Transformation toughening. Grains are held in a metastable tetragonal phase, and the stress field ahead of an advancing crack triggers them to transform to the monoclinic phase, which occupies slightly more volume. That local expansion compresses the crack and arrests it. No other common technical ceramic has an equivalent built-in mechanism.

Tetragonal zirconia polycrystal stabilised with 3 mol per cent yttria. The yttria is what holds the tetragonal phase at room temperature so that transformation toughening is available. It is the most widely stocked engineering grade; magnesia-stabilised and ceria-stabilised grades trade some strength for better resistance to hydrothermal ageing.

It is spontaneous transformation of the surface phase in warm moist conditions, roughly 150 to 400 degrees Celsius and notably in hot water or steam, causing microcracking and progressive strength loss. It matters wherever a part sees wet heat for a long service life. Grade choice controls it, so state the environment when specifying rather than ordering by strength alone.

Less well than its toughness suggests, because its thermal conductivity is around a tenth of alumina's. Heat cannot redistribute through the part, so a rapid change produces steep gradients and high internal stress. Its toughness helps it survive what would crack alumina, but silicon nitride remains the correct choice for genuine thermal shock duty.

Where failure would come from impact, edge chipping or crack propagation rather than abrasion: pump plungers and shafts, valve components, cutting blades for film and fibre, extrusion and wire-drawing dies, precision balls, and insulating spacers where its low conductivity is wanted. It costs considerably more, so it is specified for the mechanism, not by default.

It takes an exceptional polish, and that is one of its strengths. The fine grain size allows a roughness surface that holds up in sliding contact, which is why it appears in seal faces, plungers and bearing components. Finishing is by diamond, as with all fired technical ceramics.

At room temperature yes, but its resistivity falls with temperature far more sharply than alumina's, and stabilised zirconia becomes an oxygen-ion conductor when hot - the property that makes it the sensing element in oxygen sensors and fuel cells. For high-temperature electrical insulation, alumina is the correct material.