Active brazing is a method for producing reliable and cost-effective ceramic-to-ceramic and ceramic-to–metal joints. The suitability of three different active filler metals Ag98.6In1Ti0.6, Cu74.5Sn14.0Ti10.0Zr1.5 and Ag59.0Cu27.25In12.5Ti1.25 for joining Si₃N₄/TiN composite ceramics for application at elevated temperatures (up to a maximum of 350C) was studied. Ceramic-ceramic and steel-ceramic joints were brazed and tested in bending as well as in tension up to 600C. Fractographic investigations were performed to investigate the joint failure mechanism. A Scanning Electron Microscope (SEM) with an integrated Energy Dispersive X-Ray Analysis (EDX) was used for microstructural analysis of the brazed zone. In addition, the micro-hardness was measured across the brazed zone (from one joining partner to other). The micro-hardness of the various phases of the brazing filler metal in the as-melted (in crucible) and as-brazed (in a joint) was compared.
The amount of the active element titanium has to be very carefully optimised under consideration of the properties of the joining partners. Increasing amounts of Ti up to a critical value improves the wetting behaviour of the filler metal on the ceramic. However, the variation of Ti content also changes the spreading behaviour of the brazing filler and strongly affects its reactivity. As a result, undesirable brittle reaction products may form at the interface between the metallic joining partner and the filler metal, thereby altering the properties of both of the metallic joining partner and filler metal. In particular, the brazing filler may lose its ductility which is essential for reducing residual stresses - a critical aspect in metal-ceramic joints.