alumina
/ uh-LOO-mih-nuh /
Alumina is aluminium oxide, Al2O3 — the single most important engineering ceramic by volume, the reliable white workhorse behind spark-plug insulators, cutting-tool tips, wear-resistant liners, laboratory crucibles, orthopaedic hip balls, and the substrates that carry microchips. If porcelain is the ancestor of everyday ceramics, alumina is the plain, tough, do-everything member of the advanced family: not the hardest, not the toughest, not the most refractory, but a superb all-round balance of hardness, strength, electrical insulation, chemical inertness, and cost. Its natural single-crystal form is the mineral corundum; coloured by traces of chromium it is ruby, by iron and titanium it is sapphire.
The material chemistry is clean and well behaved. Stable alpha-alumina has the corundum structure: oxygen ions in a nearly hexagonal close-packed array with aluminium ions filling two-thirds of the octahedral holes, giving very strong, tightly bonded, densely packed crystals. That structure makes alumina hard (about 9 on the Mohs scale, second only to a few materials), stiff (Young's modulus near 380 GPa), a fine electrical insulator, and chemically inert to most acids, bases, and molten metals. Engineering aluminas are graded by purity: a 96 percent alumina contains a few percent of a glassy grain-boundary phase (from sintering aids) and is cheaper and easier to fire, while 99.9 percent high-purity alumina has almost no glass, fires denser, and reaches higher strength and better high-temperature and electrical performance. Fine grain size and near-full density are the levers that push strength up.
Because it hits a sweet spot of properties at a moderate price, alumina turns up almost everywhere ceramics are used: as electrical insulation in spark plugs and electronic substrates, as abrasive grit and grinding media, as wear parts (seals, valves, pump liners, textile guides), as the ball of a hip-replacement joint, and as furnace tubes and crucibles. Two honest limits keep it from being the answer to everything. First, like all ceramics it is brittle and flaw-controlled, so it is far weaker in tension than a metal and fails from its worst flaw. Second, in cheaper grades the glassy grain-boundary phase softens at high temperature, dragging down high-temperature strength and creep resistance — which is why the most demanding jobs pay for high-purity, glass-free alumina.
The white insulator inside a spark plug is high-alumina ceramic: it must hold off tens of thousands of volts, survive combustion-chamber heat and thermal shock, and resist chemical attack for years — a job alumina does cheaply, which is why billions of alumina insulators are fired every year.
Alumina (Al2O3, corundum structure): the all-round advanced ceramic — hard, stiff, insulating, inert, and affordable — used from spark plugs to hip balls.
Alumina purity grade matters more than the name suggests. A 96 percent alumina carries a glassy grain-boundary phase that softens hot and cuts high-temperature strength; only near-pure alumina (99.5 percent and up) keeps its strength and creep resistance at the top of its range.