Please could you help me to understand about main reasons that make surface of Spsed samples rougher than pressureless samples?
This due to the fact that commonly the sample is separated from the die by a thin graphite paper (0.2-0.5 mm) the graphite paper has an inhomogeneous density so that after removal of the graphite paper the sample below has a rough surface. If you use graphite paper in hot pressing you get the same effect it has nothing to do with the SPS process as such.
If you leave out the graphite paper you get a smpooth surface (but if you have bad luck the sample is so closely attached to the graphite punch that it cannot be separated).
This due to the fact that commonly the sample is separated from the die by a thin graphite paper (0.2-0.5 mm) the graphite paper has an inhomogeneous density so that after removal of the graphite paper the sample below has a rough surface. If you use graphite paper in hot pressing you get the same effect it has nothing to do with the SPS process as such.
If you leave out the graphite paper you get a smpooth surface (but if you have bad luck the sample is so closely attached to the graphite punch that it cannot be separated).
There is also the possibility to use metal foils instead of graphite paper. But you have - of course - to check the compatibility of your material and the sintering temperature. With that you will get a smooth surface as well.
Based on Frank's response, coating the graphite paper with boron nitride aerosol might help relieve the problem. We use it to make samples easier to remove and to prevent them from reacting with the graphite.
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