Usually when we hot press or sinter nano powders in order to get bulk pellets, the resulted pellets dont pertain the initial size of the grains. Whether only temperature is responsible for grain growth or pressure also does role here ? If so, how does it occur ?
Grain growth will always occur.
It is just a matter of how much. The reason for this is the fact that grain-boundaries are "high energy" interfaces and your system tries to reduce the total amount of those high energy areas by increasing the grain size.
Hot pressing typically has less grain growth, since it sinters your sample faster and in a shorter period of time.
So, reduce the temperature and time as much as you can during hot pressing and grain growth will be less. You can also run a series of experiments and see if grain growth is favored at higher or lower temperatures. Grain growth will have a different activation energy than your densification process and whatever has the higher activation energy will "gain" more by increasing the sintering temperature.
In addition, there are numerous ways to reduce grain growth through additions of other materials. E.g. grain boundary pinning by adding nano particles or by changing the chemical composition of your grain boundary phase. However, those always depend on the composition of your material and there is no general rule.
Grain growth will occur at high temperature due to increase mass transfer leading to sintering .The movement of grain boundary will cause the smaller grains to merge inside other leading growth.
It is difficult to get rid of grain growth.
But a control heating ,change in sintering pressure and atmosphere can effect the grain growth.
But the maximum effect can be achieved by adding some dopants or impurities that can hinder the grain boundary movement.
Thank you so much Herbert Giesche, Sanjay Mohan and Anirudh Karati. Can you please suggest me some fundamental books related to grain growth while sintering.
Both effects (pressure and temperature) have an influence on the crystal growth, but temperature is probably the biggest problem, especially if you keep the high temperature for long time. To avoid or reduce this problem Spark Plasma Sintering (SPS) showed to be a really effective technique, hence I strongly suggest you to try it. If you want you can also have a look to this paper:
http://www.sciencedirect.com/science/article/pii/S0167577X15000555
Hope it will be useful.
http://www.sciencedirect.com/science/article/pii/S0167577X15000555
Thank you Sir, Giacomo Cerretti for your kind suggestion. I have planned to use both FAST and compression by shock wave. So I can compare the grain size variations in each technique.
You can also read "Powder Metallurgy Science" by Randall M. German. MPIF, Princeton, New Jersey, USA. 1994
Grain growth is a thermally activated phenomenon. It follows an Arrhenius law which means that it is exponential with temperature. Time is less important. Pressure is used to get rid of porosity at lower temperatures. This is critical for grain growth. SPS, FAST technologies use this principle, since these cycles combine pressure and temperature. The problem arises from the gradients induced in the loading direction.
I aggree with the listed reasons for grain growth. There is one more important factor which influences grain growth: Particle size distribution of the powder. Coarser particles in an environment of many small particles act as a nucleus from which grain groth starts. Very often you can see under the optical microscope that grain growth is not homogeneous all over the sample but many isles with grain growth. Check your starting powders!
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