Superplastic flow stress and electronic structure in yttria-stabilized tetragonal zirconia polycrystals doped with GeO2 and TiO2

Abstract Superplatic flow in 3 mol%-yttria-doped tetragonal zirconia polycrystal (3Y-TZP) with tetravalent cation, Ge and Ti, has been examined by high temperature tensile testing. These cations are effective to suppress the flow stress of 3Y-TZP and to improve tensile ductility. First principles molecular orbital calculations using model clusters reveal that substituted Ge and Ti ions have strong covalent bonds with neighboring oxygen ions than Zr ion. Such high covalency of Ge–O and Ti–O bonds is due to large bonding components in valence band. Addition of Ti and Ge ions leads to reduction in the ionicity in 3Y-TZP. From our high temperature tensile tests and theoretical calculations for GeO 2 - and TiO 2 -doped TZP, flow stress is observed to correlate with an effective ionic bonding strength. It is expected that the ionicity is a major parameter to control the diffusivity of cation and to hence affect the superplastic flow of 3Y-TZP.