One of the most common technical ceramics that could be compared in strength with steel has been tetragonal zirconia, also called YTZP or yttria partially stabilised zirconia, which has been doped with yttrium trioxide, also called yttria.
YTZP has outstanding properties at normal temperatures. It has a structure that is predominantly tetragonal and a very high flexural strength. It also has a very fine grain size and a superior resistance to wear and corrosion, which means it can be used for precision cutting tools.
This compound is stable up to 500 degrees Celsius, but at higher temperatures suffers from a form of extreme plastic deformation. So the task has been to develop technical ceramics that could perform at temperatures higher than 500 degrees Celsius.
If magnesium oxide — magnesia — is used to dope zirconium, the result is ( Magnesia Stabilised Zirconia ) a very stable cubic structure. Where the YTZP microstructure suffers sliding along its grain boundaries at high temperatures, and so causes the plastic deformation, the magnesia-doped zirconia, also called magnesia-stabilised zirconia (MSZ), tends to develop a toughening mechanism at high temperatures. As the heat increases, the MSZ microstructure toughens and its mechanical characteristics become enhanced.
Zirconium oxide in its monoclinic form undergoes phase transformations as it heats. This leads to volume changes that cause cracking and failure of the material. The magnesia dopant stabilises the zirconia, enabling its use up to temperatures as high as 2,200 degrees C. The stabilisation occurs as the matrix structure of the material changes from tetrahedral to a more stable cubic structure.
In addition, the lens-shaped crystals of zirconia, which are tetragonal in micro form, precipitate on the cubic matrix faces and so impede any type of sliding at grain boundaries and consequently the plastic deformation.
The microstructure in this type of material can be finely tuned, heated and aged to address specific needs and to improve its thermo mechanical properties.
This ceramic is used for high-performance applications such as power-generation turbine blades and anti-ballistic protection and ceramics used in varieties of military armours.
Zirconia can be doped with other oxides to enhance its physical properties. The addition of caesium oxide and calcium oxide, also called lime or quicklime, gives zirconia an excellent thermal shock resistance at temperatures between 1,000 and 1,200 degrees C.
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