Ball milling of powder mixture almost result into some change in particles features:

1. Reduction in particle size due to fracture of particles under the effect of cold welding and impact of balls.

2. Morphology change of particles and formation of flake or plate like particles.

These two change will increase the friction between particles. By increasing the friction flow_bility of particles as well as compressibility of them decrease. As a result the ball milling has a negative effect on compressibility but improve green strength and sinterability of compact.

Dear JH Ym,

In order to get high green density for your compacted sample, the particle size distribution is preferred to be wide. This is because the coarse particles produces many voids in the sample and the fine particles increases the friction during the pressing. Thus, the wide particle size distribution provide less voids size and less friction.

If the processing parameters are set well, ball milling can produce wide particle size distribution. for this purpose, I would recommend you to try two step milling process. In the first step the rotational speed is 70-85 % of the critical speed of the mill. While, in the second step the rotational speed is 45-65 % of the critical speed of the mill.

Please note that the critical speed of the mill is calculated as follows:

Critical speed (in rpm) = 42.3/sqrt(D - d) with D the diameter of the mill in meters and d the diameter of the largest grinding ball you will use for experiment (also expressed in meters).

The milling time is set experimentally, however, the time for the first step determines the upper limit of the particle size distribution and that of the second step determines the lower limit.

Wish you all the best.

@ Thomas ' The smaller particles are the easier is filling up something with reduced cavities'

For spherical particles there is no difference in (maximum) packing density for small or large particles contrary to intuitive feeling. Hexagonal close packing or face centered cubic gives around 74% packing volume (i.e. around 26% free space). Bernal (random) packing is around 60 - 64%.

Obviously there's a shape dependence as (theoretically) one could pack perfectly cubic salt crystals with no void space whatsoever. We can imagine many other shapes where this is the case. For maximum packing density of real-world particles, one needs a distribution of particles and this theoretical best distribution can be calculated. For more information, see:

October 6th, 2016 Controlling powder bulk density by optimizing particle size and shape distribution

https://www.brainshark.com/malvern/vu?pi=271602698&b=1&tx=120486&c1=22

October 21st, 2015 In pursuit of perfect packing http://tinyurl.com/gnddyr6

how fine is you powder to begin with? is your intention to sinter or just use the green body after pressing? are you using binders (that would help in dry pressing and reach higher green density)?

very fine ceramic powders are more difficult to press to higher density but will sinter at lower temperatures, so it is a tradeoff.

Biljana Mikijelj thank you for your answer.

I am using a commercial material with its primary particle size around 100 nm. I am afraid calcining can form agglomeration, so I tried to minimize agglomeration by dry grinding.

However only poor result was obtained from dry grinding. It got stuck. Do you know better method than dry grinding for fine particles?

I am planning on both dry pressing and sintering the milled powder.