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Pure zirconia exists in three crystal phases at different temperatures. At very high temperatures (>2370 oC) the material has a cubic structure. At intermediate temperatures (1170 to 2370 oC) it has a tetragonal structure. At low temperatures (below 1170 oC) the material transforms to the monoclinic structure. The transformation from tetragonal to monoclinic is rapid and is accompanied by a 3 to 5 percent volume increase that causes extensive cracking in the material. This behavior destroys the mechanical properties of fabricated components during cooling and makes pure zirconia useless for any structural or mechanical application. Several oxides which dissolve in the zirconia crystal structure can slow down or eliminate these crystal structure changes. Commonly used effective additives are MgO, CaO, and Y2O3. With sufficient amounts added, the high temperature cubic structure can be maintained to room temperature. Cubic stabilized zirconia is a useful refractory and technical ceramic material because it does not go through destructive phase transitions during heating and cooling. The volume expansion of the tetragonal to monoclinic inversion is used to produce very high strength, hard, tough varieties of zirconia for mechanical and structural applications. There are several different mechanisms that lead to strengthening and toughness in zirconias that contain tetragonal grains. This is a complex subject matter. Simplistically, these depend on the grain sizes, the thermal history and the kind and amount of stabilizing additive in the body. These variations lead to two strong materials identified as TZP and PSZ ceramics. The PSZ is the more common commercial material and is made as a MgO partially stabilized zirconia. The second variety, TZP, is a pure tetragonal very fine grain material. This material, which is challenging to produce, has found uses in cutting and wear resistant applications due to its reliable and outstanding hardness and toughness. TZP properties degrade rapidly when the material is exposed to water vapor at 200 to 300 oC, so controlled use conditions are important for good performance. All of the toughened zirconias show a degrading of properties with increasing temperature, and this class of high strength tough materials is generally limited to use temperatures below 800 oC.Calcium oxide stabilized zirconia is popular for coarser grain refractroy bodies for crucibles, kiln furniture and kiln refractories operated in the 2000 oC range. Magnesium oxide is the most popular stabilizer added to produce a very strong and tough fine grain ceramics for mechanical and structural applications. Yittria, due to its high cost, is used in specialty applications which take advantage of the superior electronic and ionic electrical conduction of the material.
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