Metallisation of ceramics – pushing the boundaries of engineering materials
13th Jul 2010
Technical ceramics are favoured 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, they have a good thermal stability (ie a low coefficient of thermal expansion) and good thermal and electrical resistivity. They are also hard, and have excellent dimensional stability.
As a result, the list of applications for technical ceramics is long and varied, including, for example: aerospace engine blades, rings and valve components; industrial pump bearings; cutting tools and die parts; medical instruments and wide uses in the electronics industry as a substrate and in specialised vacuum components.
For many applications it is often necessary to join ceramic to metal to create the finished part.
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 by brazing. This starts with the chemical bonding ‘metallisation’ of the ceramic to create a wettable surface on the ceramic, on which braze alloy will flow between the two components during the brazing process.
Morgan Technical Ceramics is a world leader in the design and manufacture of metallised ceramic components. Expert teams at its European and US Sites are involved in production of custom parts for customers all over the world. Applications range from very small volume production runs of high value components for special projects, to high-volume manufacture of precision designs – as demonstrated by the following examples:
Example 1: A unique engineering challenge
ISIS, a world-class spallation neutron source based at the CCLRC Rutherford Appleton Laboratory, Oxfordshire, UK has recently commissioned a series of highly specialised metallised ceramic components from Morgan Technical Ceramics as part of a major expansion project to build a Second Target Station (TS-2).
The components are a fundamental part of instrumentation monitoring the intensity of the extracted proton beam (EPB). Ceramic vacuum tubes used in the first target station were sealed with Indium wire, but experience proved that these became unreliable if disturbed. Metallised ceramic offered a solution that provides a 100% reliable vacuum seal within the very tight tolerances of the design.
There were two key challenges. This first was to come up with a design and a manufacturing process that would produce a robust, high integrity vacuum seal (leak rate 10-8mbar l/s) across a large component (200mm diameter). The second was to solve the problem of the differences in thermal co-efficient 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 158mm long with two nickel-plated mild steel flanges 240mm diameter 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 850ºC to two flanges made of nickel iron cobalt steel, chosen because it provides the best thermal expansion match to the ceramic. This process is achieved by applying a moly-manganese coating which is sintered at 1400°C, then electroplating a layer of nickel. The ceramic/metal brazed sub-assembly is then 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, 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 is scheduled for June 2007.
Example 2: Precision and consistency
For another customer, Morgan Technical Ceramics manufacturers metallised ceramic components for vacuum electronic devices (VEDs) used in continuous wave and pulsed radar systems, such as those for fighter aircraft.
Here the challenge is to push the performance envelope of the materials to meet the industry’s demand for higher frequencies. This means smaller components with the same physical properties as their larger cousins and calls for very high precision engineering and close quality control to ensure consistency throughout production.
For example, the smallest part made like this is a cylinder with an internal diameter of just 0.2inches. The internal surface is metallised to a very tight thickness tolerance, within 0.007 – 0.0012inches. The metallisation process used is based on (MoMn) Molybdenum-Manganese refractory ink systems developed in-house by Morgan Technical Ceramics and is matched to specific high purity alumina ceramic bodies to ensure consistent high strength bonds. The glass phases in the MoMn metallisation bond 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.
Advanced ceramics are meeting the needs for higher performance critical components in a wide variety of applications. Through a detailed understanding of the ceramic-metal bonding techniques, such as the metallization process, designers and manufactures are better able to devise these key components.
The joining of ceramics to metals creates its own engineering challenges that require specialist expertise. Morgan Technical Ceramics is a world leader in the metallisation of ceramics. Its engineering teams in US and Europe have worked with customers all over the world to provide high integrity solutions for components of all sizes, shapes and specifications.
Created: 13th Jul 2010 @ 04:00:16
Last Updated: 20th Jan 2011 @ 08:28:04