Advanced ceramics are becoming increasingly important in all branches of engineering from microelectronic circuitry to automotive and aero engine applications. To obtain the optimum performance from specific components, it is often necessary to join ceramic parts to other materials particularly metals. The intrinsic properties of most ceramics virtually rules out all methods of fusion welding. However, solid state techniques, adhesive bonding and brazing all remain as possible means of providing strong, sound joints between ceramics and metals.
In a research programme at TWI, vacuum brazing of partially stabilised zirconia to cast iron (a typical combination required by the automotive industry) has been investigated. Conventional brazing alloys do not wet ceramics, and standard metallising procedures (using Mo Mn) are not applicable. Our work was therefore directed towards the newer active metal braze alloys based on the copper/silver eutectic with additions of titanium.
Aside from wetting, the major difficulty with metal to ceramic bonding is the difference in thermal expansion coefficients of the two materials. When combined with the rigidity of the ceramics, this difference leads to destructively high strains being generated in the joint zone. Effective ways of minimising this thermal mis match include:
• keeping joining temperature as low as possible
• introducing ductile interlayers to accommodate the stresses;
• introducing single or multiple interlayers to provide a graded joint of near matching thermal expansion coefficients.
Other external restrictions (e.g. corrosive environment, service temperature, weight, etc) often limit the extent to which any of these approaches can be adopted. At TWI we are actively pursuing techniques to minimise the joint residual stresses for zirconia/cast iron and other ceramic/metal combinations.
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