Ultra-fast sintering and grain boundary metastability in ceramics

• Rapid sintering can trigger grain boundary metastability. • Grain boundary metastability can interact with densification kinetics and microstructural evolution. • Scaling laws can exist correlating metastability with the particle size. • Grain boundary metastability can explain different phenomena observed during sintering. Grain boundaries (GB) are critical in polycrystalline ceramics. They control the material’s properties as well as the diffusional processes. While they received little attention from the ceramics processing scientists for decades, the new developments in ultra-rapid sintering technologies (flash sintering, ultrafast high-temperature sintering, fast firing, photonic sintering…) have dramatically changed the context. Recently, it has been shown that rapid consolidation can be achieved in a timeframe shorter than that required for the interfaces to relax to their equilibrium state. This provides a detectable reduction in the activation energy for diffusion and sintering (about 8% in alumina), resulting in exceptional densification. Besides sintering, this result is of broader interest, as it provides a new approach to grain boundary engineering. In this short review, we discuss the current research context and outline future perspectives related to rapid sintering with a focus on its impact on the grain boundaries. New interpretations of non-conventional microstructures and scaling laws during rapid firing are also proposed based on the concept of grain boundary metastability.