There is not a lot of information here...but here are some general pointers. Releasing the applied pressure right at the end of your run (before cooling) could help. You may also want to play around with cooling rates. I suggest you check your samples to see if maybe your sintering conditions are not adequate for thorough sintering. Everything from temperature to sample geometry makes a big difference.

Finally, be very careful when you take the sample out of your die. You need to be gentle, particularly due to the reducing nature of the SPS. Some materials seem to show unpredictable fracture behaviors.

My experience limits my answer, but I would be surprised to see a blanket statement to a problem that seems quite system-specific.

Thank you , Martin.

You are right, when I checked my sample. I found it is very interesting. the sample shows layered color in the cross section. I repeat the sintering and found it is still layered. Do you know any reason for this phenomena?

First, it may be caused by the characteristics of your ceramics powder. Bigger size of the grains make it more difficult to densify. You should try a lot of sintering conditions. Then, compare the microstructure. The sintering conditions that most affect on the densification is temperature, but, be very careful with the temperature, if the temperature is too high will result in grain growth or even the melt of your sample.

Second, please consider the heating and cooling rate. The effect of heating and cooling rate usually will result on the easily or not the crack propagate. If you can reach high densificiation of material, when you remove your sample from SPS die with damaging the SPS punch & die, there will be no affect on your sample.

For the layered color in the cross section on your sample you found, it may be because the unevenly distribution of heating which resulted in incomplete sintering process. I suggest you to consider the mass of your powder or sample geometry. The smaller sample geometry of SPS compact make more easy to distribute the heating.

This all based on my experience. I hope you can solve the SPS problem with my answer. Good luck!

HI. Lydia:

Thank you very much for your answer.

How does this sentence "If you can reach high densificiation of material, when you remove your sample from SPS die with damaging the SPS punch & die, there will be no affect on your sample. " mean?

Tomorrow, I will try to use less powder to see if the layered phenomena will disappear.

You said that the powder size has big influence on the SPSed pellet. Do you think I can downsize it into nanoscale by HEBM? In that way, I am a little bit worried that the grain size will not be "big" enough. Because I want to get the ceramic grain at micrometer grade.

Looking forward your answer.

Thank you

More information is required (i.e. material, heating rates, pressure...). However, it could be related with a non-uniform packing in the green body (and therefore inhomogeneous sintering process) due to the layered color in the final material. Of course, the heating and cooling rates have an important role. What is the relative density of your green body? and How many grams do you introduce in the graphite die?

Hi Jesus:

The material is ceria, and the heating rate we choose is 200oC per minute. Pressure various from 3mpa to 50mpa. (3mpa is the initial pressure when the graphite die is placed as alignment in the furnace). Sometimes, I do see the layered structure of my sintered cracking smaple. Every time, about 1.5g is placed in the graphite die (iner diameter is 15mm). The theoritial density of the materials is 7.25g/cubic meters.

Hope you can help me.

Thank you

Do you release the pressure during the cooling part? Maybe, the cracks are related with it.

To Ta-Lei Chou:

Sorry, answer is disappointing to both of us. I think it really depends on the materials we are trying to synthesize. The behind reason I figured out maybe the thermal expansion difference between graphite and the materials we are trying to synthesize. So, please update me if you find the graphite die works.

We just tried to SPS nanozirconia but with significantly larger samples of 50 mm diameter and 5 mm thickness, we also observed some spallation at theouter edge.

Ceria is probably the most complicated oxide material to chose for SPS, high CTE, low thermal conductivity the inherent tendency to partially reduce to trivalent (you can see the color gradient) especially in a graphite die and traces of CO. Strength is low, the only good thing is the low elastic modulus which allows for a bit of deformation.

So you have a combination of cooling stress and reduction induced stress due to Ce4+/Ce3+ gradients.

Sorry but maybe this just does not work, if you heat or cool slower the reduction will have even more time, your samples are already quite small so that the temperature gradients are probably not too high.

Just one more thing:

How does the outer rim of your sintered pellet look like, is it straight or curved ?

if it is curved you must reduce the heating rate starting from the temperature where densification starts, otherwise you are doing a combination of SPS and sinter-forging with a higher stress level in the final part (we had this problem in hotpressing of nanozirconia at 50 K/min).

@ Frank Kern

Sorry for such a delayed response. For the past several months, I did SPS on other materials systems and found this spalling problem is really a case of materials you sintred. 

As for the our rim of the pellet, it is a round shape, cause the die we used has a cylindrcal shape. 

Today, I tried at 50 k/ min of heating and 200 k/ mins of cooling. it still cracks. 

My next step is to slow the cooling rate and hope it works. 

I will update on this ongoing trial.......

You might want to make sure you are achieving good necking in your samples. You can try sintering for longer times. Depending on the oxide, and other parameters...you might get better bonding without problematic amounts of grain growth. It could be an intrinsic issue, or maybe the surface of your powder is somehow compromised, which could then lead to weak necking. Powder preparation could also help in this case.

In terms of the SPS, you might want to try different pressure profiles. High pressures can help with necking (and density). Also, you want to make sure that you apply the pressure at the right time, and that you relax it before the sample cools significantly.

Treat your sample very carefully, and anneal it as soon as possible. The reduced sample will be more brittle. I always wait for the die to cool before I remove the sample...but I am not sure this helps too much.

EDIT: For some reason I was sent this quesiton to my gmail...I forgot I already had an answer in here. 

how much minimum sample is required for SPS?? i am new to this.