Impurity effects on grain boundary strength in structural ceramics

In order to investigate impurity effects on grain boundaries in structural ceramics, several kinds of Al2O3 polycrystals were prepared by systematically doping with Ln oxides by an amount of 0.05 mol%. As another model specimens, of SiO2-doped Tetragonal Zirconia Polycrystals (TZP) were prepared by separately adding several kinds of metal oxides in the order of 1000 ppm. Creep behavior was investigated for the impurity-doped Al2O3 polycrystals at 1250 °C under a constant pressure of 50 MPa, and tensile tests were carried out at 1400 °C for the impurity-doped SiO2–TZP to evaluate their superplastic behavior. It was found that both creep and superplastic behavior in the respective specimens were much influenced by doping such impurities. The grain boundaries in all specimens were characterized by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) with a high spatial resolution less than 1 nm to reveal their atomic structure, chemical composition and chemical bonding state. There was no secondary phase along the grain boundaries, but the doped impurities tended to segregate at the grain boundaries over several nanometers in all of specimens examined. Comparing with the mechanical behavior and grain-boundary microstructural factors obtained in the present study, it is reasonable to say that the grain-boundary chemical-bonding state must be changed by the impurity segregation. This was actually confirmed by the fact that there was a small amount of chemical shift in the experimental Electron Energy Loss Near Edge Structure (ELNES), depending on the kinds of dopants. In order to quantitatively evaluate the grain-boundary chemical bonding state and ELNES profiles, the discrete variational (DV)-Xα molecular-orbital method was applied for some model clusters, calculating the bond overlap population (BOP) and net charge (NC) of neighboring atoms in the respective models. The calculated results explained well the mechanical behavior for the impurity-doped specimens as well as the experimental profiles of ELNES. The role of impurity atoms at the grain boundaries were discussed in detail on the basis of experimental and theoretical results obtained in the present study.