Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations

- Lowering Al2O3 amount, increasing Y2O3 content and adding 0.2 mol% La2O3 were all effective to improve the translucency of 3Y-TZP ceramics.
- A reduced alumina content however significantly lowered the aging stability.
- Increasing Y2O3 content had the best effect to enhance the translucency and aging stability, but sacrificing the toughness and strength.
- Adding 0.2 mol% La2O3 provided a promising combination of translucency, aging stability and mechanical properties.

ObjectiveThe aim was to evaluate the optical properties, mechanical properties and aging stability of yttria-stabilized zirconia with different compositions, highlighting the influence of the alumina addition, Y2O3 content and La2O3 doping on the translucency.MethodsFive different Y-TZP zirconia powders (3 commercially available and 2 experimentally modified) were sintered under the same conditions and characterized by X-ray diffraction with Rietveld analysis and scanning electron microscopy (SEM). Translucency (n = 6/group) was measured with a color meter, allowing to calculate the translucency parameter (TP) and the contrast ratio (CR). Mechanical properties were appraised with four-point bending strength (n = 10), single edge V-notched beam (SEVNB) fracture toughness (n = 8) and Vickers hardness (n = 10). The aging stability was evaluated by measuring the tetragonal to monoclinic transformation (n = 3) after accelerated hydrothermal aging in steam at 134 °C, and the transformation curves were fitted by the Mehl-Avrami-Johnson (MAJ) equation. Data were analyzed by one-way ANOVA, followed by Tukey's HSD test (α = 0.05).ResultsLowering the alumina content below 0.25 wt.% avoided the formation of alumina particles and therefore increased the translucency of 3Y-TZP ceramics, but the hydrothermal aging stability was reduced. A higher yttria content (5 mol%) introduced about 50% cubic zirconia phase and gave rise to the most translucent and aging-resistant Y-TZP ceramics, but the fracture toughness and strength were considerably sacrificed. 0.2 mol% La2O3 doping of 3Y-TZP tailored the grain boundary chemistry and significantly improved the aging resistance and translucency. Although the translucency improvement by La2O3 doping was less effective than for introducing a substantial amount of cubic zirconia, this strategy was able to maintain the mechanical properties of typical 3Y-TZP ceramics.SignificanceThree different approaches were compared to improve the translucency of 3Y-TZP ceramics.