Mechanical Properties and Aesthetic Performance of Novel Translucent Alumina Ceramics

Objectives: This study is aimed to evaluate three novel translucent alumina ceramics for dental applications through the detailed investigation of their mechanical properties and aesthetic performance.
Methods: Polycrystalline coarse grain alumina (CGA) was obtained through conventional pressureless sintering, fine grain alumina (FGA) through pressureless sintering followed by hot isostatic pressing (HIP), and ultrafine grain alumina (UFGA) through gel-casting approach followed by HIP treatment. After sintering, the blocks were sectioned into bar and plate specimens, then polished down to 1 μm finished for mechanical and optical characterization. The microstructure, density, flexural strength, fracture toughness, in-line transmittance, translucency parameter, and opalescence index were characterized.
Results: SEM imaging showed that the grain size of UFGA is 0.45 ± 0.02 μm, much smaller than that for FGA (0.90 ± 0.02 μm) and CGA (20.5 ± 0.1 μm). The density for sintered UFGA, FGA, and CGA is 3.97, 3.96, and 3.97 g cm^-3, respectively. The flexural strength and fracture toughness are 621 ± 19 MPa and 2.10 ± 0.12 MPa m^1/2 for UFGA, 768 ± 45 MPa and 2.07 ± 0.20 MPa m^1/2 for FGA, and 264 ± 4 MPa and 2.40 ± 0.18 MPa m^1/2 for CGA. The in-line transmittance of UFGA (14.77%) is much higher than that of FGA (6.54%) and CGA (3.83%) with a common specimen thickness of 1 mm at a wavelength of 555 nm (i.e., most sensitive to human eye). In contrast, the translucency parameter values of UFGA, FGA, and CGA are 38.46 ± 0.17, 35.12 ± 0.06, and 50.04 ± 0.71, respectively. In addition, the opalescence value of CGA (1.9) is much lower than that of UFGA (8.9) and FGA (9.2).
Conclusions: Compared with FGA and CGA, ultrafine alumina (UFGA) has desirable mechanical properties and aesthetic performance, suggesting that this material has potential applications in restorative dentistry.