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ZrO2 is a crystalline white oxide of zirconium, which can be used as an alternative to alumina in some applications. Its high melting point and refractory properties make it an excellent material for crucibles, furnace bricks, and ceramics. It is one of the most refractory materials known, and can be used to smelt aluminum, iron, nickel, platinum, and other metals and alloys. It can also be smelted to make crucibles for the treatment of silicates and acid slag.
Zro2 is a polymorphic material that can have three temperature-dependent polymorphs: monoclinic or baddeleyite (room temperature to 1170degC), tetragonal (1170-2370degC), and cubic (2370-2700degC). The monoclinic phase is stable at low temperatures while the tetragonal and cubic phases are more unstable and can crack upon cooling from high temperatures.
The tetragonal and cubic phases of zirconia are stabilized by additives such as calcia, magnesia, and yttria. This stabilisation allows the tetragonal phase to be metastable in dense bodies at room temperatures, and it can then undergo stress-induced phase transformation with concurrent toughening.
This phenomenon, known as transformation toughening, is the reason that stabilised zirconia is used in a variety of restorative applications. In these applications, a stress at a crack tip can cause the tetragonal phase to convert into a monoclinic phase with volume expansion. This phase transformation arrests a crack, retarding its growth and increasing fracture toughness. The mechanism is similar to that of alumina, which can increase the mechanical property of a ceramic by changing its crystal structure from tetragonal to monoclinic.