Tetragonal phase stabilization and densification in AC flash-sintered 1.5 mol% yttria-stabilized zirconia polycrystals with high toughness

For the first time, isothermal pressureless flash sintering of 1.5 mol% yttria-stabilized zirconia (1.5YSZ) polycrystals designed for high toughness was performed in a 1-kHz alternating current field (120 V & sdot;cm- 1). The parameters governing tetragonal phase stability and densification needed to maximize the toughness of 1.5YSZ were investigated by varying the applied current density limit and holding time. A sample with relative density exceeding 98% and indentation toughness of about 13 MPa & sdot;m0.5 was sintered at 1100 degrees C by applying 40 mA & sdot;mm- 2 to the green body for 2 min, where the average steady-state temperature during flash reached 1330 degrees C. Unlike flash sintering of conventional oxide ceramics, the applied current density and densification were not positively correlated despite pore closure. While Joule heating intensified when applying higher current densities, sample densification was hindered as the accelerated grain growth resulted in intergranular cracks which were generated by spontaneous tetragonal-to-monoclinic martensitic phase transformation during cooling.