Frequently Asked Questions
Glasses can be used as bonding media, particularly for oxide ceramics and non-oxide ceramics containing an intergranular glass phase, for example 92% alumina and sintered silicon nitride - where wetting can easily be achieved.
The brittle nature of glasses below their softening temperature, and their consequent inability to relieve stress by plastic deformation, means they are not suitable for joining materials where differences in thermal expansion exist. Consequently, glasses have mostly been considered for joining ceramics to themselves, i.e. to join a number of relatively simple shapes to produce a more complex component.
If there is sufficient glassy phase in the structure (>5%) direct bonding can often be achieved. Provided that the surfaces to be joined are flat and adequate temperature and pressure are applied, the glass will soften and allow a bond to form.
Typical conditions for alumina (<4µm surface roughness average) would be 1600-1800°C, 5-20MPa for 30-90 mins. For example, 85% pure alumina has been successfully joined using glasses based on compositions of Al2O 3-MgO-SiO2 and Al2O3-CaO-MgO-SiO2.
When there is insufficient glass present in the parent material, indirect bonding can be used. Indirect bonding places glass at the interface to be joined, usually as a glass shim or frit. This technique is used for joining silicon nitride materials and frequently uses oxy-nitride glass as the bonding medium.
The bonding glass composition is chosen to be very similar to that present in the intergranular phase in the Si3N4, such that it is possible to homogenise the silicon nitride until the original boundary disappears. This provides a joint of equivalent strength to that of the parent material and can be used on most types of silicon nitride.
The important criterion is that the thermal expansion of the glass should match that of the original material such that stresses do not form on cooling.
Advantages
- good joint strengths
- good temperature capability to 1000°C
- capable of producing many joints simultaneously
- inexpensive constituents
- simple, flexible process
Disadvantages
- unable to cope with large coefficient of thermal expansion mismatches
- precise control of glass composition required
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