densification
A freshly pressed ceramic part is mostly full, but not entirely: perhaps 40 percent of its volume is empty space between the powder particles. Densification is the process that closes that gap — the steady disappearance of porosity during firing, so the piece grows denser and, because the same amount of material now occupies less room, physically shrinks. It is the whole point of sintering: you can bond particles into a weak, porous body without densifying, but a strong, watertight, load-bearing ceramic demands that the empty space be driven out.
Density is measured against a ceiling. The theoretical density is what the material would have with zero porosity — a perfect single crystal's density. Relative density is the fraction of that ceiling actually reached, so a green body might be 55 percent and a well-sintered part over 98 percent. As the pores empty and density climbs, the body contracts by roughly the cube root of the volume change: eliminating 40 percent porosity down to near zero gives about 15 to 20 percent linear shrinkage, which is why sintered parts must be designed oversized and why uneven shrinkage warps them. The defining requirement of densification is subtle but absolute: it only happens when matter is removed from between the particle centres, so that the centres approach. That means matter drawn from the grain boundaries or the lattice — grain-boundary and lattice (volume) diffusion, or plastic and viscous flow. Matter merely shuffled over the surface fattens necks without ever pulling the centres in, and so does not densify.
This is the crux of the whole field: densification is one of two things a powder can do with its surface energy, and its rival, coarsening, is always running alongside. Both lower the surface energy, but coarsening does it by growing grains and pores rather than removing them, and once grains grow fast enough to abandon their pores, densification stalls with porosity still trapped inside grains. So more heat or more time is not a reliable path to higher density — beyond a point they feed coarsening faster than densification. The maker's craft is to keep the densifying paths ahead: fine uniform powder for a big driving force, a firing schedule that reaches the densifying temperatures quickly, grain-growth inhibitors to keep pores on boundaries, and, when surface energy alone cannot finish the job, an applied pressure to force the last pores shut.
A silicon-nitride turbine part is pressed to about 55 percent density, then sintered until it reaches over 98 percent — and in doing so it shrinks nearly a fifth in every dimension, so the mould had to be cut oversized to hit final size after firing.
Densification: porosity leaves, density rises toward theoretical, and the part shrinks about 15-20 percent linearly.
Densification is not the same as bonding. Necks can grow and a body can gain strength with almost no shrinkage if the matter came from the surface. True densification requires the particle centres to approach, which only boundary- and lattice-sourced transport achieves.