In situ formation of β‐Si ₃ N ₄ nanoheterogeneous nucleation sites for toughening and strengthening porous Si ₃ N ₄ ceramics

Abstract Resolving the inherent conflict between high porosity and superior mechanical properties in porous silicon nitride (Si 3 N 4 ) ceramics is critically important for their structural applications. This study presents a novel strategy to simultaneously enhance flexural strength and fracture toughness without compromising porosity. By introducing a small amount of silicon (Si) powder into the starting composition (α‐Si 3 N 4 and Y 2 O 3 ), a network of in situ Si 3 N 4 nanowires was formed via a gas‐phase reaction and vapor–solid mechanism under a nitrogen atmosphere. These nanowires eventually transform into nanoscale crystal nuclei, providing abundant heterogeneous nucleation sites for β‐Si 3 N 4 crystal grains. This process resulted in a refined and uniform microstructure characterized by significantly reduced grain size and pore diameter, while maintaining a high aspect ratio (∼10) of the β‐Si 3 N 4 grains. Consequently, at a constant high porosity of ∼56%, the flexural strength and fracture toughness were remarkably improved from 171.2 to 234.9 MPa and from 2.41 to 3.33 MPa·m 1 / 2 , respectively. The evolution of nanowires and the phase transformation were systematically characterized, and the underlying strengthening and toughening mechanism involving microstructure refinement and Hall–Petch strengthening is thoroughly discussed.