Ceramic brazing technology
Various brazing joints formed by ceramic metallization, due to the existence of coating or coating, restrict the use of ceramic elements at high temperature. It is difficult to give full play to the advantages of high temperature stability of ceramics. It 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 geometrical dimensions. It can greatly simplify the brazing process and meet the high temperature requirements of ceramic components. The method USES filler metals containing Ti or Zr active elements for direct brazing of metals, ceramics and ceramics, ceramics and graphite brazing ceramics.
The solder used for direct brazing has higher melting temperature and can meet the requirement of high temperature application of ceramic elements. To avoid cracking of component materials due to different thermal expansion coefficients, buffer layers can be inserted between them. Depending on the conditions of use, the filler metal shall be sandwiched between the brazed parts as far as possible, or placed in the position of filling gap with the filler metal, and then brazed like ordinary vacuum brazing
For cermet, such as sic ceramics containing 10% free silicon and less than 500ppm iron and aluminum, germanium powders can be directly brazed using germanium powders as filler metals. Germanium powder for 20 to 200 microns thick, background vacuum pump to the 10-2 pa, vacuum degree of work for 8 x 10-2 pa, brazing temperature is 1180 ℃, holding time for 10 min. The brazing seam structure for Ge - Si solid solution, remelting temperature as high as 1200 ℃, can be used in high temperature environment.
Alumina ceramics are usually brazed with alkali - resistant metals Ta, Nb and alloys in the production of heat exchangers and infrared sources. During brazing process, a layer of tungsten or molybdenum is sprayed on the surface of alumina ceramics by plasma spraying equipment. The prepared solder mixture (Ni powder + Fe powder 17% or Nb powder + Ni powder 15%) is then mixed with the binder and ground in a roller for several hours. To suspend the filler powder in the binder, apply it to the interface between the ceramic and the metal, with a thickness of 0.125-0.25mm. After a few hours of drying, the filler metal is brazed in a vacuum furnace. Cold background vacuum pump to 8 * 10-3 Pa, vacuum degree of work not less than 3 * 10-2 Pa, brazing temperature 1500-1675 ℃, holding time 5 to 10 minutes. The obtained brazing joints are compact, with tensile strength up to 140MPa and corrosion resistance of joints. Braze welding head can withstand temperatures above 1000 ° C.
Due to the poor thermal conductivity of ceramics, the brazing heating rate can not be too fast. When brazing insulation, it takes a long time to reach a uniform temperature. In addition, poor plasticity and improper heating or cooling rate can cause deformation and lead to ceramic cracking. Therefore, the correct selection of brazing parameters is more important for ceramic brazing.