Activated Semi‐Coherent CoSi–Si ₂ N ₂ O Interfaces as an Efficient Toughening Pathway in Si ₃ N ₄ Ceramics

ABSTRACT Ceramics exhibit superior stability in extreme environments but suffer from intrinsic brittleness. Here, we report a novel interfacial toughening mechanism in silicon nitride composites via cobalt microalloying. Trace cobalt promotes in situ formation of CoSi particles that establish semi‐coherent interfaces with the surrounding Si 2 N 2 O grains. Unlike conventional strategies relying on crack deflection or bridging, these tailored interfaces guide crack propagation and activate synergistic fracture modes, significantly enhancing energy dissipation. The optimized composite achieves a fracture toughness of 11.7 MPa·m 1/2 and a flexural strength of ∼710 MPa at room temperature, while maintaining a toughness of 7.5 MPa·m 1/2 at 800°C, surpassing many existing Si 3 N 4 ‐based systems. Density functional theory confirms strong interfacial bonding with partial lattice coherency, while phase‐field modeling shows that semi‐coherent interfaces guide cracks and induce transgranular fracture of CoSi for energy dissipation. This work demonstrates semi‐coherent interface engineering as an effective strategy for designing high‐strength, tough ceramics for extreme conditions.