Dissolution, liquid phase sintering

Specific surface area. The specific surface area is an important criterion for the sintering activity (solid state sintering), dissolution processes (liquid phase sintering), and reaction with gaseous or solid substances during carburization. Commonly, it is in the range of 0.01 m /g (coarse powders) up to 12m /g (very fine powders). 

This mechanism is similar to the one occurring during liquid-phase sintering, where the dissolution of crystalline material into the glassy phase occurs at the interfaces loaded in compression and their reprecipitation on interfaces loaded in tension. The rate-limiting step in this case can be either the dissolution kinetics or transport through the boundary phase, whichever is slower. This topic was discussed in some detail in Chap. 10, and will not be repeated here. 

The manufacturing process (especially the cold and hot compaction stages) of pcBN is very similar to that of pcD, summarized in Section 1.3.3. However, the types of binders used and liquid phase sintering mechanisms can be quite different. In pcBN production, reactive sintering plays a major role in terms of driving the densification process, whereas in pcD, dissolution and precipitation and/or adhesion/coalescence are the main driving mechanisms. A typical example for pcBN would be the following reaction [162]. 

Fig. 5.43 The three mechanisms of grain-shape accommodation and neck growth during solution reprecipitation controlled densification of liquid-phase sintering a contact flattening, b dissolution of small grains, and c solid-state bonding. Reproduced with permission from [73]. Copyright 2009, Springer...

For solid phase sintering, there are fonr ways of diffusion i) surface diffusion, ii) volnme diffusion (often called lattiee diffusion), iii) vapor phase transport (evaporation-eondensation), and iv) grain boundary diffusion the boundaries are very disturbed areas, which allow diffusion short-circuits . For liquid phase sintering, we must add dissolution-reprecipitation effects or a vitreous flow. Finally, for pressure sintering the pressure exerted allows the plastic deformation of the crystallized phases and the viscous flow of the amorphous phases. 

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