Review: microstructure-development mechanism during sintering in polycrystalline zirconia

Y2O3-stabilised ZrO2 (YSZ) ceramics have been used for various engineering applications since Garvie et al. discovered phase-transformation toughening in 1975. The performance of YSZ ceramics depends on the YSZ microstructure. In the present review, the tetragonal-to-cubic phase transformation and grain growth are first discussed, organised according to existing hypotheses of microstructure-development mechanisms in YSZ during sintering. We demonstrate that the phase transformation and grain growth can be most reasonably understood by a grain boundary segregation-induced phase transformation (GBSIPT) mechanism and that the solute-drag effect of Y3+ ions segregating along the grain boundaries, respectively. Next, the Al2O3-doping effect is discussed with emphasis on the microstructure-development behaviour in a small amount of Al2O3-doped YSZ, which is widely used in engineering applications. We further discuss the effect of GBSIPT on low-temperature degradation resistance and the effect of grain size on superplasticity, focusing on the nanocrystalline YSZ, which was created by applying the Al2O3-doping effect.