the final stage of sintering

Near the end of firing, the connected tunnels of the middle stage grow too thin to survive and pinch off, the way a stream in a drying puddle breaks into separate droplets. What was one continuous pore network becomes a scatter of isolated, rounded pores, most of them tucked at the corners where three or four grains meet. The final stage of sintering is this closing act — carrying a body from roughly 90 percent density up toward full density by emptying those last stranded pockets of empty space. It sounds like a formality; it is in fact the hardest part of the whole process, and the reason most ceramics fall short of the last one or two percent.

Two things make the endgame so difficult. First, the pores are now closed — sealed off from the outside surface — so any gas trapped inside them (furnace atmosphere, or gas from a decomposing additive) can no longer escape; as the pore tries to shrink, the gas compresses and pushes back, and the pore can stall at an equilibrium size or even swell (bloating). This is why the final stage is often run under vacuum or in a gas like hydrogen that dissolves and diffuses out through the solid. Second, and more fundamental, grain growth is now rapid, and a pore is only removable while it sits on a grain boundary, which gives its vacancies a fast highway to a sink. If a fast-moving boundary tears away from a pore and leaves it stranded inside a grain, the pore is now served only by slow lattice diffusion and is effectively frozen in place.

The whole craft of reaching full, pore-free density — needed for transparent alumina, for the strongest structural ceramics, for pore-sensitive dielectrics — comes down to winning the final stage by keeping pores married to boundaries until they vanish. The landmark example is Coble's translucent alumina (Lucalox), where a tiny magnesia addition slows the grain boundaries just enough that they never break away from their pores, and the last porosity is swept out to leave a body so dense it transmits light. Pressure-assisted routes (hot pressing, HIP, SPS) attack the same final stage from the other side, using an external stress to collapse the last closed pores that surface energy alone is too weak to remove.