Grain Boundary Energy and Tensile Ductility in Superplastic Cation-doped TZP

Abstract

Fine-grained 3Y-TZP has been known to show high superplasticity. Addition of a small amount of metal oxide influences the superplastic behavior in 3Y-TZP. In this study, 3Y-TZP doped with 1 mol% GeO₂, TiO₂ or BaO were fabricated, and respective grain boundary energy has been systematically measured by a thermal grooving technique with atomic force microscopy. It has been found that addition of Ge⁴⁺ or Ti⁴⁺ ions decreases the grain boundary energy to stabilize the grain boundaries in TZP whereas doping of Ba²⁺ ion increases the grain boundary energy to destabilize the grain boundaries. A change in the grain boundary energy should be due to segregation of dopant at grain boundaries. It has been also found that the elongation to failure of cation-doped 3Y-TZP is directly proportional to the stability of grain boundary. Grain boundary energy is thus one of the principal factors to determine the tensile ductility of TZP. In order to reveal the effect of dopant on the grain boundary energy, lattice static calculations and first principles molecular orbital calculations have been performed for supercells and model clusters including the present dopant, respectively. A series of results shows that substitution of Ge⁴⁺ or Ti⁴⁺ ion for Zr⁴⁺ ion increases the covalency of TZP, but the covalency of TZP is reduced by addition of Ba²⁺ ions. The grain boundary energy is found to have a relationship with covalency nearby grain boundaries in TZP.