Interfacial structure and properties of Si₃N₄/AgCu/TiAl brazing joints

The non-active AgCu filler metal was used to join Si₃N₄ ceramics to TiAl-based alloys. The typical interfacial microstructure of the joints was determined as TiAl/Ti₃Al + Ti(s,s)/AlCuTi/Ag(s,s)+ AlCu₂Ti/Ti₅Si₃+ TiN/Si₃N₄ ceramics. During the brazing process, the active element Ti, which dissolves into the non-active AgCu liquid filler metal, involves in the wetting on the Si₃N₄ ceramic and reacts to the ceramic materials. With the increase of brazing temperature and holding time, the thickness of the TiN reaction layer close to the Si₃N₄ ceramic increases and AlCu₂Ti compounds dispersed in the Ag-based solid solution gather greatly, which reduces the joining properties. The shear strength of the joints reaches a maximum of 124.6 MPa when the brazing temperature T = 860°C and the holding time t=5 min. According of the thermodynamics and kinetics calculation, the activation energy Q and the growth coefficient KP of the TiN layer are 528.7 kJ/mol and 2.7 × 10^-7 m/s^1/2, respectively.