I've also tried 5wt% PVA binder in second ball milling after calcination of a solid state route dielectric ceramics.
But the binder is like a gelly after ball milling shown in picture, and still there is cracks in green pellets
(with & without binder) after pressing at 6 ton/cm to minimum possible pressure.
Whats the possible reason of layers and diagonal cracks in green pellets?
Could any one of the respected researchers help me in this troublesome?
I'll be very thankful to give your valuable suggestions!
Horizontal cracks in compacts is the most common problem in ceramic manufacturing. The cause is the compressed air, which cannot escape from the compact and tears the green compact after release. The finer the powder and the more binder, the more difficult it is to deaerate the compact.
In your case you have too much binder. The measures you can take can be the following:
- Significantly reduce the amount of binder (in the case of fine-grained powder, the binder is completely superfluous, a slight moistening is sufficient).
- Reduce pressing pressure.
- Apply vacuum when pressing.
Uniform or fine powder is necessary for formation of pellets. Cracks can be avoided by proper amount of binder. After adding binder, grind the mixture and place it under hot bulb for 5 to 10 min and then again perform grinding to make it fine. I think..@ it will help you
It is too difficult to remove the air completely. So when you start pressing the pellets the air enters and these cracks occur. Also if you a dye for making the pellets, the powder near the walls of the dye gets sticked Or gets a little adhered to the walls, which is natural and you get a little concave surface instead of a plane one, where you apply pressure. Due to this, no matter how precisely you make the pellet, due to the concavity of the surface the pressure applied will vary radially in the pellet. It makes the pellet of varying density and also allows air cavities to form.
Thanks for your valuable suggestions sorry for my late reply this due to technical problem in email sync.
Respected @vadim verlotski this should definitely work, I've tried your suggestions mentioned in my question
Unfortunately I didn't find the exact solution.
Now I made a thick pellet then by polishing, removed the Cracked portion.
[1] I read in an old text book on PM that the clearance between the die bore and top punch may be slightly increased compared to that of the bottom punch to permit air to escape during compression to avoid cracks caused by entrapped air. Please let me know if this worked. [2] By prepressing and granulating the powder before final pressing, the entrapped air can be reduced.
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