17.8: Powder Consolidation by Cold PRessing and Sintering
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Powder processing is often an attractive alternative to casting or deformation processing in order to produce shaped components. There are many situations in which it is the only viable option - for example, if the material is difficult to melt, contain or deform. The powder is mixed with some sort of fugitive (usually polymeric) binder and moulded into the required shape, often by simply applying pressure at ambient temperature to a flexible (rubber) mould containing the mixture. The resultant "green compact" is then fired at high temperature (usually well below the melting temperature of the material), when the binder is driven off and the compact becomes consolidated as a result of sintering - see below. The powder may be just a single species, although sometimes "sintering aids" are added.
Sintering often requires extensive diffusion, so the temperature usually needs to be relatively high (>~0.6Tm). The diffusion is driven by the resultant reduction in surface area. On a local scale, diffusion tends to reduce the local curvature of the free surface. This is illustrated in the figure below, where it can be seen that the diffusion can either occur within the surface or can be "internal" - ie through the lattice or via preferred routes such as grain boundaries or dislocations. This figure highlights an important point, which is that, while both types of diffusion cause growth of necks (via transport of material to regions of high surface curvature), raising the strength of the compact, only internal diffusion leads to densification (ie movement of the centres of particles towards each other). It's sometimes desirable to create strong compacts with relatively high porosity levels (eg for lubricated bearings). In such cases, conditions are sought that will favour surface diffusion over internal diffusion.
The animation below shows schematic depictions of the processes of mould filling, cold pressing and the diffusion that causes neck growth (with or without densification). Also shown are representations of the process variants of liquid phase sintering, reactive sintering and hot isostatic pressing (HIP).