Abstract
The chemical bonding states of GeO2 and/or TiO2-doped tetragonal zirconia polycrystal (TZP) are calculated by a first principle molecular orbital method using model clusters. It is clarified that Ge4+ and Ti4+ ions, which are substituted into a lattice of TZP, have a high covalent bond with oxygen ions rather than Zr–O bond. Covalency of TZP is more increased by solution of germanium ions than that of titanium ones. In superplastic deformation of TZP, an addition of GeO2 or TiO2 enhances tensile ductility of TZP . Germanium ion is more effective to improve ductility than titanium. The increment of covalency is in a good agreement with the improvement of elongation to failure in doped TZP . Dopant cations segregate at grain boundaries and form no secondary phase. Assuming that a dopant effect on chemical bonding states in grain boundaries is similar to that in grain interior, segregation of germanium or titanium ion increases covalent bonding strength nearby grain boundaries. Such increasing of covalency is likely to enhance cohesion of grain boundaries. The enhancement of grain boundary cohesion suppresses intergranular failure during tensile deformation at elevated temperatures. This must be the reason why an addition of GeO2 and TiO2 is effective to improve the high temperature ductility of TZP . Our calculation suggests that the covalency nearby grain boundaries have a critical role in the tensile ductility of TZP.
