My sample is made by sps with two raw materials. I got two pellets,and the sps process is like 0~900℃ (9mins/18mins);900-980(4mins/8mins)980-1000(2mins/4mins)
1000(5mins/10mins).after that,i got the good looking pellets from die,polished the surface.
in order to keep them(2 pellets) homogeneous, i put them into sintering at 1200℃for 1h(heating rate is 5℃/min).Then ,one is broken when the other is ok.
I don't know why,and it happened before.sometimes it works,sometimes it break.
Do i need to do a SEM?
Besides i checked the XRD pattern ,there is no phase transition.
This effect is not SPS specific it can also happen with hot pressed samples if you have inhomogeneous density after the SPS/HP step. As you probably know from many previous publications, depending on the material you produce (conductive/non-conductive), you get temperature gradients from middle to outer rim. this may lead to inhomogeneous densification. If you do not achieve full density at the edge or in the middle and expose the sample to a higher temperature the sample will continue to densify in the non dense volume. After that you will either see warping (the sample can bear the stress by creep deformation) ot the sample will crack especially if there are some pre-existing defects. In some cases when the strength of the material is high enough it will survive but have a high level of residual stress.
My experience is that when there is slighest amount of moisture in the sample there is a possibility of cracking. Eliminate the moisture by keeping it in an oven at 120°Cfor one hour and later you can sinter it. But this is one suggestion which helped us. I don't know in your case.
Suggestions above are good start. Determining the final relative density of the pellet will help finding the cause. Did the pellets densify further after they were sintered following SPS (why were they post sintered)? Was the atmosphere different? How large were the pellets? If the pellets were cooled too fast they could have cracked from thermal shock depending on what the material was and their size.
Two raw materials are mentioned but it is not clear if they were mixed (both samples from the same mix) or if each of the samples was made from a different raw material... Was there a reaction between the raw materials?
If any air/water/moisture is trapped inside the samples, i.e., through pores and cracks , this may result in propagation of cracks and complete fracture. It is suggested to vacuum drying the samples after wet grinding or polishing.
I have also experienced the same. We are making WC pellets of dia 30mm by SPS and able to get 99% TD. But when we do EDM cutting for making 13.4mm dia cylinders, few of them undergo planar crack like a cleavage. What could be the reason?
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