Microstructure-Strength Evaluation of Nb Inserted Ti/Si3N4 Dissimilar Joints Brazed with Ag and Au-Based Fillers for Development of Hybrid Thruster

The development of next-generation higher efficiency hybrid ceramic/metal thruster at Japan Aerospace Exploration Agency (JAXA) requires robust and corrosion resistant dissimilar joints to endure severe thermal and vibrational stresses. Si₃N₄ ceramic will be utilized at the combustion chamber, whereas the nozzle skirt exposed to lower temperature will be made of thin Ti plate. In this study, the interfacial microstructure evolution and mechanical behavior at various bending temperatures of Nb inserted Ti/Si₃N₄ joints brazed with Ag68.8Cu26.7Ti4.5 and Au96.5Ni3Ti0.5 (mass%) fillers were evaluated, respectively. Compared to Ti/Si₃N₄ joint, insertion of 2 mm Nb interlayer minimized both thermal expansion mismatch and formation of hard Ti-based intermetallics around Si₃N₄. When brazed with Ag68.8Cu26.7Ti4.5 filler, thick Cu-Ti intermetallics were formed at Ti/Nb interface, but did not cause strength degradation at room temperature (RT). Instead, fracture occurred at Nb/Si₃N₄ interface due to residual stress, where the relative residual stress was characterized by the fracture morphology of Si₃N₄. Brazing at 1153 K with holding time of 10 min achieved the highest strength of 181 MPa with minimal fracture in Si₃N₄, an indication of low residual stress. Nanoindentation tests revealed that fine Ag-Cu interlamellar spacing and excessive CuTi precipitates resulted in an increase in Young’s modulus. Such microstructure restricted residual stress accommodation, eventually leading to strength degradation. When tested at elevated temperatures up to 873 K, despite degradation due to weakening of filler’s intrinsic strength and oxidation embrittlement, overall strength improved attributed to residual stress relaxation. Contrarily, when brazed using Au96.5Ni3Ti0.5 filler at 1323 K for 0 min, the highest RT strength was 53 MPa, where fracture occurred at Nb/Si₃N₄ interface due to combination of weak interfacial bonding and residual stress. When brazed with 1323 K for 10 min, thickening of Au₄Ti resulted in instantaneous fracture at Ti/Nb interface due to its intrinsic brittleness.