Effect of chemical composition of intergranular glass on superplastic compressive deformation of β-silicon nitride

The effect of chemical composition of Y2O3–Al2O3–SiO2-based intergranular glass on superplastic deformation of β-Si3N4 was studied by compression tests at 1873 K. All hot isostatically pressed Si3N4 materials had essentially the same microstructure and the same amount of glass phase, which was different in composition only. The relation between flow stress and glass composition qualitatively corresponded to the effect of chemical composition on viscosity of Y2O3–Al2O3–SiO2 glass. However, the flow stress was not proportional to the viscosity of Y2O3–Al2O3–SiO2 glass, probably because the composition of intergranular glass phase had changed by dissolving Si3N4. The strain hardening (increase of flow stress with deformation) was dependent on the chemical composition of intergranular glass. Actually, the apparent strain hardening was not proportional to the strain but was proportional to time. The crystallization of Si2N2O was also proportional to time, and was dependent on the chemical composition of the intergranular glass in a similar way to the strain hardening. Thus, it was suggested that the crystallization of Si2N2O reduced the amount of the intergranular glass, thereby increasing flow stress.