From conventional ac flash-sintering of YSZ to hyper-flash and double flash

Flash-sintering experiments were performed on conventional 8 mol% Y2O3-doped ZrO2. By varying the initial power spike, the steady state limited current density, the flash duration and furnace temperature, various microstructures corresponding to different sintering stages were observed. It is shown that operating the flash-sintering under the power dissipation regime may lead to microstructure heterogeneity. Under a quasi-adiabatic regime a better homogeneity is obtained. The major part of the shrinkage strain occurs within a short time interval of about 1 s, after the incubatory period. The remaining part appears to be a conventional shrinkage process induced by Joule heating. Hyper-flashes were performed over this short time interval, yielding relative densities up to 90%. Additional flashes can be subsequently applied up to full density. It is stressed that densification is not an absolute quality parameter in the optimization of the experimental conditions; inappropriate conditions may lead to fairly dense but brittle materials.