The enhancement of superplastic flow in tetragonal zirconia polycrystals with SiO2-doping

Superplasticity in SiO2-doped tetragonal zirconia polycrystal (2.5Y-TZP) is investigated by means of tensile testing in a temperature range 1,200--1,500 C. The grain boundary SiO2 phase reduces the flow stress and greatly enhances the tensile ductility in TZP. The stress and grain size exponents take a value close to 2 and 3, respectively, in this temperature range, but there is an abrupt change in activation energy at 1,380 C in SiO2-doped TZP. The activation energy for superplastic flow above and below this temperature is estimated to be 182 kJ/mol and 635 kJ/mol. The enhancement of superplasticity due to SiO2-doping is explained in terms of the accelerated plastic flow in the grain boundary SiO2 phase to accommodate the stress concentration generated by grain boundary sliding. The elongation to failure in the SiO2-doped TZP is phenomenologically described as a function of the Zener-Hollomon parameter. At high temperatures and low strain rates, the cavities tend to align and interlink parallel to the tensile axis, and the huge elongation in excess of several hundred percent is obtained