Solid-State Sintering

1.5 Solid-State Sintering
1.5.1 Driving Forces
It is generally accepted that the reduction in the surface free energy of a sintering compact, due to the elimination of internal surface area associated with the pores, provides the driving force for sintering. When compared to other processes (e.g. chemical reactions), the decrease in the surface free energy during sintering is rather small (~100J/mol for particles with an initial diameter of 1 ?m) but the distance that matter has to be transported is also small (of the order of the particle size), so that sintering occurs at a reasonable rate at sufficiently high temperatures.
The specific energy and curvature of the particle surfaces provide an effective stress on the atoms under the surface.
1.5.2 Stages of Sintering
The microstructure of a powder compact, consisting initially of discrete particles, evolves continuously during sintering. However, it is sometimes convenient to divide the process into three idealized stages defined in terms of the microstructure, to force correspondence between simple, established sintering models.