Abstract: The aim of this study was to synthesize an experimental bilayer zirconia composed of second-generation 3Y-TZP and ultra-translucent 4Y-PSZ, as well as to characterize its microstructure, optical, and mechanical properties, and compare it with its monolithic counterparts before and after hydrothermal aging. Disc-shaped specimens (ISO 6872:2015) were obtained by uniaxial pressing of commercial powders (Zpex and Zpex4; Tosoh Corporation). Then, the discs were sintered at 1550°C for 2 h and divided into 3 groups: bilayer 3Y/4Y, monolithic 3Y and 4Y. Half of the samples of each group were subjected to hydrothermal reactor aging (20 h, 2.2 bar and 134°C). Specimens were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Optical properties were determined by contrast ratio (CR) and translucency parameter (TP). Mechanical properties were assessed by biaxial flexural strength. XRD evidenced 66 %, 32 %, 66 %, and 40 % of monoclinic phase for aged 3Y-bilayer, 4Y-bilayer, 3Y-control and 4Y-control, respectively. Raman spectra presented monoclinic content on the aged surface of 87 %, 45 %, 87 %, and 47 % for 3Y-bilayer, 4Y-bilayer, 3Y-control, and 4Y-control, respectively. SEM exhibited dense and homogeneous microstructure with smaller grains in bilayer groups, unaffected by aging. Hydrothermal aging did not influence TP and CR, regardless of the system. 3Y demonstrated lower TP and higher CR compared to 4Y and bilayer groups. Aging increased the characteristic strength of all groups. Fractographic marks indicated the origin of fracture and direction of crack propagation from tensile side defects to the compression surface. Hydrothermal aging triggered alterations in the crystalline content, microstructure, and mechanical properties of the experimental bilayer zirconia system as well as of their monolithic controls.
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