Vacuum brazing samples of ceramic, copper and Kovar Alloy

Due to the existence of coating or coating, ceramic components are limited in the use of high temperature, so it is difficult to play the advantages of high temperature stability of ceramics. People are required to look for direct brazing methods.

Direct brazing of ceramics can solve the connection problem of parts which can not be completely metallized because of blind hole or geometric size. It can greatly simplify the brazing process and meet the requirements of high temperature use of ceramic components. This method is to directly braze ceramics and metals, ceramics and ceramics, ceramics and graphite with filler metals containing Ti or Zr active elements. The melting temperature of brazing filler metal for direct brazing is higher, which can meet the requirements of ceramic components in high temperature state. In order to avoid cracks caused by different thermal expansion coefficients, a buffer layer can be sandwiched between them. According to the service conditions, the brazing filler metal should be clamped between the two parts to be brazed as far as possible or the position where the filler metal is used to fill the gap, and then the brazing is carried out like the general vacuum brazing.

For cermets, such as silicon carbide ceramics containing 10% free silicon and less than 500ppm iron and aluminum, germanium powder can be directly brazed. The thickness of germanium powder is 20-200 μ m, the background vacuum degree is 10-2Pa, the working vacuum degree is 8 × 10-2Pa, the brazing temperature is 1180 ℃, and the holding time is 10 min. The microstructure of brazing joint is Ge Si solid solution, The remelting temperature is as high as 1200 ℃, which can be used in high temperature environment. In the production of heat exchangers and infrared radiation sources, alumina ceramics are often brazed with alkali resistant metals TA, Nb and alloys. Plasma spraying equipment is used to spray a layer of tungsten or molybdenum on the surface of alumina ceramics. Then, the prepared mixed solder (Ni powder + Fe powder 17% or Nb powder + Ni powder 15%, mixed with binder) is put into the roller and ground for several hours, After drying for a few hours, the vacuum is pumped to 8 × 10 – 3 Pa, the working vacuum degree is not less than 3 × 10 – 2 PA, the brazing temperature is 1500 ℃～ 1675 ℃, and the holding time is 5 ～ 10 min, The joint has good corrosion resistance and the brazing seam can withstand the high temperature above 1000 ℃.

Due to the poor thermal conductivity of ceramics, the heating rate of brazing can not be too fast, and it takes a long time to achieve the temperature uniformity during brazing; in addition, poor plasticity, improper heating or cooling rate will cause deformation and ceramic cracking. Therefore, the correct selection of brazing process parameters is more important for ceramic brazing.