Because it resists the two things that destroy a seal simultaneously: abrasive wear and chemical attack. It is nearly as hard as diamond, so entrained particles do not score it, and it is inert to most process chemistry. Its high thermal conductivity also removes frictional heat from the face, which is what causes dry-running failures in less conductive materials.

Sintered grades are fully reacted with no free silicon, giving the best chemical resistance and the widest service range. Reaction-bonded grades are infiltrated with silicon that remains in the structure - cheaper, easier to produce in complex shapes, and nearly equivalent mechanically, but the free silicon is attacked by strong alkalis and hydrofluoric acid.

The best of the commonly stocked ceramics apart from fused silica. High thermal conductivity spreads heat through the section rather than letting it concentrate, and low thermal expansion means the resulting gradients generate modest stress. This is why it dominates kiln furniture, burner components and radiant tubes where parts are cycled routinely.

It is a semiconductor, so unlike alumina it cannot be treated as an insulator - conductivity varies considerably with grade and doping, and rises with temperature. Where a part must insulate electrically as well as resist heat, alumina or machinable glass ceramic is the appropriate material.

Low fracture toughness. It is superb against abrasion and poor against impact or point loading, and it gives no warning before failure. Most in-service breakages trace to mounting rather than to the material: clamping on a point, or restraining a part in a metal housing whose thermal expansion is several times higher.

Diamond grinding only, and it is slower and costlier than for alumina because of the hardness. Stock is supplied close to finished dimensions - seal face blanks, rings, tube, plate and rod - so that only lapping or light grinding is needed. Designing to a stocked size is normally far cheaper than machining from a larger blank.

Sintered silicon carbide resists essentially all mineral acids, including hot concentrated sulphuric and nitric, and most alkalis, which is a major reason it is used in chemical pumps and seals. Reaction-bonded grades share this except where their free silicon is exposed to strong caustic or hydrofluoric acid.