Brazing of Ceramics (Alumina) and Metal
Brazing ceramics and metals – especially the joining of alumina (Al₂O₃) to metallic components – is one of the core competencies of Alumina Systems. In high-tech applications, ceramic-to-metal joints must not only withstand mechanical loads but also be vacuum- or gas-tight, temperature-resistant, and corrosion-proof. This is where brazing provides an ideal solution: it creates strong, durable, and hermetic bonds between ceramics and metals.
Why Alumina (Al₂O₃)? Properties and Challenges
Alumina ceramics (Al₂O₃) – often simply called alumina – is one of the most widely used technical ceramics thanks to its outstanding material properties:
Extremely high hardness and wear resistance
Excellent temperature and thermal stability
Low coefficient of thermal expansion (CTE) compared to most metals – crucial for minimizing stress within the joint
The main challenge when brazing alumina to metal lies in overcoming the very different thermal expansion coefficients and the dissimilar chemical and mechanical surface behaviors. Without the proper surface preparation, brazing alloys, and process control, stresses and cracks may occur, compromising the integrity and tightness of the ceramic-metal joint.
What Is Brazing? Definition and Classification
According to ISO standards, brazing refers to joining processes with a working temperature above 450 °C. Typical brazing alloys (brazes) contain silver, copper, or nickel – sometimes with active elements like titanium or zirconium that enable direct wetting of ceramic surfaces (so-called active brazing alloys).
Two main process routes are used at Alumina Systems:
Conventional (passive) brazing – The ceramic is first metallized to allow wetting by the braze alloy.
Active brazing – Special brazing alloys with reactive elements (e.g., Ti, Zr, Hf) create a direct bond to the ceramic surface without prior metallization.
Both processes are mastered and optimized in-house by Alumina Systems.
Key Parameters in Alumina–Metal Brazing
To achieve high-quality, long-lasting ceramic-metal joints, the following process parameters must be tightly controlled:
Coefficient of Thermal Expansion (CTE)
Choosing a metallic partner with a similar low CTE minimizes thermal stress. Preferred alloys: Ni42 or Kovar.Ceramic Metallization
Application of Mo-Mn or W-Mn paste, fired at ~1400 °C in forming gas.
Subsequent galvanic nickel plating for corrosion resistance and improved wettability.
Brazing Alloy and Temperature
Silver-copper eutectic alloys (AgCu) with a melting point around 779 °C are common.
Active brazes with titanium or zirconium require higher temperatures and vacuum conditions.
Atmosphere Control
Oxygen-free environments (vacuum, inert gas, or forming gas) prevent oxidation.
Clean, oxide-free, and grease-free surfaces are essential for wetting and bonding.
Joint Design and Gap Geometry
Optimal brazing gap ensures capillary flow of the braze.
Well-shaped fillets increase mechanical strength and relieve local stress.
Proper design of component thickness and transition zones reduces residual stress.
Cooling and Stress Management
Controlled cooling avoids thermal shock.
Flexible intermediate layers (e.g., thin copper foils) may absorb residual stresses—an approach used successfully by Alumina Systems.
Typical Applications
Vacuum-tight ceramic-metal feedthroughs for high-vacuum and electron-tube applications
High-temperature components requiring electrical insulation and structural stability
Housings and sensor interfaces combining electrical isolation with metallic connection
Hermetic seals for power electronics, medical devices, and aerospace components
Advantages and Limitations of Brazing
Advantages
Strong, durable, and hermetic bonds between ceramic and metal
Excellent resistance to temperature, pressure, and aggressive media
Stable performance under thermal cycling
Proven reliability in demanding industrial applications
Limitations
Process complexity and higher cost due to special materials and equipment
Thermal stress risks if design or material selection is inadequate
Active brazing requires precise atmosphere and temperature control
Quality Control at Alumina Systems
Each brazed component undergoes extensive testing to ensure maximum reliability:
Mechanical strength (tensile, shear, and peel tests)
Leak tightness – vacuum and gas leak rates down to < 1×10⁻¹¹ Pa·m³/s
Thermal cycling – to verify temperature-change resistance
Corrosion resistance – testing under humidity and harsh environments
Microstructural analysis – examination of the brazing fillet and interface for cracks, porosity, and reaction layers
Conclusion
Brazing alumina to metal is a complex but highly reliable joining technology that demands precise control of materials, atmospheres, and process parameters. Alumina Systems combines decades of know-how in metallization, passive and active brazing, and quality testing to produce robust, vacuum-tight ceramic-metal assemblies that meet the highest technical requirements in industry, research, and high-tech engineering.