solution-reprecipitation
In liquid-phase sintering the fast initial squeeze comes from particles simply sliding into tighter packing, but that alone cannot close all the space — the grains are still awkward shapes that do not fit together. Solution-reprecipitation is the clever middle stage that solves this: the solid literally dissolves away where grains are jammed against each other and grows back where there is room, so the grains reshape themselves to fill space and pack densely. Material is moved from the crowded contact points to the open pockets, through the liquid, atom by atom.
The mechanism turns on a pressure trick. Where two grains press together through the thin liquid film, that contact carries a high stress — the capillary force of the liquid plus any applied pressure, focused on a small area. A solid under pressure is slightly more soluble than the same solid unstressed (the same reason ice melts under a skate blade), so the loaded contact points dissolve preferentially. The dissolved material diffuses through the surrounding liquid down the concentration gradient and reprecipitates onto the unstressed grain surfaces and onto larger grains. The net effect is threefold: the grains flatten and interlock at their contacts so the body shrinks and densifies; the grain shapes accommodate to fill the pores; and because small grains are more soluble than large ones, small grains dissolve and feed the growth of large ones — Ostwald ripening, which coarsens the grain size. The rate is governed by whichever step is slowest: the solubility of the solid, its diffusion through the liquid, or the reaction at the interface.
Solution-reprecipitation is what makes liquid-phase sintering so effective at reaching full density at modest temperature — it removes material from between grain centres (true densification) while a liquid, not slow solid diffusion, carries the matter. It is the densifying heart of firing porcelain, cemented carbides, silicon nitride with oxide additives, and many electroceramics. But it comes with an honest cost. The grain coarsening it drives can be excessive, and it demands appreciable solubility of the solid in the liquid — which is precisely why some very covalent solids need carefully chosen additives to work at all. And like all liquid-phase routes, the liquid that carried the matter freezes on cooling into a grain-boundary phase whose properties then haunt the ceramic at high temperature.
In cemented tungsten carbide, molten cobalt dissolves WC at the pressed grain contacts and reprecipitates it elsewhere; the carbide grains reshape and pack to near-full density, and on cooling the cobalt sets as a tough binder threading between them — a classic solution-reprecipitation densification.
Solid dissolves at stressed contacts, travels through the liquid, and reprecipitates in the pores — grains reshape and densify.
Solution-reprecipitation densifies and coarsens at once: the same dissolving of small grains that lets contacts flatten also feeds the growth of large grains, so pushing it hard for density can leave you with a coarse microstructure.