In technical terms, sintering is the solidification and compaction of a green compact or powder forming a compact material after temperature treatment. It is characteristic for this process that not all components melt and thus the outer shape is maintained. Compaction may only result in shrinkage which is as even and reproducible as possible. In the course of this process, a homogenous and crack-free material is to be produced with a highly microcrystalline structure and a certain degree of porosity (in many cases as low as possible)
Sintering is categorized according to two processes:
These processes may be accompanied by chemical reactions which highly influence the sintering process and the microstructure.
Sintering without liquid phase
The sintering temperature for solid phase sintering is below the melting point of all phases involved in the process. Sintering can be triggered only on the basis of diffusion processes, vaporization and condensation mechanisms and, if pressure is applied from the outside, also of crystal-plastic flow. During this process, the free energy of the system decreases by the reduced surface and boundary surface energy.
Sintering with liquid phase
Sintering agents are added for liquid phase sintering and heated to a temperature which is higher than the solidus temperature of the material system resulting in liquid phases. The following types melting phase generation are classified: Limited melting of homogenous sintering material, melting of a phase in a heterogeneous sintering material without reaction, and generation of a melting phase in a heterogeneous material caused by chemical reaction.
It must be noted that burnt refractory products always have a consistent sintering state. Refractory materials, in contrast, which are not sintered until applied onsite, e.g. after furnace lining, have an inconsistent sintering state depending on the temperature gradient in the material layer from the inside of the furnace towards the outside.