I have noticed the presence of few cracks while looking at optical microscopy. I wonder if anyone has had similar experience. This sample contains mainly alumina and zirconia and it‘s been consolidated in Spark plasma sintering and thermally treated for few minutes.
Did you also find residual porosities? They look like localized collapses caused by underlying porosites but I don't have enough informations... Is this a polished cross-section? Is this the material surface? How many magnifications did you use? What's the size of the sample?
It seems to me there are both cracks and pores in the sample. The reason might be that the green compact was not treated well before SPS.
@Elia, I rarely saw residual porosity. It is polished surface after thermal etch. The length of the crack was about 46 micron.
Ok, it's just a guess but if you brutally enhance the contrast in the second picture (as an example), you can see the porosity underneath. This is not how a simple crack looks like.
Try polishing a little more and see what happens to the cracked areas.
Dear Sami
I agree with Prof. Zhou there are some pores in the magnified picture given in previous answer by Prof. Marin, and the cracks almost dissappeared and very fine parllel scraches are seen with higher manifcation when looking at the preveously metioned picture. Using o.5 micron or less of diamond paste in the final polshing step will remove these fine scraches and no cracks will appear, but the pores can not be removed, because they seem to be quiet deep pores.
Good luck.
These cracks are generating to accommodate the stress gradient created during sintering. Residual created during loading, densification process and unloading of sample. Remedy to this problem is proper filling of powder in die and die should be initial pressed and powder should be uniformly filled in the die...Best wishes
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