Processing and characterization of an experimental bilayer first-generation 3Y-TZP and super-translucent 4Y-PSZ zirconia subjected to hydrothermal aging

The aim of this study was to develop an experimental bilayer zirconia system composed of first-generation 3Y-TZP and super-translucent 4Y-PSZ, and to characterize its microstructural, optical, and mechanical properties before and after hydrothermal aging, comparing them with its monolithic controls. Disc specimens were produced through uniaxial pressing of commercial 3Y-SBE and ZPEX 4 powders (Tosoh Corporation) and sintered at 1550ºC for 2 hours. Hydrothermal aging was performed in a hydrothermal reactor for 20 hours at 134ºC and 2.2 bar. Microstructural characterization by scanning electron microscopy revealed smaller grains in the bilayer group compared to the control groups. X-ray diffraction indicated a lower susceptibility to hydrothermal degradation for the bilayer group, while Raman spectroscopy showed that degradation occurred only in the outermost layer. Optical characterization demonstrated that the bilayer system successfully combined the higher translucency of 4Y with the superior opacity of 3Y, resulting in an effective aesthetic balance between the layers. Mechanical evaluation indicated that the bilayer system remained stable before and after hydrothermal aging, with strength values exceeding 800 MPa. Fractographic analysis revealed that cracks originated on the tensile side and propagated towards the compressive side. It is concluded that the 3Y/4Y bilayer zirconia system presents a promising balance between aesthetics and strength, making it a viable solution for various dental applications.