Ceramic Brazing

Contrary to popular belief, ceramics can be brazed.  Given that it is very difficult to machine ceramics, brazing offers a unique opportunity to produce a composite where the ceramic component is exposed to the harsh environment whereas on the other side a metallic component is able to facilitate easy connectivity.  Ceramics can also be brazed to glass and to other ceramics.  Of course, the ideal solution will depend on the geometry, materials, and application requirements.  Alloys used to braze metals are unable to react and bond to ceramics and hence the need for active brazing alloys.  As the name suggests, active metal brazes have an active component which is able to react with the ceramic.  The most common active metal is Ti used in the Ag-Cu-Ti ternary composition.  Other active metals such as Zr have been experimented as well.  The active element is usually limited to less than 5% and is sufficient to uniformly attack the ceramic surface, reduce the ceramic and form a strong bond.  Because the active element is very reactive, the brazing process has to be carried out in either vacuum or an inert environment such as under Argon.

Examples include brazing of ceramic turbines to steel shafts in turbochargers.  Some race-cars also have a engine valves made of ceramic on the hot end and a metal shaft on other; ceramic valves allow high temperature operation whereas the metal end allows impact loads against the rocker arms.  Ceramic-to-ceramic brazing has been successfully used in producing high voltage vacuum interrupters for power stations.

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