Phase-transition-regulated precipitation–densification to achieve a hardness–toughness synergy in in situ composite ceramics of triphase silicon nitride

Structural materials generally struggle to simultaneously achieve high hardness and toughness. Composites, as a “complementary advantages” strategy, aim to optimize the synergy between hardness and toughness through the regulation of microstructure and composition. However, conventional composite strategies commonly suffer from insufficient interfacial compatibility and an inhomogeneous distribution of reinforcing phases, limiting the full realization of multiphase synergistic strengthening effects. Here, we successfully synthesized homogeneous triphase silicon nitride ceramics via an in situ composite strategy under high-temperature and high-pressure (HPHT) conditions, achieving synergistic enhancement in hardness and toughness with a Vickers hardness of (26.34±0.61) GPa and a fracture toughness of (6.02±0.48) MPa·m^1/2. The superior mechanical properties are attributed to the precise regulation of microstructural evolution through the phase-transition pathway, establishing a sequential precipitation–densification mechanism that regulates γ phase precipitation and transformation kinetics while allowing concurrent α-to-β transformation and progressive densification. Specifically, nanoscale γ phase particles significantly enhance the hardness, whereas high-aspect-ratio β grains effectively improve fracture toughness through crack bridging, crack arrest, and reinitiation mechanisms. This phase-transition-regulated in situ composite strategy based on silicon nitride polymorphs provides new insights into microstructural engineering for synergistic hardening and toughening of advanced structural ceramics.