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  • R. Smith says:

    The short answer is “yes”… but that deserves some explanation to your points to explain how active brazing of ceramics work.

    Active brazing depends on a “substitution” reaction between the metal atoms in a ceramic and the active metal atom in the braze filler. In the case of aluminum oxide and CuAgTi active braze fillers, the Ti atom is substituted in a reaction with the Al atom in the aluminum oxide. The literature shows that this reaction needs temperatures of at least 800C to drive the reaction. As mentioned in the other post in this blog, active brazing by definition uses reactive elements such as Ti, Hf, and Zr. All with strong affinity to react with oxygen, nitrogen and/or carbon. These elements when reacted with a ceramic containing these elements, then when another less reactive atomic element is in the structure of the ceramic, the “active” elements will take the place of the other metal to locally form a oxide, nitride or carbide with it. When this reaction happens, the active element (Ti) in the brae filler then enables the other metals in the braze filler to wet and adhere to the ceramic. Active braze bonds will occur over a very narrow zones, normally much less the 1 micron, which is very different than metal braze bonds which occur over 50 – 100 microns. Diffusion of the active metals into the ceramic are limited by the formation of the new compounds such as Ti oxide as the Ti reacts with aluminum oxides.

    Now to the need for vacuum. Yes clean, high vacuum is needed, normally well below 10 – 4 torr. Also metal wall furnaces are superior to graphite lined vacuum furnaces since porous graphite linings can offgass and for contamination, even if the vacuum is “high”. The reason for clean, high vacuum is that 800C and above, the presence of active elements such as Ti, Hf and Zr, will immediately create reactions with the oxygen and/or nitrogen in the furnace atmosphere (even in vacuum and use up the active elements in the molten braze filler and leave contaminating phases which prevent the reactions of the active phases with the ceramic and /or leave contaminated layer that will prevent the wetting of the metal filler to the ceramic surfaces. VERY low oxygen and/or nitrogen levels (measured in the PPM level or below) are required and only high vacuum has shown itself to work consistently.

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