Brazing joins ceramics and metals in a component used in nuclear particle research.

Subassemblies that have ceramics and metals joined by brazing.

Technical ceramics are used in a wide range of electronics and engineering applications for their chemical and mechanical properties. Compared to metals, they are stronger in compression, especially at higher temperatures, have good thermal stability, a low coefficient of thermal expansion, and good thermal and electrical resistance. They also are hard, and have excellent dimensional stability.

Because of their desirable properties, the list of applications for technical ceramics is long and varied, including, for example, aerospace engine blades, ring and valve components, industrial pump bearings, cutting tools and die parts, and medical instruments, and they are widely used in the electronics industry as a substrate and in specialized vacuum components.

Ceramic-metal bonding
For many applications it is often necessary to join ceramic to metal to create the finished part, and ceramic- metal bonding is one of the biggest challenges that has faced manufacturers and users over the years because of the inherent differences in the thermal expansion coefficients of the two types of materials. Various methods are available including mechanical fasteners, friction welding and adhesive bonding but by far the most widely used and effective method for creating a leak-tight, robust joint between ceramic and metal is brazing.

Brazing starts with the chemical bonding – the metallization – of the ceramic to create a wettable surface on the ceramic, on which braze alloy will flow between the two components.

Here are two examples of applications in which the design and manufacture of ceramic components used brazing to produce custom components that range from parts produced in very small volumes to the high volume production of precision parts.

A unique engineering challenge
ISIS, a world-class spallation neutron source based at the CCLRC Rutherford Appleton Laboratory, in Oxfordshire, UK recently commissioned a series of highly specialized metallized ceramic components as part of a major expansion project to build a Second Target Station (TS-2). ISIS is a particle accelerator that is designed to produce neutrons. The target station is the point within the accelerator at which atomic collisions occur.

ISIS commissioned the parts from Morgan Advanced Ceramics.

The components are a fundamental part of the target station’s instrumentation, and are used to monitor the intensity of the extracted proton beam. Ceramic vacuum tubes used in the first target station were sealed with indium wire, but those became unreliable if they were disturbed. Metallized ceramic provided a reliable vacuum seal within the very tight tolerances of the design.

There were two key challenges:
The first was to develop a design and a manufacturing process that would produce a robust, high integrity vacuum seal with a leak rate of 10-8mbar l/s across a component with a 200 mm diameter-(7.87 in.).
The second was to solve the problem of the differences in thermal coefficient between the alumina ceramics of the tube and its mild steel flanges. Further, a very tight specification was set for the physical dimensions and cleanliness of the components because of the nature of the project.

The ISIS assembly is 158 mm (6.22 in.) long with two nickel-plated mild steel flanges that are 240 mm diameter (9.45 in.) and are insulated from each other by a pre-formed, diamond-ground alumina ceramic insulator.

To ensure hermetic integrity of the assembly the ceramic is brazed in a hydrogen/nitrogen furnace at 850 degrees C. to two flanges made of nickel iron cobalt steel. The steel alloys was chosen because it provides the best thermal expansion match to the ceramic. The brazing process is achieved by applying a molymanganese coating that is sintered at 1400 degrees C., onto which a layer of nickel is electroplated.

The ceramic/metal brazed sub-assembly then is welded to the mild steel flanges with a stainless steel interface and machined to the final dimensions.

The order from ISIS was for 13 components, that were to be supplied by the end of 2006.

As is usually the case with this sort of project, there isn’t the time or budget available to produce a prototype to refine the process – so it relied heavily on the experience and expertise of the specialist team to get it right first time. Working together, problems were solved as they arose and all the components have now been delivered. Construction of TS-2 began in July 2003 and first neutron production started in June.

Precision and consistency
In another application, Morgan Advanced Ceramics manufacturers metallized ceramic components for vacuum electronic devices (VEDs) that are used in continuous wave and pulsed radar systems, such as those for fighter aircraft. The challenge was to push the performance envelope of the materials to meet the demand for higher frequencies. This means smaller components that have the same physical properties as their larger counterparts, high precision engineering and close quality control to ensure consistency throughout production. The smallest part made for this application is a cylinder with an internal diameter of 0.2 in. The internal surface is metallized to a very tight thickness tolerance, 0.007 in. to 0.0012 in. by a metallization process based on molybdenum-manganese refractory ink systems developed by Morgan Advanced Ceramics. The metallization process is matched to specific high purity alumina ceramic bodies to ensure consistent high strength bonds. The glass phases in the molybdenum-manganese metallization link with the glass phases in the ceramic to form the bond. The metallized surface receives a secondary coating of nickel to seal and improve wettability for later brazing.

In these applications, advanced ceramics met the needs for higher performance critical components In the future, a detailed understanding of the ceramicmetal bonding techniques, such as the metallization process, could provide designers and manufactures ways to devise these and other components with the help of special expertise to overcome the challenges of joining ceramics to metals.

Morgan Advanced Ceramics designs and manufactures metallized ceramic components at sites in Europe and the United States.

For more information visit www.morganadvancedceramics.com or contact one of our sales offices.