Enhanced Fracture Toughness of Dental Zirconia through Nb Incorporation into the Surface

Background: Our previous study revealed that the addition of pentavalent cations, such as niobium (Nb), to zirconia stabilized with trivalent cations, such as yttrium (Y), increased fracture toughness. However, the coefficient of thermal expansion (CTE) and opacity also increased undesirably. Thus, a novel surface treatment is required to enhance fracture toughness without changing the CTE and the translucency. Methods: The surfaces of pre-sintered 3-mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 4.2-mol% yttria-stabilized partially stabilized zirconia (4.2Y-PSZ) were treated with an Nb sol solution containing Nb2O5 nanoparticles. After drying and final sintering, a surface layer with high Nb content formed to a depth of approximately 1 mm. Results: Nb was equivalent to that of the bulk uniformly containing 1 mol% Nb2O5. The tetragonality of tetragonal zirconia near the surface increased with the addition of Nb, leading to improve fracture toughness in the surface layer and enhancing the fracture toughness of the entire specimen. However, the CTE and the translucency remained unchanged. Conclusions: The addition of pentavalent cations such as Nb, confined to the surface vicinity, improved fracture toughness without affecting the CTE and the translucency. By selectively adding Nb to the surface of weak parts of zirconia prosthesis, the overall strength can be improved, making it possible to produce highly reliable dental restorations.