Grain boundary structure and chemical bonding state of superplastic SiO2-doped TZP

Superplastic SiO²-doped TZP (tetragonal zirconia polycrystal) was fabricated by sintering at 1300°C in air. The grain boundaries were investigated by high-resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). It was found that no amorphous phase was present between two adjacent grains, but an amorphous pocket was formed at multiple junctions of grain boundaries. An experimental HREM image of σ17a grain boundary was compared with the computer-simulated image to identify the periodic channel structure along the boundary. EDS analysis revealed that yttrium and silicon were cosegregated in the vicinity of grain boundaries. The bonding state at a grain boundary was measured by EELS to reveal that O-Kedge spectrum was shifted by –4 ^eV toward high energy loss side. Electron energy loss near edge structure (ELNES) of O-Kedge was investigated by a first principles molecular orbital (MO) calculation using the discrete-variational (DV)-Xα method both for ZrO²,Y²O³ and SiO² model clusters. Comparison between the experimental and theoretical spectra by the present MO calculation found that the bonding state at the grain boundaries was similar to that in SiO², although no silicious glassy film was present