coarsening
There is more than one way for a powder to lower its wasteful surface energy, and not all of them help you. Coarsening is the unhelpful one: instead of removing surface by squeezing out the pores, the microstructure removes surface by growing its features — small particles and pores dissolve or shrink away while large ones grow, so you end up with fewer, bigger grains and fewer, bigger pores. The total surface area drops, the energy bill falls, and yet the fraction of empty space barely changes. The body has rearranged itself without getting any denser. Think of the ice crystals in ice cream left too long in the freezer: no new water, but the fine crystals ripen into coarse, gritty ones.
The engine of coarsening is curvature, acting through the same Kelvin and Gibbs-Thomson relations that drive everything in sintering: a small particle, being more sharply curved, has a higher chemical potential (or vapour pressure, or solubility) than a large one, so matter drifts from small to large. It is carried by exactly the transport paths that source their matter from the surface or the vapour — surface diffusion and evaporation-condensation in solid-state sintering, and dissolution-and-reprecipitation through a liquid in liquid-phase sintering. Because these paths move material around the outside of particles or across pores without ever emptying the space between particle centres, they grow the microstructure without densifying it. In the final stage, coarsening also shows up as grain growth that enlarges the grains and drags or abandons the pores.
Coarsening is the villain of the densification story, and understanding it dissolves the beginner's puzzle of why hotter and longer is not always denser. Every degree and every minute feeds both densification and coarsening, since they share the same driving force; the question is only which wins. Coarsening hurts twice over: it spends the driving force that densification needed, and by enlarging pores and grains it makes the remaining porosity slower to remove and more likely to be stranded inside a grain. That said, coarsening is not always the enemy — it is deliberately used to grow grains for some electrical ceramics, to develop the interlocked microstructures that toughen silicon nitride, and wherever a controlled grain size is the goal. The art of sintering is largely the art of tilting the balance: fine powder, fast heating through the coarsening-prone low temperatures, and dopants that slow grain boundaries all serve to let densification outrun coarsening.
Two magnesia powders fired to the same temperature: the coarser one ends up with big grains and big pores at only 90 percent density, while the finer one reaches 98 percent — the coarse powder spent its driving force on ripening its features instead of eliminating the empty space.
Coarsening lowers surface energy by growing grains and pores, not by removing them — the rival of densification.
Densification and coarsening are not sequential; they compete for the same driving force at every moment of firing. This is why simply raising temperature or extending time can lower final density: past a point, coarsening outruns densification and locks porosity in.