All kinds of brazing joints formed by ceramic metallization, because of the existence of coating or coating, restrict the use of ceramic components at high temperature. It is difficult to give full play to the superiority of high temperature stability of ceramics. It requires people to find a direct brazing method.
Direct brazing of ceramics can solve the problem of connection of parts that can not be metallized completely because of blind hole or geometric dimension factors. It can greatly simplify the brazing process and meet the requirements of high temperature use of ceramic components. This method uses brazing fillers containing Ti or Zr active elements to braze ceramics directly with metals, ceramics and ceramics, ceramics and graphite.
The solder used for direct brazing has higher melting temperature, which can meet the requirements of high temperature application of ceramic components. In order to avoid cracks in component materials due to different thermal expansion coefficients, a buffer layer can be inserted between them. According to the use conditions, the filler metal should be clamped between two brazed parts as far as possible or placed in the position of filling gap with filler metal, and then brazed like general vacuum brazing
For metal ceramics, such as silicon carbide ceramics containing 10% free silicon and less than 500 ppm of iron and aluminium, germanium powder can be directly brazed by using germanium powder as filler metal. The thickness of germanium powder is 20-200 um, the background vacuum is pumped to 10-2 Pa, the working vacuum is 8 x 10-2 Pa, the brazing temperature is 1180 °C and the holding time is 10 min. The obtained brazing seam structure is Ge-Si solid solution, and the remelting temperature is as high as 1200. It can be used in higher 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. 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 solder (Ni powder + Fe powder 17% or Nb powder + Ni powder 15%) is mixed with binder and ground in the drum for several hours. In order to make the filler powder suspended in the binder) coated on the interface between ceramics and metals, the thickness is 0.125-0.25 mm. After several hours of drying, the filler metal is brazed in vacuum furnace. The vacuum degree of cold background 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 °C, and the holding time is 5-10 min. The obtained brazing seam is compact, the tensile strength can reach 140 MPa, and the joint corrosion resistance can be achieved. The brazing joint can withstand high temperature above 1000 °C.
Because of the poor thermal conductivity of ceramics, the heating rate of brazing can not be too fast. It takes a long time to achieve uniform temperature when brazing is kept warm. In addition, poor plasticity, inappropriate heating or cooling rate will cause deformation and lead to cracking of ceramics. Therefore, the correct selection of brazing parameters is more important for ceramic brazing.