High-Temperature Behavior of SiO2 at Grain Boundaries in TZP

The chemical reaction between SiO₂ and tetragonal zirconia polycrystal (TZP) was directly observed using a TEM in-situ heating technique in order to understand the behavior of SiO₂ in TZP at high temperatures. Their dynamic interaction was recorded up to about 1400°C using a CCD camera-video system connected to the TEM. Most of SiO₂ phase dissolved into the ZrO₂ grains above 1300°C. On the other hand, during cooling from the high temperature to around 400°C, amorphous SiO₂ reprecipitated from the surface of ZrO₂ grains and formed a thin layer around the ZrO₂ grains. This result agrees well with the fact that silicon segregates in the vicinity of grain boundaries in SiO₂-doped TZP. In order to confirm the grain boundary segregation at high temperatures, we investigated grain boundaries in quenched specimens 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 in the quenched samples. EDS analysis revealed that silicon segregated at the grain boundaries and that the segregation layer was wider than that in as-sintered specimens. The electron energy loss near edge structure (ELNES) of O K-edge was measured from both grain boundary and grain interior in quenched specimen, and their spectra were interpreted by a first principles molecular-orbital (MO) calculation using the discrete-variational (DV)-X method.