Zirconia

Zirconia is unusual among ceramics in being tough. Ceramics are hard and brittle because a crack, once started, has nothing to stop it; zirconia has something.

Pure zirconia changes crystal structure on cooling, from tetragonal to monoclinic, and the change involves a volume increase of a few per cent — enough to shatter the piece. Adding a few per cent of yttria holds the tetragonal phase in place at room temperature, metastably. When a crack then starts, the stress at its tip triggers the transformation locally, the expansion squeezes the crack closed, and it stops.

That is transformation toughening, and it gives zirconia several times the fracture toughness of alumina. It is one of the few genuinely elegant mechanisms in materials engineering: a phase change that was the material's defect, harnessed as its defence.

Sintered from fine powder with a stabiliser — yttria for the toughened grades, more of it for the fully cubic ones. Dental restorations are machined from a partly sintered blank, which is soft enough to cut, and then fired to full density, shrinking by about a fifth in the process; the machining is scaled up to allow for it.

Cubic zirconia for jewellery is grown as a single crystal by skull melting, which uses the material's own unmelted outer shell as the crucible because nothing else survives 2750 °C.

Dental crowns and bridges, which is now a very large market and where its combination of toughness, colour and biocompatibility is hard to beat. Hip joint heads. Cutting tools and ceramic knives. Oxygen sensors in every petrol engine, exploiting a different property entirely — hot zirconia conducts oxygen ions, and the voltage that generates measures the exhaust mixture.

Thermal barrier coatings on turbine blades, sprayed a fraction of a millimetre thick, which is what lets a turbine run hotter than its alloy could otherwise survive.

And cubic zirconia as a gemstone simulant, which is the use most people have met: harder than almost anything else it will encounter, dispersing light more than diamond does, and about three hundred times cheaper.

The mineral baddeleyite was described in 1892 and the oxide has been used as a refractory since the early twentieth century. Stabilisation with yttria and calcia was developed to stop the destructive phase change, and transformation toughening — turning that change into an advantage — was described in 1975 by Garvie, Hannink and Pascoe in a paper titled 'Ceramic steel?'.

Soviet researchers at the Lebedev Institute developed skull melting for single crystals in the 1970s, and the gemstone industry has used the result ever since.