A technique of bonding materials by heating below their melting
points and pressing them to achieve solid state adherence by the mutual diffusion of their atoms.
As the materials are bonded in a solid state, far more accurate
bonding is possible than with fusion bonding. This method is mainly used for bonding metals or
bonding a ceramic to a metal. After bonding dissimilar materials, thermal stress occurs during
cooling because of the difference in the coefficients of the thermal expansion of the materials. To
avoid cracking caused by this stress, most diffusion bonding research is concerned with ways of
reducing thermal stress. Methods of achieving this include sandwiching a third material with a
coefficient of thermal expansion roughly halfway between that of the two materials, or a readily
deformable material between them. Much research is being done into the insertion of a material
whose coefficient of thermal expansion changes gradually across the thickness (functionally gradient
material, i.e. FGM).