hot pressing
Sometimes the gentle nudge of surface energy is simply too weak to squeeze out the last pores of a stubborn ceramic. Hot pressing brute-forces the problem: heat the powder and press on it at the same time. The powder sits in a die — usually graphite, since it stays strong when red-hot — and a ram pushes down on it while the whole assembly is held at sintering temperature, so densification is driven by the external pressure on top of the surface-energy driving force. It is the ceramic equivalent of a blacksmith forging hot metal rather than waiting for it to settle on its own.
The power of the method is in the numbers. The intrinsic capillary stress from surface curvature in a fine powder is only a few MPa; a hot press applies perhaps 20 to 50 MPa, an order of magnitude more, and that external stress adds directly to the driving force for densification (it enters the sintering equations as an extra term alongside the surface-energy stress). The consequences are all the things you want: full density is reached at a lower temperature and in a shorter time than pressureless sintering, which means grains have less chance to grow, giving a finer, stronger microstructure; and materials that flatly refuse to densify without pressure — the hard covalent solids like silicon carbide, silicon nitride, and boron carbide — can be pushed to near-full density. The pressure also crushes shut the residual closed pores of the final stage that surface energy alone cannot remove.
The price of hot pressing is shape and throughput. Because the pressure is uniaxial — pushed in one direction through a die — you are limited to simple shapes like discs, plates, and blocks, and only one part (or a short stack) is made per cycle, slowly, with the die heated and cooled each time. Graphite dies also react with or contaminate some ceramics and limit the atmosphere. So hot pressing is reserved for demanding materials and high-value parts: cutting-tool inserts, armour tiles, sputtering targets, research-grade dense ceramics. Where complex shapes or full density beyond what a uniaxial die can reach are needed, its cousins take over — hot isostatic pressing applies gas pressure from all sides, and spark-plasma sintering adds a fast internal current.
Boron carbide is nearly impossible to sinter dense without pressure, so armour-grade B4C tiles are hot pressed: powder in a graphite die at roughly 2100 degrees C under about 30 MPa reaches near-full density in a couple of hours, as flat plates that are then ground to shape.
Hot pressing adds a large external stress to the weak surface-energy driving force — dense, fine-grained, but simple shapes only.
Hot pressing's applied stress dwarfs the intrinsic capillary stress, so it densifies hard covalent ceramics that pressureless sintering cannot. But it is uniaxial: only simple shapes, one slow part at a time, often with graphite contamination.