Combined experimental and numerical study to understand electrical and thermal behaviors of ZrN flash sintering

Flash sintering (FS) is a comparatively new technique for rapid densification of ceramic materials at significantly lower ambient temperatures. In this study, FS of a high temperature ceramics - ZrN was investigated utilizing both experimental and numerical approaches, aiming to address the knowledge gap with respect to flash sinter-forging of PTC-type ceramics (i.e., those with positive temperature coefficient for bulk resistivity). Different assumptions about sample effective electrical conductivity vs temperature were explored and the best fit to experimental data was explained from a particle compact model. In addition, the effects of several process parameters such as electrode material, sample assembly geometric configuration, as well as the thermal insulation were studied. The results show that electrical responses are largely in line with expectation, but the influence on densification is much more nuanced, reflecting complexity of the system. It was found that electrode heating can play a crucial role in raising sample temperature under the flash sinter-forging configuration for materials like ZrN, distinguishing it from conventional FS for oxides such as YSZ where heating is almost exclusively confined to the sample. On the other hand, thermal insulation is found to play a very significant role in the observed flash behavior and densification. Finally, the limitations as well as future research directions for FS of high temperature ceramics like ZrN are discussed.