Vacuum brazing of high temperature ceramic and metal joints

1. Vacuum brazing of structural ceramics
Structural ceramics (such as Si₃N₄, SiC, Al₂O₃) have been widely used in mechanical processing, chemical industry, electronics, aerospace and other fields due to their excellent high temperature resistance, corrosion resistance and wear resistance. The reliable connection between ceramics and metals can give full play to the performance advantages of both and has become a hot topic of current research. In recent years, the application of ceramic-metal joints in electric vacuum devices and medium and low temperature environments has been relatively mature. However, applications at higher temperatures (>700℃) and greater stress environments still face challenges, such as thermal expansion mismatch, insufficient joint strength and poor long-term high temperature stability. Therefore, it is of great significance to develop new processes and new materials for high temperature resistant ceramic-metal joints.
2. Vacuum brazing of high temperature resistant ceramic-metal joints
Vacuum brazing is one of the most commonly used methods for ceramic-metal connection. The latest research shows that the optimized design and process improvement of high temperature resistant brazing filler metals significantly improve the performance of the joints.
3. Problems with vacuum brazing
High connection temperature: usually above 1000℃, even up to 1450℃, which may cause ceramic grain boundary softening or decomposition; Strict environmental requirements: high vacuum or protective atmosphere (such as Ar, N₂) is required, and the equipment cost is high; Large performance dispersion: Especially in large-area joints, uneven wetting leads to inconsistent strength; Cost issues: Brazing materials containing precious metals (such as Au, Ag, Pt) are expensive, which limits large-scale applications.
4. Vacuum diffusion welding of high-temperature resistant ceramic-metal joints
Vacuum diffusion welding forms a metallurgical bond through solid-state diffusion, which is an important method for high-temperature resistant joints. In recent years, breakthroughs have been made in the optimization of the intermediate layer.
Characteristics of vacuum diffusion welding process
Under optimized temperature and pressure, the joint strength is significantly improved. For example, Si₃N₄-Mo uses a V intermediate layer with a shear strength of 70 MPa at 700°C; Si₃N₄-Si₃N₄ uses a Ni-20Cr intermediate layer, and the room temperature strength is still 347 MPa after 900°C × 100 hours of heat treatment; New process: Si₃N₄ and wear-resistant steel are optimized at 1200°C, and the shear strength is increased to 97.5 MPa.
5. Problems with vacuum diffusion welding
High temperature requirements: The connection temperature is usually between 1100°C and 1700°C, which is easy to damage the ceramic or metal base material; Complex process: High vacuum and precise pressurization are required, and the production efficiency is low; Insufficient high temperature performance: When the joint is in service for a long time above 700°C, the strength decays significantly, which is difficult to meet the needs of aircraft engines, etc.
According to the latest technical standards and research progress, the following conclusions can be drawn:
Vacuum brazing: The process is mature and the joint performance is good, but high temperature (>1000℃), high cost (precious metals) and narrow application range limit its development. Low-cost, low-melting point brazing materials need to be developed in the future.
Vacuum diffusion welding: The joint has strong high-temperature stability, but the high process temperature (1100℃～1700℃) and complexity make it difficult to mass produce. Optimizing the middle layer and reducing the temperature are the key directions.

Vacuum Brazing Furnace