Spark Plasma Sintering (SPS) is normally carried out to obtain consolidated materials of low residual porosity. SPS can also be used for the preparation of partially densified (porous) materials. Partial densification is achieved during SPS by using relatively low sintering temperatures or pressureless conditions. For a pressureless assembly, short punches are used; in addition, sintering without the upper punch is possible. It was shown that the number of contacts between the nickel and synthetic diamond particles was higher in the compacts partially sintered by SPS than in those cold-pressed and vacuum-annealed at the same temperature. During pressureless SPS, conditions for non-uniform current distribution can be realized causing non-uniformities in the microstructure of the compacts. It was shown that SPS of metals without the upper punch produces porous gradient structures. Electric current passing through the porous compacts and interfaces between the compacts and the punches/foils under pressureless conditions induces specific local effects evidenced by the morphology evolution of the particles and microstructure of the sintered material in those areas of the compacts.
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