|Definition:|| technique of bonding materials by heating them to below their melting points and pressing them together to achieve solid state adherence by the mutual diffusion of their atoms
Note 1 to entry: As the materials are bonded in a solid state, far more accurate bonding is possible than with fusion bonding. Diffusion bonding 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 either a third material with a coefficient of thermal expansion roughly halfway between that of the two bonding 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 its thickness (functionally gradient material, i.e. FGM).