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Ceramic Direct Brazing Process
- Aug 28, 2018 -

Various brazing joints formed by ceramic metallization are limited in the use of ceramic components at high temperatures due to the existence of coatings or coatings, which makes it difficult to give full play to the advantages of high temperature stability of ceramics and requires people to find direct brazing methods.

Direct brazing of ceramics can solve the connection problem of parts that cannot be completely metallized due to blind holes or geometric size factors, greatly simplify brazing process and meet the requirements of high temperature use of ceramic components.  This method uses brazing filler metal containing ti or Zr active elements to directly braze ceramics and metals, ceramics and ceramics, ceramics and graphite, etc.  The brazing filler metal selected for direct brazing has a relatively high melting temperature, which can meet the requirements of high temperature use of ceramic components.  In order to avoid cracks caused by different thermal expansion coefficients of component materials, a buffer layer can be sandwiched between them.  According to the usage conditions, the brazing filler metal should be sandwiched between the two parts to be brazed as far as possible or placed in a position where the gap is filled with brazing filler metal, and then brazing is carried out like general vacuum brazing.

For cermets, such as silicon carbide ceramics containing 10 % free silicon and less than 500 ppm iron and aluminum, germanium powder can be used as brazing filler metal for direct brazing. germanium powder is sandwiched between ceramic parts. the thickness of germanium powder is 20 - 200 μ m, the background vacuum degree is 10 - 2 pa, the working vacuum degree is 8× 10 - 2 pa, the brazing temperature is 1180 ℃ and the heat preservation is 10 min. the obtained brazing seam structure is ge - si solid solution. the remelting temperature is as high as 1200 ℃ and can be used in higher temperature environments.  In the production of heat exchangers and infrared radiation sources, alumina ceramics and alkali-resistant corrosion metals ta, nb and alloys are often brazed together. the brazing process uses plasma spraying equipment to spray a layer of tungsten or molybdenum on the surface of alumina ceramics. then the prepared mixed brazing filler metal ( ni powder + Fe powder 17 % or nb powder + ni powder 15 % ) is mixed with binder and placed in a roller for grinding for several hours so as to promote the brazing filler metal powder to be suspended in binder ) is coated on the bonding surface of ceramics and metals. the thickness is 0.125 - 0.25 mm. after drying for several hours, it is brazed in a vacuum furnace. the cold state version is obtained.

Due to the poor thermal conductivity of ceramics, the rate of brazing heating cannot be too fast. it takes a long time to achieve uniform temperature when brazing and heat preservation.  In addition, poor plasticity and improper heating or cooling rate will cause deformation and crack of ceramics.  Therefore, the correct selection of brazing process parameters has more important significance for ceramic brazing.

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