Zirconia polymorphic transformations

Borothermic reduction was confirmed at a lower temperature, and the carbothermic reduction occurred at a higher temperature (33), which favors the production of high-purity powders. TiB2 and ZrBj powders were also synthesized by mechanochemical treatment of titania and zirconia powders with amorphous boron followed by a relatively low-temperature annealing (1100°C) (34). The occurrence of polymorphic transformations has been observed with milling time for both systems, together with a decrease upon annealing of the temperature of the boro-... 

One of the key features of zirconia lies in its polymorphism. Zirconia exhibits three polymorphs. The monoclinic phase is stable up to 1170°C where it transforms to the tetragonal phase, which is Itself stable up to 2370 C. Above this temperature, zirconia exists as a cubic Cap2 type phase. The reversible m- to t-Zr02 transformation is key to the use of zirconia in ceramics. First, it is reversible but occurs with a thermal hysteresis. Second, it is rapid and takes place by a diffusionless shear process similar to that of a martensitic transformation. Finally, it is dependent on particle size and occurs with a volume change (3 to 5%) [82]. 

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