SIntering was attempted at 1800C/40 MPa and 1600 C/50 MPa. In both cases, densification was good but the graphite die broke during the removal of sintered sample from die. I would be grateful if someone could provide any explanation for this occurrence.
I would presume you are using graphite foils inside the die. Likely due to the reaction between carbon in the foil or die and your sample, your die is completely stuck with the sample. When you are trying to release the sample from the die, you have to apply excessive pressure to detach the sample and the die punch is breaking. One thing you can try is that applying a thin coating of hexagonal boron nitride (powder in methyl alcohol solution) using a paint brush between the graphite foil and the graphite die. This should work.
Sorry no patented solution available. I think Dr. Chakrabarti is right, there must be some reaction between yoour powder mixture and the mold. However you must be very careful with application of HBN, because this completely changes the electric characteristics of your mold especially in the beginning befor the powder mixture becomes electrically conductive. In ZrB2-SiC depending on your powder chemistry you may form silica, zirconia and ZrSi2 which readily reacts with the carbon die. Maybe use a different graphite foil or a thicker foil (this will also change the SPS characteristics).
I also concur with Dr. Chakrabarti and Dr. Kern that reaction of ZrB2-SiC with graphite die is causing the sintered sample to stick. An application of h-BN coating with alcohol should help. Secondly, you may want to keep the pressure applied till the process ends, and increase the cooling rate as well. That way, thermal stresses may help in smooth release of the samples.
Respected Sir, try using graphite spacers in the inner curved surface of the graphite die as well as at the top and bottom(you may use single and double papers) between your powder and punches. If this doesn't work, try boron nitride spray to make a thin coating on the inner sides of the die and the punches. Please check that the punches aren't too tight for the die, and on the other hand it should not be too lose as well. I hope this will work.
Hello, I agree with the previous comments. If you want to apply a hBN coating in the inner graphite foil face minimizing the impact on the current distribution, There is a trick that I often use. You applied first the hBN coating on all your inner graphite foil and while it is still wet remove the hBN coating on the upper and lower area of the punches. doing that you will have a hBN coating on the sample area only, but you allow the current to get across the punch/die interface. I hope it will help.
Please check the graphite grade used as die. This is very important and always choose high density graphite. Use high density graphite material with correct CTE will also help you to prevent the die breakage. Though the hBN coating helps you to prevent the reaction between the material and die wall and to provide good lubrication, but it affects the SPS process as the current flow will changes. I would suggest to use the plain graphite foil (2-3 layers on each side-top & bottom; two layers on the die internal wall overlapping the joint so that it prevent the direct contact with powder and graphite die. You may also provide good tolerances on the die ID and punches to accommodate the die expansion upon heating / cooling cycle. .
I also had the problem but the trick we used was to use a carbon foil which is around 0.35 mm in thickness. We made all sorts of ceramics, UHTC including B4C, SiC and also different shapes of the sintered discs, never had any problem.
You can get the foil from SGL Sigmaflex TH Foil
I hope it helps
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