Grain boundary characterization of superplastic ceramics by HREM and AEM

Grain boundary structure, chemical composition and bonding state in superplastic TZP were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) with a spatial resolution in a field emission type transmission electron microscope (FE-TEM). Superplastic ceramics characterized in this study were TZP, SiO2-doped TZP and SiO2 including a small amount of metal oxide (MgO, Al2O3 and TiO2) doped TZP. It was found that there was no amorphous phase at any grain boundary faces but yttrium was segregated at grain boundary faces with the distribution of 3-5 nm across the boundaries for all of the materials examined. The segregation of silicon was also observed over a width of 4-6 nm across the boundaries for SiO2-doped TZP. Magnesium, aluminum and titanium were distributed within a width of several nm across the boundaries for (SiO2-metal oxide)doped materials. O-K edge EELS spectrum taken from a grain boundary in SiO2-doped TZP was shifted by 3-4eV to high energy loss side, comparing with that from grain interior. This indicates that the ionicity of the grain boundary is higher than that of grain interior, resulting in the occurrence of low grain boundary energy in SiO2-doped TZP. This agrees well with the fact that an average dihedral angle between grain boundaries in SiO2-doped TZP is relatively as high as 80 degree. Superplastic deformation behavior was discussed on the basis of the microstructural examinations.