Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.