Ceramics and joining technology
– Ceramic-ceramic and ceramic-metal glass soldering
CERAMICS AND JOINING TECHNOLOGY // RAW MATERIALS PREPERATION // GRANULATION // SHAPING // FIRING PROCESSES // HARD MACHINING // CONTROLS // LABORATORY // CERAMIC-METAL BRAZING // CERAMIC-CERAMIC AND CERAMIC-METAL SOLDERING
Metallic soldering processes performed above 450 °C are conventionally classified under the term “brazing”. Ceramic-metal composite components are usually brazed with the help of silver-based brazing solder such as AgCu28 or AgCu26,6Pd5 between 800 and 850 °C.
As the metallic materials used are not resistant to oxidation, for brazing processes electrically heated furnaces are operated either with inert gases such as H2, N2, Ar or under vacuum. In such furnaces, for example, oxide ceramic components for thermocouples or electric feed-throughs are used, the purpose of which is the electrical insulation of the thermocouple elements often consisting of W-Re alloys.
Dense-sintered Al2O3 ceramic is generally an electrical insulating material that has proven effective over many years and exhibits the following properties:
- Purity: >95 %
- Density: >98 % of the theoretical value
- High compressive strength
- High thermal and thermomechanical strength
- Stability in reducing inert gas and in vacuum
- Dielectric strength: >20 kV/mm
- Specific electrical resistance at 1500 °C: 106 Ω*cm
Beyond applications as electrically insulating material, in special cases the fixtures that hold the components to be joined in geometrically exact positions relative to each other during brazing are sometimes made of oxide ceramic materials like Al2O3 or ZrO2. When standard fixtures made of graphite are not sufficient in respect of their strength, their thermal expansion or the geometric tolerance values realizable with this, the use of oxide-ceramic fixtures can be recommended.
The glass soldering technology for joining ceramic to ceramic is far less complex than the brazing technique described above with regard to the technical equipment required as it can be generally performed in air in an electrically heated chamber or tunnel furnace of relatively simple design.
For soldering fixtures, however, only oxidation-resistant materials, primarily Al2O3 ceramic in porous or dense-sintered form can be used. Special attention is given in this case to the risk of the product being soldered to the fixtures with the glass solder, which, however, can generally be avoided with design measures.
The glass soldering technique for joining a ceramic to a metal part requires a soldering furnace operated with an inert gas atmosphere. If soldering fixtures are necessary, components made of the above-mentioned oxide ceramic materials are also suitable for this technology.