Chemical behavior in diffusion bonding of Si3N4-Ni and Si3N4-superalloy IN-738

Solid state diffusion bonding of Ni/Si3N4 and Ni-based superalloy IN-738/Si3N4 were investigated by TEM. At 1123 K, no reaction products were found at the Ni/Si3N4 bonding interface, even the specimen with a 25 h heat treatment time, while at the IN-738/Si3N4 bonding interface, Cr3Ni2Si, NI31Si12, δ-Ni2Si, and Cr3Si were found. According to thermodynamics theory, the N2 pressure near Ni/Si3N4 interface might be over 102 Pa, and be lower than 10-3 Pa for IN-738/Si3N4. The Cr3Ni2Si was characterized by TEM as the η-phase whose composition has not been determined in the Cr-Ni-Si system. Existence of Cr3Ni2Si was thought to be one of the reasons that N2 atmosphere decreased in IN-738/Si3N4 diffusion bonding. Standard formation energy of Cr3Ni2Si was estimated to be lower than -136 kJ/mole.