Ceramics, copper, Kovar alloy vacuum brazing samples

Various brazing joints formed by ceramic metallization have coatings or coatings, which limit the use of ceramic components at high temperatures, and it is difficult to exert the advantages of ceramic high-temperature stability. People are required to find direct brazing methods.

Ceramic direct brazing can solve the connection problem of parts that cannot be fully metallized due to blind holes or geometric dimensions, and can greatly simplify the brazing process and meet the requirements of high-temperature use of ceramic components. This method uses brazing filler metals containing Ti or Zr active elements to directly braze ceramics and metals, ceramics and ceramics, ceramics and graphite. The brazing material selected for direct brazing has a higher 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 the two. According to the conditions of use, the solder should be clamped between the two parts to be brazed or placed in a position where the gap is filled with the solder as much as possible when selecting the solder, and then brazing is performed like 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 used as the solder for direct brazing, and the germanium powder is sandwiched between the ceramic parts. The thickness of the germanium powder is 20～ 200 μm, the background vacuum is pumped to 10-2Pa, the working vacuum is 8×10-2Pa, the brazing temperature is 1180℃, and the holding time is 10min. The obtained brazing seam structure is Ge-Si solid solution, and the remelting temperature is as high as 1200℃. Can be used in higher temperature environments. In the production of heat exchangers and infrared radiation sources, alumina ceramics are often brazed with alkali corrosion resistant metals Ta, Nb and alloys. The brazing process uses plasma spraying equipment to spray a layer on the surface of alumina ceramics. Tungsten or molybdenum, then mix the prepared mixed solder (Ni powder + Fe powder 17% or Nb powder + Ni powder 15%), mix it with a binder, and place it in the drum for several hours to suspend the solder powder Coated on the bonding surface of ceramic and metal with a thickness of 0.125～0.25 mm. After drying for a few hours, put it into a vacuum furnace for brazing. The cold background 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 heat preservation is 5～10 min. The brazing seam obtained in this way is dense, the tensile strength can reach 140MPa, the joint corrosion resistance is good, and the brazing seam Can withstand high temperatures above 1000°C.

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