boundary breakaway
Imagine dragging a wide net through water with a few floating corks caught in its mesh. Pull gently and the corks come along; pull too hard and the net tears past them, leaving the corks bobbing behind. Boundary breakaway is exactly this in a firing ceramic: a grain boundary that had been dragging a pore along with it moves too fast, detaches, and leaves the pore stranded inside the newly enlarged grain. It is the single most common reason a ceramic fails to reach full density, and the moment a pore goes from 'about to be removed' to 'trapped for good'.
The physics is a race of forces. A boundary migrating during grain growth is pushed by its own curvature; the pore sitting on it pushes back with a drag force, because the boundary would have to re-create erased area to leave the pore. As long as the drag can match the driving pressure, the pore and boundary travel together and the pore keeps its fast grain-boundary diffusion route to shrink away. But the drag a pore can supply is limited by how fast the pore itself can move (by surface diffusion around it, or evaporation across it). When the boundary's driving pressure exceeds the maximum pore drag — most dangerously during abnormal grain growth, when one grain balloons and its boundary sweeps fast across many pores — the boundary breaks away. The pore is now inside a grain, far from any boundary, served only by sluggish lattice diffusion, and for all practical purposes it can no longer be removed.
Boundary breakaway is why 'fire it hotter to finish densifying' so often backfires, and why controlling grain growth is inseparable from reaching full density. The canonical triumph over it is Coble's translucent alumina: adding a small amount of MgO slows the alumina grain boundaries just enough that they stay attached to their pores throughout the final stage, so no pore is ever stranded and the last porosity is swept out, yielding a body dense enough to pass light (the basis of high-pressure sodium lamp envelopes). The same lesson governs any pore-free ceramic — transparent armour, the strongest structural grades, low-loss dielectrics: keep the boundaries slow, keep the pores on them, and never let the boundary win the race until the pores are gone. When surface energy alone cannot keep pace, pressure-assisted sintering (hot pressing, HIP, SPS) sidesteps breakaway by collapsing pores with external stress before grains can grow away from them.
Look at a ceramic that was overfired: under the microscope you find large grains each with a rounded pore sitting near its centre. Those pores mark boundaries that raced ahead and broke away — the pores were caught mid-removal and stranded where no boundary can now reach them.
Boundary breakaway strands a pore inside a grain, out of reach of the fast grain-boundary path — density locks up.
A pore trapped inside a grain by boundary breakaway is essentially permanent under normal firing — only slow lattice diffusion can serve it. Preventing breakaway (fine powder, grain-growth dopants, or applied pressure) is far easier than curing it.