Solute segregation at grain boundaries in superplastic SiO2-doped TZP

Grain boundary structure, chemical composition, and bonding state in superplastic SiO2-doped TZP and undoped TZP were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a field emission type transmission electron microscope (FE-TEM). No amorphous phase was observed at any grain boundaries in either SiO2-doped TZP or undoped TZP. Yttrium ions segregated over a width of 4--6 nm at grain boundaries in both materials, and silicon ions segregated over a width of 5--8 nm at grain boundaries in SiO2-doped TZP. The average dihedral angle between grain boundaries in SiO2-doped TZP was as high as 80 degree, which agreed well with the fact that no grain boundary had glass phase. The strain energy is accumulated by the dissolution of silicon ions into the tetragonal zirconia lattice. However, the grain boundary energy of SiO2-doped TZP is likely to be low enough to compensate the increase of strain energy near grain boundaries. O-K-edge EELS spectra taken from grain boundaries in SiO2-doped TZP were shifted 3--4 eV to the higher energy side in comparison with those from the grain interior. This may suggest that the chemical bonding is strengthened at grain boundaries by the presence of solute silicon. The strengthening may be responsible for the enhanced superplasticity in SiO2-doped TZP