One of the major problems in ceramic/metal joining is the thermal stresses due to large mismatch in coefficients of thermal expansion (CTE) between the ceramic and metal. These stresses affect the strength and failure behavior of the bonded joint. If too high, they can cause fracture in the joint or in the ceramic, either during cooling from the joining temperature or during thermal cycling in service. Many investigations have been carried out to evaluate the residual stress distributions and explore methods to reduce the stresses in ceramic/metal joints. Soft metals such as Cu, Al and Ni and metals with low thermal expansivity (e.g. W and Mo) as well as their combination were used as interlayers in brazed ceramic/metal joints for this purpose. Functionally gradient material (FGM), a new type of metal-ceramic composite in which the composition and structure and hence properties vary smoothly from one side to the other, was first proposed in 1987 to develop super heat-resistant materials for the propulsion system and air-frame of space plane. Recently, much research work has been done to develop FGM for various applications by using gradient in chemical, biochemical, physical and mechanical properties. The concept of FGM can also be applied for the purpose of reducing the thermal stresses in bonded ceramic/metal joints. In these joints, the FGM interlayers are sandwiched between the ceramic and metal to be joined with gradual compositional variations from a composition that is compatible with the ceramic to a composition that is compatible with the metal. Such FGM joints can be produced via conventional hot pressing joining or by means of the newly-developed pressurized combustion synthesis (PCS) joining, also called self-propagating high-temperature synthesis (SHS) joining.
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