SINTERING (DENSIFICATION)
After compaction the ceramics, was densified by sintering. The green bodies should have a certain minimum density before they can be sintered. The desired shape and minimum green density can be provided by various techniques like dry pressing, wet pressing, and cold isotatic pressing (CIP), slip casting, and extrusion.
In the sintering process, reactions between the un-reacted constituent phases (if any) take place at high temperatures.
Sintering commonly refers to a process involving the heat treatment of powder compacts at elevated temperatures, usually at T>0.5Tm K, in the temperature range where diffusional mass transport is appreciable. Successful sintering usually results in a dense polycrystalline solid.
Ceramic processing is based on the sintering of powder compacts rather than melting/solidification/cold working because (a) ceramics melt at high temperatures, (b) as solidified microstructures cannot be modified through additional plastic deformation and recrystallisation due to brittleness of ceramics, (c) the resulting coarse grains would act as fracture initiation sites,(d) low thermal conductivities of ceramics (
The reduction in the surface free energy of a powder compact, due to the elimination of internal surface area associated with pores, provides driving force for sintering. During sintering of polycrystalline materials part of the energy decrease due to elimination of internal surface area associated with the pores goes into creating new grain boundary area.
The grains also have a tendency to grow, by migration of the boundaries, to reduce the energy associated with the grain boundaries.
home.agh.edu.pl/~nmos1/TPCP/spring2015/Sintering.pdf
In practical sintering you always have to except some grain growth. A large pore is always stable during sintering. The reason is that a large pore is surronded by a high number of grains. As the grains grow (and merge) the number of grains around a pores decreases and the angle between the grains become shaper. With a shaper angle more surface engergy is recovered as the space between the grains fill up with material. So the large pore eventually becomes unstable and disapears.
So you can minimize grain growth by having a small particle size, a very good particle packing with small pores between the particles and no agglomeration. But you must also control temperature and avoid overtemperature during sintering.
You cannot avoid some grain growth during sintering but you can control and minimize it.
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