxynitride, Zr(N,O). This study performed the microstructure analysis primarily using scanning transmission electron microscopy for 8 mol%Y2O3-ZrO2 (8YSZ) nitrided by isothermal flash sintering protocol under DC electric fields in air. The findings confirmed the existence of a threshold current density for Zr(N,O) formation. The nitrided 8YSZ grains comprised crystal-oriented Zr(N,O) fine ellipsoid-shaped grains with 10 - 20 nm in size. The formation of this structure is attributed to the nucleation manner of the Zr(N,O) grains to align their {001} planes to those of the original 8YSZ grains. The transformation of 8YSZ into Zr(N,O) during nitridation could be considered to precede by a replacement of their anions from oxygen to nitrogen while maintaining the cation framework of the respective crystal structures, as predicted by Bechteler et al.