TWI Knowledge Summary
Diffusion bonding - Ceramics and ceramic/metal joints
Diffusion bonding is a solid phase process achieved via atomic migration with no macrodeformation of the components. Initial surface flatness and cleanliness are essential. Surface roughness values of less than 0.4microns are required and the samples must be cleaned in acetone prior to bonding. Typically the process variables range from several hours at moderate temperatures (0.6T m ) to minutes at higher temperatures (0.8T m ), with applied pressure. The process is most commonly used for titanium in the aerospace industry. However, ceramics can be diffusion bonded to themselves and to metals.
Ceramic-ceramic diffusion bonding is difficult to achieve unless either diffusion aids or (more commonly) secondary phases are present. These are most typically glassy phases at grain boundaries.
Bonding of thin shims to build up complex 3D structures is one of the most up-to-date applications for this technology. This has been achieved for rapid prototyping of alumina components. Some may argue that this is sintering; and in reality there is little difference in the mechanics of diffusion bonding and sintering.
Diffusion bonding usually takes place in a uniaxial press (hot isostatic pressing can be used but requires more complex fixturing) heated via elements or induction units. This also presents a restriction on the size of components that be processed. However, a more recent innovation uses microwave heating and this has been shown to produce excellent bonds in a matter of minutes, but still for components no bigger than a shoebox.
It is also possible to produce ceramic-metal diffusion bonds; and, as for ceramic-ceramic diffusion bonding, a combination of time, temperature and pressure are required. Figure 1 shows a schematic of the bonding process, note that the metal deforms at the macro level to the ceramic. Even so, good flat mating surfaces are beneficial.
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Fig.1. Sequence for diffusion bonding of ceramics to metals
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There is usually an added complication when joining dissimilar materials - the differences in coefficient of thermal expansion (CTE). This can cause strains to develop at the interface which can cause premature failure of the bond. This can be overcome by ensuring correct design of the bond line and/or by using interlayers, which have CTE's intermediate to those of the materials to be joined and often have a low elastic modulus. Appropriate stacking of interlayers (ranging in thickness from microns to mm's) can allow diffusion bonding to take place.
In the electronics industry, alumina and aluminium nitride have been bonded to copper using a thin gold interlayer. Gold is an exceptionally good interlayer and diffusion bonds can be produced at temperatures as low as 300°C.
Diffusion - or more accurately ionic migration - forms the basis of electrostatic bonding. Electrostatic bonding requires that the surfaces being joined must be as flat and clean as possible. The components to be bonded are heated in a vacuum and pressure is applied. Typical conditions would be 3MNm -2 and 400°C. When the required temperature has been achieved a DC voltage of about 100V is applied and the metallic component is held positive. The nonmetallic component must contain mobile ions such as Na+. The process has been successfully applied to glass and ceramics such as beta-alumina. Figure 2 shows an electrostatically bonded silicon-glass transducer.
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Fig.2. Electrostatically bonded silicon glass transducer |
Electrostatic bonding offers several advantages for joining in the electronics and optics industries. These include bonding without interlayers, which is a source of contamination, and relatively low bonding temperatures.
A further variation of the conventional diffusion bonding process involves the formation of a transient liquid phase. The best example of this is the bonding of copper to alumina. The process, known as direct copper bonding, involves copper in contact with the alumina in a slightly oxidising atmosphere at a temperature of 1070°C. Under these conditions the surface of the copper oxidises and forms an eutectic liquid. The liquid reacts with the alumina and on solidification a copper aluminate phase is formed at the interface, producing a bond between the copper and alumina.
Further information
Joining ceramics - a guide to best practice. Section 5. Solid state bonding of ceramics contains further information on this topic. Industrial Members can also access a number of Industrial Member Reports and articles from TWI Bulletin on diffusion bonding of ceramics.
You can use the Weldasearch literature database to supplement what you find in JoinIT.
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