Microstructure and crystal phase development of Y₂O ₃-stabilized ZrO₂ polycrystal fabricated by the solid phase mixing and sintering method

The microstructure and crystal phase in 2.8 mol% yttria-stabilized zirconia polycrystals were investigated. Starting powders were prepared by wet dispersing and mixing using an agitator mill or by the liquid phase precipitation method. The composites powders were sintered at 1275-1600°C and then analyzed. Solid phase method (SPM) powders showed higher sinterability than those of the liquid phase precipitation method (LPM) powders, and high density samples consisting of fine grains could be obtained at lower sintering temperatures than from LPM powders. Unlike the LPM sample, the cubic and tetragonal phase was formed in the SPM sample at low sintering temperature, because there was the region of high and low Y³⁺ ion concentration in the material. As the sintering temperature increased, the cubic phase decreased and tetragonal phase increased by the progress of the diffusion of Y ³⁺ ions. And then, Y³⁺ ions segregated at grain boundaries, and tetragonal grains containing cubic phase at grain boundary regions were formed. The thermal expansion curve for the SPM sample samples sintered at 1275°C exhibited a monoclinic to tetragonal transformation during the heating stage; however, the expansion was linear during the cooling stage, as was the case for both SPM samples sintered at higher temperatures and the LPM samples. The thermal expansion behavior of the SPM sample suggested that the monoclinic phase was essentially not present within the sample, although the transformed monoclinic phase in a surface layer by grinding stresses was exist.