Reduced interfacial stresses in laminated ceramics during flash sintering enabled by an extremely low uniaxial viscosity

Constrained sintering is a crucial issue for the development of laminated or composite ceramics. It had been suggested that flash sintering (FS) may eliminate the defects caused by constrained sintering. In this work, the interfacial tensile stresses of laminated ceramics during conventional sintering (CS), field assisted sintering technique (FAST) and FS were estimated and compared by combining a set of free sintering, bending creep and constrained sintering experiments with theoretical analysis. The interfacial tensile stress in constrained FS is significantly lower than those of both CS and FAST, and can only be explained by an extremely low uniaxial viscosity of the materials achieved during FS. The possible effects of temperature, density and microstructure on the uniaxial viscosity reduction have been carefully assessed to show that the electrical field itself is the key factor for the extremely low viscosity. Plausible mechanisms for the reduction in the uniaxial viscosity under different levels of electrical fields are also discussed.