1. Overview
Ceramics in the welding of ceramics and metals basically refer to new ceramics that are artificially synthesized from various metals, oxygen, nitrogen, carbon, etc. It has the characteristics of high strength, high temperature resistance, wear resistance, corrosion resistance, super hardness, etc., and is widely used; commonly used ones include alumina, silicon nitride, zirconia ceramics, etc.
2. Difficulties in welding ceramics and metals
1. The linear expansion coefficient of ceramics is small, while the linear expansion coefficient of metal is relatively large, resulting in easy cracking of the connection. Generally, the thermal stress problem of the metal intermediate layer must be well handled.
2. The thermal conductivity of ceramic itself is low and its thermal shock resistance is weak. During welding, the temperature gradient in and around the welding part should be reduced as much as possible, and the cooling rate should be controlled after welding.
3. Most ceramics have poor conductivity or even no conductivity, making it difficult to use electric welding. Special process measures need to be taken for this purpose.
4. Since ceramic materials have stable electronic coordination, it is unlikely that metals and ceramics will be connected. Ceramics need to be metallized or soldered with active filler metal.
5. Since ceramic materials are mostly covalent crystals, they are not prone to deformation and brittle fracture often occurs. At present, most use the intermediate layer to reduce the welding temperature and use the indirect diffusion method for welding.
6. The structural design of ceramic and metal welding is different from ordinary welding. It is usually divided into flat sealing structure, enveloping structure, pin sealing structure and counter-sealing structure. Among them, the encapsulating structure effect Best of all, these joint structures are very demanding to make.
3. General process for welding ceramics and metals
1. Cleaning: The surface of metal and solder must be cleaned. Ceramics commonly use detergents and ultrasonic cleaning.
2. Paste: The paste is mostly composed of pure metal powder and appropriate metal oxide powder. The particle size is mostly between 1~5um. It is prepared with an organic binder to have a certain viscosity. of ointment. Then use a painting tool to evenly apply the paste on the ceramic surface to be metallized. The coating thickness is generally 30~60um.
3. Metallization: Send the pseudo-ceramic piece coated with paste into a hydrogen furnace and keep it at a temperature of 1300~1500℃ for 1 hour.
4. Nickel plating: For better solder wetting, a nickel layer about 5um thick is electroplated on the metallized layer. When the brazing temperature is lower than 1000°C, the electroplated layer needs to be pre-sintered in a hydrogen furnace at 1000°C for 15~20 minutes.
5. Rack assembly: Assemble the processed metal parts and ceramic parts into a whole using stainless steel, graphite, and ceramic molds, and install brazing on the joints; during the entire operation The parts to be welded should be kept clean and should not be touched with bare hands.
6. Brazing: Brazing is performed in a furnace with argon gas or hydrogen gas or in a vacuum furnace. The temperature selection, heating rate selection, etc. should be based on the use. Determined by the characteristics of the solder, special attention should be paid to the cooling speed not being too fast to prevent the ceramic parts from cracking due to temperature stress.
7. Post-weld inspection: After welding of ceramics and metals, first visual inspection is required, and then necessary inspections are carried out according to the requirements of the technical documents.
