Zirconia as a pure oxide does not occur in nature but it is found in baddeleyite and zircon (ZrSiO4) which form the main sources for the material. Of the two of these, zircon is by far the most widespread but it is less pure and requires a significant amount of processing to yield zirconia.
The processing of zirconia involves the separation and removal of undesirable materials and impurities - in the case of zircon - silica, and for baddeleyite, iron and titanium oxides. There are several routes to the extraction of zirconia from zircon including:
• Alkali oxide decomposition
• Lime fusion
• Plasma dissociation
Pure zirconia exists in the monoclinic form at room temperature. Cubic and tetragonal phases are also stable at higher temperatures. The transformation of monoclinic to cubic zirconia occurs at 800-1000°C and is accompanied by a large change in lattice size. A consequence of this phase change is a large volume expansion on cooling which make the fabrication of pure zirconia ceramics impossible.
In order to manufacture zirconia components, it is necessary to 'lock' the material wholly or partially into the cubic form by the use of additives or stabilising agents.
The addition of varying amounts of cubic stabilisers such as CaO, MgO, and Y2O3 allows the formation of partially stabilised zirconias which combined with processing variations can result in ceramics which demonstrate exceptional properties and have found a number of application areas.