When a green ceramic is subjected to firing, the sintering process is expected to result in shrinkage, densification, reduction in porosity, improvement in strength, hardness and other properties. What changes are expected when the once sintered ceramic is again sintered? The need for re-sintering may perhaps arise due to for example, hypothetically, failure of power to the sintering furnace or unexpected behavior when there is a change in raw materials or consumables or equipment.
This is a question arise in a practical situation. Thank you for throwing this question in this platform. We really need a discussion on this topic. We really become worried in a situation of power cut while the sintering process is on in furnace.
What happens when the sample is heated to certain extent and then there is a power cut, the sample cool down may be or not to room temperature. Then again the sample is heated to your desire temperature. So you are doing a three stage sintering, initial heating, cooling and then again heating. So sintering must be different with respect to one go sintering. In an a one stage sintering desification is better than three stage sintering as in the later case curing occur. Therefore grain growth is stopped while withdrawing the heating.
The influence of an interruption of the sintering process on ceramic quality depends in the first place on the material. For traditional clay-based ceramics, this influence is minimal as long as the unexpected cooling did not cause cracks. It will, well, nothing bad happen when the part of clay-based ceramic undergoes a three-stage sintering.
For other materials it could be very different. I suspect that all silica-based refractory products are very sensitive to interruption of the sintering process. This is due to volume changes in polymorphic transformations of beta quartz to alfa quartz and from alfa quartz to cristobalite. Even other ceramics with multiple crystal lattice modifications can break down during power outage.
Vadim already added a very good comment. Phase transitions are indeed a big problem if the sintering cycle is interrupted. In case of zirconia this may also lead to problems due to tetragonal-monoclinic phase transformation at ~1100°C which is associated with ~5% expansion.
For alumina, SiC and Si3N4 it is not a general problem.
If the sintering cycle fails during the dwell a subsequent cycle with identical firing conditions may lead to different properties due to grain growth and different resulting microstructure. In some cases interrupted firing cycles may lead to loss of sintering activity.
On the other hand multiple firing is a standard process e.g. in sinter-HIP processes where the ceramic is prefired until open porosity is eliminated and subsequently HIPed (very often an annealing process at lower temperature follows to eliminate residual stress).
During sintering of a compacted powder, densification and grain growth occur simultaneously through atomic diffusion mechanisms. The temperature profile during sintering process has a major effect on final product density and microstructure. If the furnace cooling during power failure does not cause structural damage to sintered bodies, resuming heat treatment could be considered as another stage of the process. Heating curve control will be probably needed to manipulate the relationship between densification rate and grain growth rate to obtain similar results as the original process. https://www.nature.com/articles/35004548 http://cdn.intechopen.com/pdfs/29792/intech-mechanisms_of_microstructure_control_in_conventional_sintering.pdf
This is a general question, thus, the answer will be general too. The expectations depend on the type of the pre-sintered ceramic, its structural properties after the first sintering, and the details of the second heat treatment. The expectations should be one of the followings:
1- No changes.
2- Physical/structural changes including the changes in the dimensions, porosity, flaws, grain size, internal stresses, crystallinity, homogeneity, phases, and so on.
3- Chemical changes including the thermal decomposition and the phase changes due to chemical reactions.
4- Chemical and physical/structural changes.
Based on that, the properties of the ceramic will be altered.
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