hardness

Hardness is a material's resistance to being scratched or dented, and it is the property that makes ceramics the natural choice for cutting tools, grinding abrasives, armour, and scratch-proof watch faces. Rub two minerals together and the softer one is scratched; that everyday test is the basis of the old Mohs scale, which runs from soft talc at 1 up to diamond at 10. Ceramics dominate the hard end of that scale because their strong, short, directional ionic-covalent bonds fiercely resist any rearrangement of atoms, so it is very hard to push their surface out of the way.

The engineering measure presses a hard diamond tip into the surface under a known load and measures the size of the permanent dent it leaves: the smaller the dent, the harder the material. A Vickers test uses a square pyramid and reports HV = 1.854 times F / d^2, where F is the load and d the diagonal of the impression, giving a hardness in GPa or kg/mm2. The values span a wide range: window glass around 5 to 6 GPa, alumina 15 to 20, silicon carbide about 25, boron carbide near 30, and diamond an unmatched 70 to 100 GPa. Because a sharp indenter also cracks a brittle ceramic, the lengths of the cracks radiating from the corners of the dent can even be used to estimate the fracture toughness, tying hardness and toughness together in one quick test.

Hardness matters wherever surfaces must survive contact and wear: alumina and silicon-nitride cutting inserts, silicon-carbide and boron-carbide abrasives, ceramic bearings, and boron-carbide or silicon-carbide armour plate. But there is a blunt honesty here, one worth remembering. Hardness usually trades off against toughness, because the very immobility of dislocations that makes a ceramic hard is what also makes it brittle. The hardest ceramics, boron carbide and diamond, are among the least tough and crack readily. So hardness is not strength and it is not toughness; it measures resistance to denting, and a superbly hard material can still shatter from a single sharp blow.