TWI Knowledge Summary

Ceramic adhesives

by Alan Taylor

Ceramics can be readily bonded using conventional organic adhesives. However, since organic materials combust and degrade, the temperature capability of these adhesives limits the operational conditions in which the joint can be utilised to temperatures below 200°C. Ceramic adhesives are inorganic and do not burn, which means that they can operate at much higher temperatures.

A number of ceramic adhesives have been developed which offer a service temperature of up to 2200°C. These are based on inorganic binding compounds such as alkali silicates and various metal phosphates, with a carbon, alumina, silica, magnesia or zirconia powder filler. These ingredients differ from conventional cements in their level of purity and their temperature capability. The general principles behind how these adhesives work is, however, the same as for cements. Once all the ingredients have been mixed together (usually in a water-based slurry), chemical reactions occur within the mixture and give rise to refractory bonds. This process will occur slowly at room temperature, but can be speeded-up by heating. It is sometimes necessary to heat to temperatures in excess of 1000°C before complete curing of the adhesive is achieved.

Ceramic adhesives are mainly available in two-part systems (although some one-part systems are available) and are typically separated as binder solution and ceramic filler. The slurry that is created when these are combined is readily applied to a joint region. Suitable selection of the binder and, more importantly, the filler allows the coefficient of thermal expansion of the adhesive to be matched to that of a substrate to minimise stresses during heating and cooling.

Ceramic adhesives are suitable for bonding metals or ceramics that do not require a hermetic seal or a high degree of reliability. However, their use is limited because they do not match engineering ceramics in terms of durability and reliability, and once cured are brittle and unable to relive stresses generated during thermal cycling. Used in an appropriate situation, such as a gas burner nozzle, a ceramic adhesive can provide an inexpensive and easy joining method with a high temperature capability.

Further information

Also, you can use the Weldasearch literature database to supplement what you find in JoinIT.

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