a boride ceramic
A boride ceramic joins a metal to boron. The best-known are titanium diboride TiB2 and zirconium diboride ZrB2, dense, hard, and among the most heat-tolerant solids humans can make, melting above 3000 degrees C. If oxides are the stable branch, carbides the hard branch, and nitrides the tough branch, borides are the extreme-temperature branch, built for the leading edge of a hypersonic vehicle.
Borides have an unusual mixed bonding: covalent boron-boron networks plus metallic bonding through the metal atoms. So, oddly for ceramics, they conduct electricity and heat rather well while staying extremely hard and refractory. The transition-metal diborides (TiB2, ZrB2, HfB2) are the headline members of the ultra-high-temperature ceramics (UHTCs), the class defined by melting points above about 3000 degrees C.
Borides appear as UHTC leading edges and nozzles for hypersonic and re-entry vehicles, as electrodes and evaporation boats (because they conduct), and as very hard wear and armour components, often combined with SiC to improve oxidation resistance. The honest limits: pure diborides are hard to densify (they need very high temperature or pressure-assisted sintering) and they oxidise in hot air, forming liquid boria B2O3 that can either protect or, above its useful range, evaporate away, so UHTC design is as much about the oxide scale as about the boride itself.
The leading edge of a sharp hypersonic vehicle, glowing past 2000 degrees C, may be zirconium diboride (ZrB2) mixed with silicon carbide, a boride ultra-high-temperature ceramic chosen because almost nothing else stays solid and holds its shape that hot.
Metal-plus-boron: ultra-refractory (melting above 3000 degrees C) and, unusually for a ceramic, electrically conductive.
Borides sit outside the 'ceramics are insulators' stereotype, their metallic bonding makes many of them good conductors, a useful reminder that the property rules are tendencies, not laws.