Toughening of silicon nitride ceramics through stress-induced phase transformation

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.