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 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 and crack arrest and reinitiation mechanisms. This phase-transition-regulated in-situ composite strategy based on silicon nitride polymorphs provides new insights into the microstructural engineering for synergistic hardening and toughening of advanced structural ceramics.

Ceramics, particularly covalently bonded ceramics, exhibit intrinsic brittleness resulting from the strong directional nature of their chemical bonds, significantly limiting their applications. Despite extensive research, effective strategies for substantially enhancing the toughness of covalently bonded ceramics without compromising their exceptional properties have remained elusive. Here, we present a breakthrough in the transformation toughening of dual-phase α/β-Si₃N₄ ceramics, driven by stress-induced β-to-α phase transformation. Through a nucleation and growth method, we successfully synthesized dense, dual-phase α/β-Si₃N₄ ceramics with coherent interfaces under high-pressure and high-temperature (HPHT) conditions. The dual-phase α/β-Si₃N₄ ceramics demonstrate exceptional toughness, reaching 7.30 MPa·m^1/2, which is more than double that of β-phase silicon nitride. The significant toughening is attributed to localized plastic deformation caused by stress-induced phase transformation, which hinders crack propagation. This represents the demonstration of stress-induced transformation toughening in covalently bonded ceramics, highlighting the transformative potential of interface engineering for enhancing the toughness of brittle ceramics.