A number of factors such as the brazing filler composition, brazing process parameters, gap thickness, properties of the joining partners, joint geometry, etc., can influence brazed joint performance. The complex relationships between these parameters make joint optimization a difficult task. The effects of some of these factors can be predicted, others have to be determined after processing and joint characterization. The fractographic analysis of joints provide important information on the failure origin and hence about the “weakest link” in the joint. The comparison of the crack path with the simulated stress distribution helps to distinguish between the processing oriented and design based failures.
Ceramic-ceramic and ceramic-metal joints were brazed using different Cu- and Ag-based active brazing filler metals and characterized by four point bending tests. Si3N4/TiN was used as the ceramic partner and 14 NiCr 14 steel as the metal joining partner. AgCuInTi, AgInTi and CuSnTiZr brazing filler metals with differing amounts of the active element titanium varying between 0.6 and 10.2 wt.% and different working temperatures and ductilities. To minimize the residual stresses the AgCuInTi filler was modified with SiC particles, the brazing gap thickness was controlled in the range between 0.1 and 1 mm.
From each brazing batch the specimens with the highest and lowest values of bending strength were fractographically analyzed. Different joint failure behavior patterns were observed depending on the filler metal used. High residual stresses and weakening of the ceramic partner near the braze-ceramic interface were found to be the main causes for the failure of ceramic-metal joints brazed with CuSnTiZr. The stresses were reduced and the strength optimized by applying a Cu-interlayer. In the joints with AgCuInTi the influence of residual stresses on the mechanical strength could also be observed. Under these conditions the joints were successfully optimized.

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