Flash Sintering (FS) is a field assisted sintering technique which enables a sustainable densification of ceramics within a very short time compared to conventional methods. Even if the application of this process is possible and various materials are densified by it, the fundamental mechanisms of the rapid densification remain partly unclear. Beside Joule heating, other mechanisms, like electrochemical reduction or phase transitions during FS of yttria stabilised zirconia (YSZ) are proposed by some researchers. However, their connection and importance to FS are still under debate.

In this investigation the proposed fundamental mechanisms in FS of YSZ, such as electrochemical reduction, phase transition and Joule heating are investigated in detail by DC FS of YSZ under varying conditions and atmospheres. Changing atmospheres during FS enabled the formation of new electrochemically reduced phases which were retained at room temperature for the first time. Subsequently, the phases were characterized by various techniques such as XRD, Raman spectroscopy, XPS and EPR to determine the fundamental structure of this new phase and its importance to FS of YSZ.

The characterization of the new phases, which form during FS of YSZ, showed that the original YSZ material forms two separate phases. One of the phases can be specified as cubic zirconia, the other shows a face centred cubic crystal structure with a significantly higher conductivity and in general a more intermetallic or even metallic behaviour was detected in this phase.

In this presentation the experiments about FS of YSZ conducted will be explained and particularly the influence of various atmospheres will be elaborated. The second focus of this talk will be the electrochemical reduction of YSZ under FS conditions and the following phase transition and its importance and influence on FS of YSZ in general. As a third point, the properties of this new phase, which was retained at room temperature for the first time, will be presented and explained in the context of FS.