Diffusion bonding is a solid state joining process in which a force is applied to two or more flat and smooth surfaces at elevated temperature (0.5-0.8 Tm where Tm is the melting temperature of the material). A permanent bond is created between the two materials, similar or dissimilar, by the formation of interatomic bonds. The application of both appropriate temperature and pressure is necessary to plastically deform the surface asperities.
The general mechanism of diffusion bonding can be seen in the diagram below. In the first stage, the two surfaces are brought together and the micro-asperities came into contact. Indeed, even if the surfaces had been polished and seem to be smooth, some slight projections as bump, ridge or other vertical deviation remain; they are called micro-asperities. The stresses within these micro-asperities will be above the yield stress of the material. Plastic yielding will occur in asperities until the stresses are reduced to lower than the yield stress leading to the formation of an interfacial boundary. Then the grain boundaries migrate away from the initial interfacial boundaries and some pores are removed. Finally, volume diffusion, diffusion within the materials, occurs eliminating remaining pores.
The diffusion bonding is governed by many variables:
- bonding pressure and temperature
- how the pressure is applied
- time of the process
- surface finish and cleaning
- presence and nature of interlayers
- environment of the process: vacuum or argon