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Evolution of Dental Zirconia

2022-01-19 Page view : 57 views

Zirconia as a dental material, zirconia, under various forms, is used worldwide in order to replace metal-ceramic restorations. It is obvious that the usage of ceramic-zirconia versus metal-ceramic restorations it is more suitable due to biocompatibility, the appearance as close as possible to the natural teeth. Monolithic zirconia restorations proved better mechanical properties, but they offer a limited tooth color reproduction, while the surface state and wear behavior still raise some questions.

The experimental observations on ageing have been briefed. Thus, it seems that the degradation depends on the time and occurs most rapidly at temperatures of 200-300°C. Even it is caused by the tetragonal to mono-clinic transformation, the phenomenon it is always accompanied by micro- and macro-cracking. The process develops from the exterior and continues inside the sample. In the presence of water the transformation develops faster. The transformation may be slowed of it is used a lower grain size and a bigger amount of stabilizer, we did the same case and test for 3D Pro multilayer zirconia.

 

In pure zirconia (Zr02), three crystal phase systems (monoclinic, tetragonal, and cubic)

transform with temperature, with the monoclinic phase being stable at room temperature. When zirconia is solidly dissolved in yttrium (Y), or other ions with a larger ionic radius than that of zirconium (Zr), the tetragonal and cubic phase systems become stable at room temperature.

When the amount of yttria (Y203) added and the cubic phase is stable at room temperature, which is called cubic-stabilized zirconia. When yttria is 3 to 8 mol%, tetragonal and cubic phases are mixed at room temperature and it is called partially stabilized zirconia (PSz). When yttria is around 3 mol%, the tetragonal phases are close to 100% at room temperature, and it is called tetragonal zirconia polycrystal (TZP), also called toughened zirconia. This yttria 3 mol% tetragonal zirconia polycrystal (3Y-TZP) was an early zirconia that was applied to dentistry as “white metal”.

 

PSZ and TZP exhibit a very peculiar phenomenon, i.e., when stress is applied and a crack is formed, the phase transforms from tetragonal to monoclinic near the crack tip, forming a transition zone. This phase transformation is accompanied by a large volume change of about 4%. It is believed that the accumulation of strain energy due to the increase in volume lowers the stress at the crack tip and prevents the crack propagation. This phenomenon is called “stress-induced phase transformation” and is the reason why zirconia has extremely high strength despite being a ceramic.