Processing, fine powders preforming

A rather different approach has been developed for reaction sintered silicon carbide (RSSC), first developed in the former Soviet Union. In this process a powder preform of mixed graphite and silicon carbide is immersed in a liquid bath of molten silicon. The silicon wets and infiltrates the preform, reacting with the finely-divided graphitic component (carbon black). In the best case, all graphite is reacted and the residual sihcon content is no more than a few percent. The product of the reaction, silicon carbide, firmly bonds the silicon carbide preform powder... 

Open-pore microcellular aluminium foams can be produced by a process known as replication . This consists in infiltration of NaCl powder preforms by a melt, which is then solidified to form a composite. The NaCl is subsequently leached out with water, to leave a network of open pores, of volume fraction roughly varying between 65 and 90% [15], The foams can be produced to feature good microstructural homogeneity over a comparatively wide range of metal alloy compositions, pore size and component shape. They furthermore serve as attractive model materials for the investigation of microstructure/property relations in metal foams because of their macroscopically uniform and fine-scale microstructure, and because the metal making the foam can be varied with relatively wide latitude and produced free of internal defects. 

Drawbacks of these processes are capital cost and limited flexibility. Use of plasma spray deposition techniques requires sufficiently smooth surface of the underlying layer to be coated. Preformed perforated alloy sheets are thus preferably used for these processes. If the alloy is to be processed by powder metallurgy methods, fine-grained alloy powder is more favorable to produce substrate with small pore size and surface roughness. On the other hand, the resulting enhanced specific surface area may yield, after a certain operating time to Cr depletion in the porous substrate. This might induce non-protective breakaway oxidation if the critical Cr threshold is reached and subsequent breakdown in electrochemical performance. 

Polytetrafluoroethylene is the polymer resulting from the free radical polymerisation of tetrafluoroethylene CF2=CF2. The reaction is usually carried out in an aqueous emulsion which produces sub-micron particles of the polymer, or in a suspension which gives rise to particles of 100 xm or greater. The polymer is processed from various resin grades, which differ in fineness, by means of preforming followed by sintering as in powder metallurgy. 

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