Vacuum Brazing of Ceramic Metals

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Engineering ceramics has developed into a generally recognized high-performance structural material due to its excellent high temperature resistance, corrosion resistance and wear resistance, but its application is limited due to its poor plasticity and impact resistance. Only by combining the strength and toughness of metal with the wear resistance, corrosion resistance and high temperature resistance of ceramics can it become an ideal structural material. To solve the connection problem between ceramics and metals, vacuum brazing is one of the most ideal ways.

However, ordinary metal materials neither wet nor adhere to the surface of the ceramic. Active metals must be added to make the liquid metal solder wet and adhere to the ceramic through the interfacial chemical reaction. The presence of a highly gradient stress-affected zone weakens the joint strength. Due to the existence of these two chemical and physical factors, it has become a great obstacle to the wide application of ceramic/metal materials.

The main difficulty of ceramic/metal vacuum brazing is metallurgical incompatibility and physical property mismatch. The former is that the brazing filler metal does not wet the ceramic after melting, and it is difficult to achieve metallurgical bonding between the atoms of the ceramic in the welding zone; the latter is that the thermal expansion coefficient of the ceramic/metal is too different, and there is a large stress gradient in the brazing joint area. The thermal stress generated by welding makes the connection joints low in strength and difficult to meet the quality requirements.

To solve the mismatch of physical properties of ceramic/metal vacuum brazing, the method of adding a buffer layer is generally used. The buffer layer is divided into a soft buffer layer, a hard buffer layer and a soft/hard double-layer buffer layer. The thermal expansion coefficient of the soft buffer layer is high, and the residual stress problem caused by thermal expansion mismatch can be solved by being sandwiched between the metal brazing filler metal and the ceramic. In order to solve the problem of mismatch of physical properties between ceramics and metals, some people use self-propagation technology to form a transition layer with gradient changes in chemical composition and mechanical properties between ceramics and metal materials through the thermochemical reaction of powder, and the effect is also ideal.

The vacuum brazing mechanism of ceramics/metals is generally a “wetting” mechanism, that is, the connection between ceramics and metals lies in the degree of infiltration of the molten solder to the ceramics. According to the combination of ceramic/metal, the wetting process can be divided into non-reactive wetting and reactive wetting.

For non-reactive wetting, the driving forces for wetting are mainly van der Waals and dispersion forces. In this wetting system, the wetting is generally poor, and the wetting proceeds very quickly, reaching equilibrium within about 10-3s. Wettability does not have much relationship with temperature change, but has a great relationship with ceramic metallization and orientation and addition of alloying elements.

The broad application prospects of ceramic-metal connection materials make the research on ceramic/metal vacuum brazing connection technology and theory receive widespread attention. To solve the problem of vacuum brazing of ceramics/metals, the “wetting” mechanism can explain some practical problems in welding. For example, only the brazing filler metal that can fully wet the ceramics is suitable for the connection between ceramics and metals.

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