Mechanical and optical properties of multilayer yttria-stabilized zirconia for dental prostheses

Dental multilayer zirconia ceramics are attractive materials for restorative dentistry because they offer significant esthetic advantages over monolithic, white, or monocolored zirconia. They enable anatomical adjustments in dental prostheses, making them more similar to natural teeth. New categories of dental ceramics have been developed using varying translucency and color within the same piece to meet esthetic requirements. This technique employs zirconia with different yttria (Y2O3) contents to achieve varying levels of translucency. This study investigated two types of dental ceramics based on multilayer zirconia, correlating their mechanical properties, translucency, and color variations concerning their application in dental prostheses. Two types of multilayer zirconia pre-sintered blocks with different Y2O3 content were examined: 1) blocks with a Y2O3 gradient chemical composition between 3 mol.% and 4 mol.%, named M3-4YSZ; 2) blocks with a Y2O3 gradient chemical composition between 4 mol.% and 5 mol.%, named as M4-5YSZ. Samples were cut in cross-section to separate the layers based on Y2O3 content and color and were then sintered at 1530 °C for 2 h. The sintered samples were characterized by relative density, crystalline phases, microstructure, Vickers hardness, fracture toughness (KIC), and optical properties (color and contrast ratio). Fully dense ceramics were obtained after sintering. Layers containing a greater amount of Y2O3 in the zirconia matrix have a higher content of the ZrO2 cubic phase, while layers with a lower content of Y2O3 in the matrix are predominantly tetragonal ZrO2. No statistical variation in hardness was observed depending on the layers of the material. However, fracture toughness values showed significant statistical variations, with an increase in the tetragonal phase content responsible for the increase in fracture toughness. KIC in M3-4YSZ samples varied between 3.63 and 7.73 MPa.m1/2, while M4-5YSZ samples ranged from 2.70 to 5.65 MPa.m1/2. The smooth and gradual variations in color and translucency observed between the layers of the two groups of dental ceramics studied indicate stability and homogeneity between the layers, contributing significantly to esthetic effects in dental applications.