What is the main driving force for sintering

Sintering does not usually involve chemical reactions, and the driving force is a reduction in the surface area and the associated reduction in surface energy. This driving force can be illustrated for a flat surface that contains a spherical promberance and a similar spherical depression. The vapour pressure over a curved surface is related to the vapour pressure over a flat surface by the Kelvin equation. This shows that the vapour pressure over a protuberance will be greater than the vapour pressure over the flat surface and will increase as the radius of the curved surface decreases. Similarly, the vapour pressure over a depression will be less than the vapour pressure over a flat surface. When a solid is heated, vapour transfer of matter will take place from a protuberance to a depression, and the surface will tend to become flat. 

A body to be sintered consists of many small approximately spherical grains. Material transport occurs, for a similar reason, when two spheres touch to form a neck. The spheres will have a positive and relatively large radius of curvature, and the neck region a smaller negative radius of curvature. Matter will thus tend to be transported via the vapour phase from the larger spheres into the neck region, causing the particles to join. 

The transport of material to achieve reduction in surface area is not restricted to transport via the vapour phase. Bulk or surface diffusion can also be called into play to achieve the same result. 

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