TWI Knowledge Summary

Adhesive bonding of ceramics

by Alan Taylor

Engineering ceramics such as silicon nitride, silicon carbide and a large number of oxides are used in industries ranging from aerospace to automotive and biomedical to electronics. These materials are used because they possess a range of properties that are attractive for particular applications. These include:

• Chemical inertness
• High hardness
• High stiffness
• Strength at high temperature

The excellent stability ceramics possess under extreme chemical and thermal environments is often the primary reason for their selection. However, the ceramic component must be joined to the rest of the device. There are many joining techniques that can be utilised, these range from mechanical attachment to direct bonding methods such as brazing or adhesive bonding. With all of these methods the correct design criteria for ceramic materials must be followed. These criteria must address issues such as:

• The inherent brittle nature of ceramics
• Their low fracture toughness
• Their low tolerance to high shear and tensile stresses
• Their low coefficient of thermal expansion compared to other materials

The technique selected depends on whether the ceramic is to be joined to a similar or dissimilar material, and on the expected operational conditions at the joint. If the joint temperature is not expected to exceed 150°C or to only have very short-term excursions to ~200°C, and the environment is not too chemically aggressive, organic adhesives offer an attractive joining solution.

There is a wide range of adhesives commercially available, such as epoxy compounds or cyanoacrylates, which can be used to bond ceramics. Each of these has its optimum application method and curing regime to give maximum performance. Optimisation may involve the use of a primer or other additive, for example, oxide ceramics are generally porous structures with slightly acidic surfaces. This acidity tends to inhibit the polymerisation of cyanoacrylate adhesives, whilst the porosity requires these surface initiating species to extend across relatively large gaps. Both problems can be overcome by the use of small quantities of basic species such as amines, which activate polymerisation of the cyanoacrylate. Other adhesives systems also give enhanced bonding properties when used in conjunction with surface modifying primers, or keying agents, such as silane compounds.

The use of adhesives bonding of ceramics has both pros and cons:

Advantages

• Uniform stress distribution at the joint
• No finishing costs
• Easily automated
• Adhesives seal and join in one operation
• Good fatigue resistance
• Small areas can be bonded accurately

Disadvantages

• Joints can be weak when subjected to peel load
• Limited service temperature, typically <150°C or <200°C in special applications
• Poor electrical and thermal conduction: although loading with metal particles improves performance
• Joint integrity is sensitive to cleanliness of the mating surfaces and service environment
• Surface preparation can be critical
• Joints are not hermetic

With correct joint design and material selection and consideration of operational conditions, adhesive bonding of ceramics can be used highly successfully. Probably the most famous use of advanced ceramics utilised adhesive bonding. The NASA Space Shuttle employs 24 000 ceramic tiles as a thermal protection system to keep the temperature inside the vehicle relatively constant. The external temperature of the tiles can vary between -80°C during orbit to 1250°C during re-entry. These tiles are adhesively bonded via a strain isolation pad to the aluminium skin of the shuttle, and the skin has a design limit of 175°C. The success of this system not only allows the shuttles to operate but permits them to be re-used many times.

Further information

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