Lately, the market of dental Yttria-Stabilized Zirconia (YSZ) has shown a growing interest in the development of grades with higher translucency in order to meet the aesthetic appearance of natural teeth, without compromising mechanical properties and Low Temperature Degradation (LTD) resistance. The limited transparency of this family of ceramics is caused by scattering, both on porosity and grain boundaries, due to the birefringence inherent to the tetragonal structure. To reduce the negative effect of birefringence on the optical properties, it is possible to decrease the grain size down to 100nm to limit the scattering cross section of the nano-grains.
The mechanical and optical properties and the aging resistance of YSZ doped with 1.5 mol.% and 3 mol. % of Y2O3 with ultra-fine grain size (around 100 nm) are characterized and compared to standard Tetragonal Zirconia Polycrystal doped with 3 mol. % of Y2O3 (3Y-TZP). Their microstructure, chemical and phase composition are correlated to their performances.
Nanometric 3YSZ shows improved translucency and opalescence, together with increased flexural resistance and LTD resistance. Only toughness is slightly reduced, if compared to benchmark 3Y-TZP. This grade is almost entirely composed of tetragonal phase, which is stable thanks to the reduced grain size, preventing aging but also the activation of transformation toughening mechanisms. On the other hand, nanometric 1.5YSZ exhibits very high toughness, strength, resistance to contact damage and LTD resistance, superior to both benchmark 3Y-TZP and nanometric 3YSZ. This grade is composed of highly transformable tetragonal phase, thanks to the combination of very small grain size and low content of Yttria. This tetragonal phase easily undergoes stress-induced phase-transformation when the sample is subjected to mechanical stress but it is, instead, more resistant to aging than 3Y-TZP thanks to the lower content of oxygen vacancies because of the reduced amount of stabilizer. Nanometric 1.5YSZ shows improved transparency and opalescence, if compared to 3Y-TZP. However, it does not achieve the translucency of nanometric 3YSZ. This is related both to the scattering on the residual nano-porosity, which is higher than in the other ceramics, and to the larger scattering cross section of its tetragonal grains with 1.5 mol.% Yttria, which have a higher tetragonality and, therefore, a larger birefringence.
Both these nano-zirconias show a superior compromise between mechanical properties, LTD resistance and translucency than the zirconias currently used in the dentistry field. They overcome the limitations of other sub-micrometric Yttria-richer grades, which show a high transparency and LTD resistance at the expense of toughness and mechanical strength, because of their high content in cubic-phase. The strategy of grain-size reduction is, therefore, extremely promising.