Enhanced joining of silicon nitride and molybdenum via functionally graded structures: Reaction mechanism and microstructural optimization

A reliable joining between silicon nitride (Si 3 N 4 ) ceramics and metallic molybdenum (Mo), which possess significantly different physical properties, is successfully achieved through powder metallurgy and gradient composite technology. The interfacial reaction mechanisms between Si 3 N 4 and Mo at various compositional layers are systematically investigated under a sintering temperature of 1500 °C. Based on the elucidated reaction pathways and the concentration distribution index (p), the structure of the transition layer in Si 3 N 4 /Mo x Si y /Mo functionally graded materials (FGMs) is effectively optimized. By eliminating the intermediate layers where severe reactions induce excessive porosity, the overall densification and interfacial integrity of the graded structure are significantly enhanced, thereby improving the mechanical performance of the composite. The results indicate that Si 3 N 4 and Mo primarily react via diffusion between Mo and Si to form molybdenum silicides, following the reaction sequence: (Mo + Si) → (Mo 3 Si/MoSi 2  + Si) → (Mo 5 Si 3 ). At a concentration distribution index of p = 1.5, the flexural strength of the Si 3 N 4 /Mo x Si y /Mo FGMs reaches a maximum value of 371.4 MPa. Furthermore, the elemental distribution across the optimized transition layers exhibits a quasi-continuous gradient profile, indicating excellent structural continuity and robust interfacial bonding.