The Mn-Mo Braze Process vs. Active Filler Metal Braze Joining

The Mo-Mn process for ceramic:metal bonding has been used since the 1930’s to join metal surfaces to ceramics. The Mo-Mn process forms a composite layer that is reacted in-situ to form a glass:metal composite layer that can be nickel plated to form a metallized base layer. The glass metal layer is formed by applying a Mo, MoO3, Mn, and MnO3 mixture with various glass forming compounds into a slurry that is applied to the ceramic (alumina) surface. The slurry layer is then reduced / oxidized in a wet H2 atmosphere to create a Mo-Mn metal with glass composite that can be plated. After Ni-plating conventional Cu-Ag braze filler metals that melt from 780 – 900 C can be reacted to bonded to the Ni layer of the joint.

Active metal fillers on the other hand melt and react directly with the alumina (ceramic) surfaces. Active brazes are those braze filler that have had reactive elements such as Ti, Hf or Zr added to Cu-Ag or other braze filler metal bases. When active braze fillers with reactive Ti, Hf and/or Zirconium melt on the ceramic above 850C in vacuum (to prevent reactive metals from oxidizing) the active element substitutes itself into the atomic structure of the ceramic to create a binding chemical compound to form a chemical bond between the active braze and the ceramic. The metals in the molten braze filler also interact and from compounds and/or interdiffuse into the metal side of the joint and thus chemically bond to the metal side at the same time as the braze binds to the ceramic to form a ceramic:metal bond.

Either Mo-Mn braze process or active brave process have been used and continue to be used for ceramic:metal bonding. However the choice between the two depend on a lot of factors and design considerations.

  1. Mo-Mn process braze joint are generally more tolerant of stress and cyclic service conditions. Due to the composite layered nature of the joint, CTE mismatch and stresses are accommodated better.
  2. Active brazing is a simpler process with less steps and not dependent on
  3. Mo-Mn process brazing can be done in conventional braze atmospheres on under good controls may be flame brazed with the presence of a fluxing compound.
  4. Active brazes are “fluxless” but require brazing in clean, high vacuum atmospheres to protect the reactive elements in the braze filler metals.
  5. Actively brazed joint are less tolerant thermal cycles in service… since there is a direct ceramic:metal bond, the narrow reaction zones that bond the ceramic to metal can fracture.
  6. Active braze filler metals are readily available commercially while long processing steps to get the Mo-Mn layer applied by “qualified” suppliers with the right mixtures and equipment are not as readily available… hence active metal brazing is a more available for faster turn times and easier to arrange for small braze runs.

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