In biomedical field, for the prosthetic restorations, the material considered more performant for its characteristics is zirconium oxide (ZrO2) - commonly called ZIRCONIA - which is an metal-free ceramic oxide, not to be confused with the zirconium (Zr) that is a metal instead.
The zirconium oxide ZrO2 is obtained from the zirconium silicate mineral, found in deposits located in Australia, Russia, USA, Brazil, India and South Africa, or Baddeleyte, whose deposits are found mainly in South Africa and Sri Lanka
Zirconium DiOxide (ZrO2), known as Zirconia (non to be confused with Zircon metal, is a polymorphous substance, meaning its crystalline structure may alter without altering the original chemical composition. At atmospheric pressure, Zirconia has three polymorphous phases:
At room temperature, up to 1170 ° C, it has a monoclinic crystalline structure.
From 1170° C to 2370° C, the crystalline structure transforms from a monoclinic to a tetragonal structure, with a volumetric contraction of approximately 5%.
Above 2370° C, it transforms into a cubic crystalline structure.
The phase transformations are reversible and influence the performance of the material, which is modest in the monoclinic and cubic phases, but become more interesting in the tetragonal phase.
Tetragonal to monoclinic transformation is martensitic: during cooling, the tetragonal structure transforms into a monoclinic structure, with an expansion in volume varying between 3% and 5% and exceeding the elastic limit of Baddeleyite, causing microcracking in the structure.
This mechanism, which could, at first glance, appear to cause a negative effect on performance of the material, is actually used to increase resistance and toughness.
In fact, although cracks form in ceramic under loads, the surrounding Zirconia crystals transform into the monoclinic structure. The pressure tensions which form close to the crack, due to expansion of the material, stop it from spreading. These “crack-blocking functions” give high values of resistance to breakage and toughness.
It is possible to maintain the tetragonal phase at room temperature through partial stabilisation of the Zirconium with the addition of 3-6% of Magnesium Oxide (MgO), Yttrium Oxide (Y2O3), Calcium Oxide (CaO) and Cerium Oxide (Ce2O3).