alumina

Alumina is aluminum oxide, Al2O3, and it is the workhorse of engineering ceramics, the everyday high-tech ceramic. You have met it without knowing: it is the white ceramic in spark plugs, the substrate under many electronic chips, the beads inside cheap sandpaper, and, as its natural single-crystal form corundum, it is ruby and sapphire (coloured by tiny impurities). It is cheap, extremely hard, stiff, and chemically almost indestructible.

Its power comes from a compact ionic-covalent crystal in which every aluminum is surrounded by six oxygens, all held by strong short bonds. That packing gives a Young's modulus around 380 GPa (nearly double that of steel), a hardness of 9 on the Mohs scale (just under diamond), a melting point above 2000 degrees C, and it is an excellent electrical insulator that still conducts heat reasonably well. Engineers make it by pressing very fine Al2O3 powder into shape and sintering it near 1600 degrees C until the grains fuse into a dense body; purity and fine grain size push flexural strength up to roughly 300 to 500 MPa.

Alumina is the honest workhorse: strong, hard, and heat-resistant, but like every ceramic it is brittle, with low fracture toughness, so it fails suddenly from flaws and cannot tolerate impact or sharp thermal shock. Where you need more toughness, engineers turn to zirconia or to alumina-zirconia composites. Its combination of wear resistance, chemical inertness, and low cost is why alumina appears in cutting-tool inserts, grinding media, wear-resistant liners, biomedical hip-joint balls, and electrical insulators.