Since mass and volume of a body both shows an additive relationship, i.e. if two volumes are combined, the new volume of final body will be the sum of the initial volumes of the two bodies. Since, mass and volume doesn't change on mixing (if no phase transformation), how sintering helps in increase in density?
Sintering of crystalline solids can occur by several mechanisms: vapour transport, surface diffusion, lattice (volume) diffusion, grain boundary diffusion and dislocation motion.
Grain boundary diffusion and lattice diffusion from boundary to the neck are the most important densifying mechanisms. Diffusion fro the grain boundaries to the pores allow neck growth and shrinkage (densification). Plastic flow due to dislocation motion is involved in neck growth and densification through particles deformation (creep) under sintering stress. For glass densification and neck growth occurs by viscous flow.
Vapour transport, surface diffusion and lattice diffusion from the particles to the neck lead to neck growth and coarsening of the particles without densification.
See the reference:
http://home.agh.edu.pl/~nmos1/TPCP/spring2015/Sintering.pdf
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 (
Densification means there is an increase in density. As the mass is constant, bulk volume of the sample need to be reduced for densification. One can increase the densification by compacting a mass, like pressing. As long as we can remove the pores from the sample there will be densification. So pressing can also be a densification process.
Now, sintering is a heat treatment process in which a large quantity of loose aggregates convert to a single compact solid mass when subjected to a high temperature. Sintering can be of different types, like solid state sintering, liquid phase sintering and reactive liquid sintering. In presence of liquid phase the removal of pores are easier, hence densification occurs. In solid state sintering, the sintering occurs by mass transfer through solid phase and is a diffusion controlled process. There are total six different paths for diffusion in solid state diffusion and only three of them are involved with pore removal and so are associated with densification. SO IT NOT TRUE THAT ANY SINTERING RESULTS IN DENSIFICATION.
Suggested books, 1)Introduction to Ceramics by W D Kingery and others 2) Physical Ceramic by Y. M. Chiang and others.
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