sintering

Sintering is how you turn a loose pile of powder into a solid object without ever melting it, the way nearly every ceramic and many metal parts are made. Think of a snowman: press fresh snow into a ball and the crystals weld together at their contact points into one solid lump, no melting needed. Sintering does the same to a pressed powder by heating it hot enough that the particles fuse and the whole thing shrinks into a dense body.

The driving force is the powder's huge surface area: countless tiny particles have an enormous amount of energy-costly surface, and the system can lower that energy by joining particles so their surfaces merge. At high temperature (typically two thirds to three quarters of the melting temperature in kelvin, but still solid), atoms become mobile enough to diffuse. They migrate to the necks where particles touch, growing bonds there and gradually filling in the pores between particles. As the pores shrink and vanish, the part densifies and shrinks overall, often by 10 to 20 percent in each dimension, which is why sintered parts must be made oversized. Grains grow and merge across old particle boundaries into a solid polycrystal.

This is the single most important step in ceramic processing, because you cannot cast a ceramic like a metal (its melting point is absurdly high and it would crack on cooling), so you shape a powder and fire it. The final strength depends fiercely on how completely the pores are removed: leftover pores act as flaws that lower strength and let liquids through, so makers use fine powders, applied pressure (hot pressing), or sintering aids to reach full density. The same idea builds powder-metallurgy gears, tungsten-carbide tool tips, and 3D-printed metal parts.