a grain

Break a fired ceramic, polish the broken face, etch it lightly and look under a microscope, and you almost never see one smooth crystal. You see a mosaic, like a dry-stone wall or a stained-glass window made of countless little tiles pressed together. Each tile is a grain: a single small crystal in which the atoms are stacked in one continuous, orderly pattern with one orientation. Its neighbour is the same material with the same recipe, but its atomic rows point a different way, so a visible seam runs between them. Almost every engineering ceramic — alumina, zirconia, silicon carbide — is polycrystalline, an aggregate of millions of these grains, because it is built by welding a powder of tiny crystals together in the fire rather than growing one big single crystal.

A grain is born from one powder particle (or a cluster that fused early) and grows as the piece is fired. Inside it the lattice is (ideally) perfect and periodic; the grain ends abruptly where its orientation no longer matches the neighbour, and that surface is the grain boundary. Grain size is usually reported as an average diameter: a well-fired structural alumina runs about 1 to 10 microns (a micron is a thousandth of a millimetre), a coarse refractory can be tens or hundreds of microns, and a nanoceramic under 100 nm. As a rough feel, a 1 micron grain contains on the order of a few billion atoms across roughly ten thousand atoms per edge.

Grain size is one of the master dials of ceramic behaviour, which is why we bother to see and measure it. Finer grains generally mean higher strength, because the biggest grain (or a pore trapped at a grain) tends to set the size of the worst flaw, and strength falls as the square root of flaw size (the Griffith idea). Finer grains also mean more grain-boundary area, which changes creep, electrical conduction and how a crack travels. So a huge part of ceramic engineering is really grain engineering: choosing a powder and a firing schedule that end in the grain size you want.