Linking microstructure evolution and impedance behaviors in spark plasma sintered Si3N4/TiC and Si3N4/TiN ceramic nanocomposites

We proposed a novel approach to investigate the three-dimensional microstructures and sintering behaviors of Si3N4-based ceramic nanocomposites by electrochemical impedance spectroscopy. Si3N4/TiC and Si3N4/TiN with various weight percentages of conductive phases were prepared by spark plasma sintering (SPS) at different temperatures and dwell times. The incorporation of TiC and TiN into β-Si3N4 provides pulse current paths inside the ceramics due to their higher conductivity. These paths enable the localized Joule heating and mass transport, facilitating the densification and grain growth of ceramic compact. The electrochemical study of such nanocomposites has revealed three-dimensional information of the evolution of their microstructures, and the capacitive and resistive characteristics of the nanocomposites reflect the densification, grain growth, and element distribution in the compact. The impedance model presented in this work suggests isolated distribution of TiN in Si3N4 while Si3N4/TiC of the same amount of additives at the same sintering conditions formed conductive network. This impedance analysis further explained the differences in densification mechanism of SPS in Si3N4/TiN and Si3N4/TiC.