For many years, glass-workers have striven to develop glass compositions which would not crystallise or devitrify during processing. The presence of crystalline phases leads to a deterioration in the workability of the glass. If this crystallisation is not controlled, it could result in the formation of a weak or brittle material. It was recognised, however, that if the crystallisation is controlled, using strict temperature-time profiles, then the product would be a group of materials, i.e. glass-ceramics, with a wide range of coefficients of thermal expansion (CTE) and other properties. As such, this makes them particularly useful as potential joining media.
For bonding between glass-ceramics and metal, similar preparations must be observed as for glass-metal seals and joints. The metal is usually roughened by grit blasting and then pre-oxidised. The glass is then applied as a slurry and the assembly fired in a suitable atmosphere (to protect the metal from further oxidation). If necessary, the glass-ceramic may also be pre-glazed. It is important when using thin interlayers of this type to allow a period of setting of the intermediate glass interlayer (similar to an annealing hold), such that the glass can stress-relieve.
For direct bonding of glass-ceramics to metals, the joints are usually of sleeve or concentric ring design.
- good joint strength
- good temperature capability to 1000°C
- capable of producing many joints simultaneously
- inexpensive constituents
- simple, flexible process
- unable to cope with large CTE mismatches
- precise control of glass composition is required
- controlled recrystallisation of glasses is critical
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