Gas-pressure-sintered silicon nitride (Si 3 N 4 ) ceramics were increased in density and mechanical properties without impairing the thermal conductivity by the addition of TiH 2 . After annealing treatment of the sintered ceramics, the inherent properties were further improved, the grain boundary phases on the surface were evaporated, and the rugged surface microstructure was formed. Si 3 N 4 ceramics before and after the annealing treatment were then joined with oxygen-free copper (OFC) foil using Ag-Cu-TiH 2 filler. Two interfacial structures, planar and rugged, were formed in the Si 3 N 4 ceramic/OFC foil joints, respectively. The peeling strength of the joints brazed with annealed Si 3 N 4 ceramics was improved, attributed to the increased inherent strength of the ceramics and rugged interface of the joints. The joint reached a maximum peeling strength of 25.08 N/mm, in which the fracture occurred mainly in the surface layer of the Si 3 N 4 ceramic as well as between the brazing alloy and the OFC foil.
Gas-pressure sintered silicon nitride (Si 3 N 4 ) ceramic was brazed to oxygen-free copper (OFC) foil using 20.22 ± 0.93 mg/cm 2 of Ag-Cu-TiH 2 filler at 875 ◦ C for 0.5 h. The effect of TiH 2 content on the 3D morphology, including the cross-section microstructure and etched surface morphology of Si 3 N 4 ceramic/OFC foil joints, was analyzed. An evolution model of the interfacial microstructure for joints was proposed based on 3D morphological analysis. The reaction between the brazing alloy and Si 3 N 4 ceramic formed the interfacial reaction layer and the inside reaction zones during brazing. The growth mechanism of the interfacial reaction layer was discussed in detail. The peeling test was used to evaluate the bonding strength of the Si 3 N 4 ceramic/OFC foil joints, and the optimum peeling strength of 25.1 N/mm was achieved at 5 wt% TiH 2 content. The interfacial microstructure of the joints changed with TiH 2 content, leading to different main fracture mechanisms and corresponding peeling strengths.
Complex liquid–solid reactions occur during brazing. A deep understanding of brazing liquid evolution is therefore of important concern for the bonding between filler and substrates. The joining of gas-pressure-sintered silicon nitride (Si 3 N 4 ) ceramic to oxygen-free copper (OFC) foil was prepared using Ag–Cu–TiH 2 filler at 875°C for 0.5 h. The effect of the Ag:Cu ratio on the interfacial microstructure and peeling strength of the joints was analyzed. The reaction of brazing liquid with Si 3 N 4 ceramic includes reactions with both the β-Si 3 N 4 and the grain boundary phases (Y–Si–Mg–O–N). The resulting interfacial reaction products include TiN, Ti 5 Si 3 , and potential Cu 3 Ti 3 O. Based on the phase composition and three-dimensional morphology of the interfacial reaction products in the Si 3 N 4 ceramic/OFC foil joints, a model for the evolution of the brazing liquid as the reaction proceeds was proposed. Peeling test results indicated that the peeling strength of the joints rises unilaterally with the increase of the Ag:Cu ratio in the filler until reaching the peak value of 24.3 N/mm at the maximum Ag:Cu ratio of 90:10. The mechanisms of the joint peeling fracture were discussed in detail.
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