For CIP, the higher pressure is the better.

However, for the normal dry pressing, if the pressure is too high, a larger density difference may be induced into the green body. A further CIP is therefore needed.

Dear Harshitha Gandham

The final density of the ceramics (after sintering) is strongly affected by green density. At first, we should prepare the green pellets with optimized binder percentage, pressure, dwell time etc. High pressure will not guarantee always high green density or crack free sample. When we apply more and more pressure layers will form inside the pellets which may be or may not be visible after pressing but will be visible after sintering. Press some pellets at different pressure and check density by dimension method. Check the surface carefully. Optimize the pressure.

You may want to check the following related RG discussion: https://www.researchgate.net/post/What_are_the_major_reasons_for_the_bending_of_pressed_powder_pellet_after_sintering

Dear Harshitha,

Generally speaking, the applied pressure affects the green density of your compacts: the higher the pressure the higher the density. However, in the case of ceramics, no plastic deformation of the particles does occur, so use of high compaction pressures is counterproductive. Not only it results in significant wear of the die tools but also in lamination and other defects caused by excessive friction between the compact and the die. So, the best option is to use the lowest pressure possible to obtain compacts that can be handled without damage. A practical way is either to lubricate the die using for instance Microwax C (Hoechst) or granulate your powder by spray drying.

The second part of your question is related to cracking after sintering. One should bear in mind that all defects introduced during powder technology process route show out at the sintering stage. This means that at any stage of your process either handling, mixing or size distribution sieving of your raw materials or compaction you have introduced defects into your green bodies that were revealed upon the heat treatment. Sometimes, cracking of sintered samples may also occur if your furnace furniture is not the most adequate one. If considerable shrinkage occurs upon sintering, you would better use a non-sticky surface to minimise the interaction between the sintered body and the support plate. Another possibility is to use a powder bed to avoid such interaction. In brief, cracks origin on sintering can either be related to defects caused by inadequate compaction or thermal stresses imposed on the material due to interaction with the supporting media or simply some disruptive transformation occurring upon cooling. If that is the case, consider optimising the sintering cycle, for instance, by means of tight control of cooling rate.

Kind regards,

Fernando Costa Oliveira

Lisboa, 25th October 2017

I thank all Ji Zou , Ajeet Kumar, Carlos Araújo Queiroz, Fernando Almeida Costa Oliveira for your valuable suggestions.

Dear Steffy

To avoid the pellet cracking you has to decrease the particle size of the calcined powder properly. Then you has to use PVA to make the pellet strong during the making pallets. Then go for slow heating for binder evaporation 2oC/min till 500oC. Then put your pellets above a platinum mesh and go for sintering. This action also happens with me but i used the above process and i got a successful sintered pellets.

Regards

M.S.Alkathy

I agree with Ajit Kumar’s discussion. There so many factors one has to take for obtaining crack free pellets. 1. The die should made of Hiichromium Hicarbon steel die mirror polished inside along with lower and upper plungers. 2. The binder PVA,PVP, PEG etc only 2% dissoled in a solvent would be sufficient. 3. Lubrication of the die with 3-4% solution of Stearic acid, Pamtic acid or Lauric acid is necessary. This solution is prepared by dissolving the above in acetone. With this solution inner side of the die and its plungers should be wiped out to have good lubrication and to get the pellets easily. 4. Lastly the hydraulic pressure applied should not be very high (as pointed out by Kumar) it should be 4-5 tons.

All these things will ensure you good and highly compacted pellets after sintering. The concave shape of pellets can be avoided by placing them on ‘V’ type grooved alumina plates and then sintered.

This our experiece.