the zirconia polymorphs

Zirconia, zirconium dioxide (ZrO2), is the textbook case of ceramic polymorphism because it changes structure three times between room temperature and its melting point. At room temperature it is monoclinic; above about 1170 degrees C it becomes tetragonal; above about 2370 degrees C it becomes cubic (the fluorite structure), and it melts near 2680 degrees C.

The troublesome step is the tetragonal-to-monoclinic change on cooling. It is a fast, shearing, diffusionless (martensitic) transformation accompanied by a 3 to 5 percent volume expansion, and that sudden swelling cracks a pure zirconia part to pieces, which is why you cannot simply fire ZrO2 like alumina. The fix is to add a stabiliser, yttria (Y2O3), magnesia or ceria, which creates oxygen vacancies and holds the tetragonal or cubic form all the way down to room temperature; yttria-stabilised zirconia (YSZ) is the best-known result.

That trapped, metastable tetragonal phase is then turned into an advantage. When a crack tries to run through the ceramic, the stress at its tip triggers the tetragonal grains to transform to monoclinic; their volume expansion squeezes the crack shut, like an airbag deploying in front of it, and this transformation toughening gives zirconia the highest fracture toughness of any oxide ceramic (earning it the nickname ceramic steel). Honest caveat: the same transformation is a liability, in warm, wet service the surface can transform spontaneously, a slow degradation called ageing that has cracked zirconia hip implants; and the cubic form of zirconia is separately prized as an oxygen-ion conductor for fuel cells.