TWI Industrial Member Report Summary 651/1998
W B Hanson and R S Dobedoe*
If a designer was given a blank piece of paper and asked to define the perfect ceramic-metal joining medium, a request would be made for a material with a range of coefficient of thermal expansions (CTE), a high temperature capability and the ability to form a refractory bond. The material the designer has just requested already exists - the material is a glass-ceramic. This report considers exploitation of glass-ceramics in this role.
The main problems encountered when bonding ceramics to metals include the relative chemical inertness and low thermal expansion coefficient of the ceramic material. When combined with the brittle nature of ceramics, these problems present significant difficulties, particularly when the bond must withstand high, or sudden changes, in temperature and mechanical loading.
Procedures which are being investigated include graded interlayers (metal/ceramic mixes), produced via HIPping, and brazing with mechanically compliant interlayers which are able to accommodate interfacial strains. A further solution is to take advantage of a class of materials known as glass-ceramics. These materials can be developed to provide a wide range of thermal expansion coefficients, dependent on composition, and thus can be used as interlayer materials.
Glass-ceramics are crystalline inorganic materials formed from the controlled crystallisation of a parent glass. They are usually based on silicate materials, typically MO-SiO 2-Al 2O 3, where MO is a metal oxide. In the glass phase, these materials are easy to process either as a melt or in powder form. As such, they offer significant scope as interlayer materials. Few data currently exist on the appropriate compositions for ceramic-ceramic or ceramic-metal joining, and on the processing requirements for achieving high temperature applications (up to 1200°C).
This programme of work was undertaken to investigate the requirements for the production of ceramic-ceramic and ceramic-metal joints.
The objectives of this project were:
- To develop glass-ceramics, of suitable thermal expansion coefficient, as interlayers for bonding silicon nitride to itself.
- To determine the feasibility of using glass-ceramics of suitable thermal expansion coefficient, as interlayers for bonding silicon nitride to metals.