Four reasons together: about 40 per cent the density of steel, so centrifugal loading falls sharply at speed; no tendency to micro-weld to steel races, so adhesive wear stops; hardness that resists debris denting; and electrical insulation, which prevents the shaft-current pitting that destroys motor bearings on variable-frequency drives.

Silicon carbide is harder, more thermally conductive and better against pure abrasion. Silicon nitride is considerably tougher and better in rolling contact and under mechanical load, with comparable thermal shock resistance. Choose carbide for abrasive sliding and chemical service, nitride where the part carries load or takes impact.

It is a standard choice for handling molten aluminium in particular, which is not wetted by molten aluminium, so metal does not adhere, and it survives the thermal cycling of repeated immersion. Riser tubes, thermocouple protection sheaths, heater tubes and degassing components are conventional applications.

Principally sintered, gas-pressure sintered and hot-pressed, differing in density, strength, toughness and cost. Bearing-grade material is a separate specification with tighter limits on inclusions and surface finish. The variation between grades is larger than for most ceramics, so ordering by material name alone is not sufficient.

Yes, and unlike silicon carbide it can be relied on as an insulator. That property is what makes hybrid bearings effective against electrical discharge damage, and it also suits welding fixtures and locating pins, where the part must position the work without carrying current across it.

It retains useful strength to around 1000 to 1200 degrees Celsius depending on grade, which is less than alumina's ceiling but with far better resistance to thermal cycling on the way there. In oxidising air at the top of that range a protective oxide layer forms, and long-term service should be assessed against the specific grade.

Processing. Silicon nitride does not sinter readily and needs sintering aids, pressure or controlled nitrogen atmospheres to reach full density, and bearing-grade material adds inspection and finishing requirements. It is specified where the combination of toughness and thermal shock resistance is genuinely needed rather than as a general-purpose ceramic.