A very typical problem in hot pressed and SPSed samples. In axial pressing there are slight density gradients from the middle to the rim. Moreover we have large temperature gradients during cooling, this results in high stress, stresses scale with size.

Thermophysical boundary conditions for thermoshock sensitivity  in ZnS are no so bad the CTE is not very high and the thermal conductivity is also acceptable however the strength of the material is low.

The larger the mold the higher the residual stress in the sample. Use slower cooling, this may help.

Thanks a lot for replying me.

I also suggest using a slower cooling.

Here we also have a similar situation. I can tell you I tried several different sintering parameters, including add a stress relieving segment during cooling. However, none of those works, and next step I am going to try a slower cooling. Right now, I am using 200 k/ mins. and intended to lowered it to 50 k/mins. BTW, the material I am sintering is oxides.

Update on my former answer. 

A wholesome pellet was obtained by me this afternoon by lowering the cooling rate to 50 k/ min.....So, lower the cooling rate is definitely a right direction.

Hope you can get it soon. 

you may try this option also:

1. as suggested by other researchers, try with low cooling rate

2. try to unload at much lower rate compared to earlier case.  

by the way, what is the pellet thickness you are fabricating ?

even with cooling rate  10 k/min it breaks!!!!!

Shall we use a lower cooling rate ?

the thickness is about 3 mm. 

Could you please explain more about "unload at much lower rate compared to earlier case"?

It could be CTE mis-match between your sample and the die. Try looking up the values for the CTE and compare them. I know a friend who was was using SPS to hotpress MgSi2 and had cracking issues. He also lowered the cooling rate but it wasn't until he switch die material (can't remember what he switched to) that he was able to get crack free pellets. 

Dear Elnaz,

Please specify the loading and unloading pattern that you have adopted for your experiments. i.e., loading rate, temperature at which you start applying load, holding period, max load applied,  unloading rate.

Few tips. Phase change, grain size. Perform DTA to see when the phase change takes place if any?