boron carbide
Boron carbide is B4C — one of the hardest materials known, sitting just behind diamond and cubic boron nitride, and among the very hardest that can be made cheaply enough to use in bulk. Combine that extreme hardness with a very low density (it is lighter than aluminium) and you get the ideal recipe for lightweight armour: boron carbide is the ceramic in bulletproof plates and helicopter seat armour, where every gram counts. It is also the tough, wear-defying grit in the hardest lapping and polishing compounds, the material of sandblast and spray nozzles, and, because boron greedily absorbs neutrons, a control-rod and shielding material in nuclear reactors.
Boron carbide has a complex covalently bonded crystal structure — twelve-atom boron icosahedra linked by short atom chains — and it is not strictly stoichiometric, its carbon content varying over a range around B4C. That stiff covalent bonding is why it is so hard (about 9.5 to 9.75 on the Mohs scale, third only to diamond and cubic BN) and so light (density near 2.5 g/cm3, versus 3.2 for SiC and 3.9 for alumina). Its low weight per unit of hardness is the number that sells it for armour: a boron-carbide plate stops a projectile at a fraction of the mass of a steel one. It is also extremely resistant to abrasion and chemically stable, and its boron makes it an excellent absorber of thermal neutrons.
The honest limits are as sharp as the material. Boron carbide is very hard but brittle and comparatively low in fracture toughness, and it is difficult and expensive to sinter to full density because those strong covalent bonds resist mass transport — dense parts usually require hot pressing or special sintering. A subtler and important issue for armour is that under very high-velocity impact boron carbide can suffer a sudden loss of strength known as amorphization, where the crystal structure locally collapses to a glassy phase along the shock, which is one reason it is often used against certain threats but paired or replaced with silicon carbide against others. In short: unbeatable hardness-per-weight, but a brittle, costly ceramic with real behavioural quirks under extreme loading.
A body-armour plate uses boron carbide backed by a tough fibre layer: the ceramic's extreme hardness blunts and shatters an incoming bullet while its very low density keeps the plate light enough to wear all day — the hardness-per-weight advantage that steel simply cannot match.
Boron carbide (B4C): among the hardest and lightest ceramics — the armour material — but brittle, costly to densify, and prone to amorphization under extreme shock.
Boron carbide's hardness-per-weight makes it the armour ceramic, but it is not universally the best. Under very high-velocity impact it can amorphize and lose strength suddenly, which is why silicon carbide is sometimes chosen against harder or faster threats.