Gradient Joining of Si₃N₄–Mo and Reaction Bonding Mechanism

The effective joining between silicon nitride ceramics and metals is crucial for making full use of their excellent properties and meeting the service requirements of materials or components in complex environments. The gradient joining of silicon nitride ceramic (Si₃N₄) and molybdenum (Mo) with large physical properties difference was realized by a powder metallurgy gradient composite technology. The reaction mechanism of Si₃N₄ and Mo in each gradient layer under different sintering temperatures and Si₃N₄ additions was investigated. The transition layer structure of Si₃N₄/Mo_xSi_y/Mo gradient material was optimized based on the reaction mechanism and the concentration distribution index p, obtaining the gradient connection of Si₃N₄ to Mo and improving the mechanical properties. The results show that Si₃N₄ and Mo mainly form molybdenum–silicon compounds through the diffusion reaction between Mo and Si. The reaction process follows (Mo+Si)→(Mo₃Si/MoSi₂+Si)→(Mo₅Si₃). The bending strength of Si₃N₄/Mo_xSi_y/Mo gradient material reaches the maximum value of 371.42 MPa, the shear strength reaches the maximum value of 30.58 MPa, and the elements in the transition layer appear a quasi-continuous gradient distribution when p=1.5.