Changes in properties of monolithic and conventional zirconia during aging process

Abstract

This study aimed to evaluate and compare the change in the various properties between monolithic and conventional zirconia during hydrothermal aging process.Four zirconia ceramics (KaVo Everest ZS-Blank: Group CE, Lava Frame Zirconia: Group CL, BruxZir: Group MB, Prettau Zirconia: Group MP) were investigated. Ninety disk-shaped specimens (diameter 15.0, thickness 1.2 mm) of each group were fabricated and randomly divided into six subgroups (n = 15). All specimens were autoclaved (134 °C, 0.2 MPa) for 0, 1, 2, 3, 4, 5 h, respectively. Confocal laser scanning microscopy, atomic force microscopy, X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy were used to analyze the surface properties. Biaxial flexural strength (BFS), nanoindentation hardness, elastic modulus, and Vickers hardness were measured for the analysis of mechanical properties. All data were statistically analyzed using ANOVA and multiple comparison Scheffé test (α = 0.05).During aging, all groups exhibited a significant increase in surface roughness (Ra, Sa, and Sq) and grain boundary height. The monoclinic phase (m-phase) of all groups increased gradually relative to the tetragonal phase. Groups MB and MP were more sensitive to aging than the others. SEM images of Groups MB and MP showed microcracks and grain push-out, which increased gradually with aging. Oxygen levels in all groups increased with aging, whereas zirconium and yttrium levels decreased. All groups showed a decrease in BFS, but no significant differences between groups were found. The elastic modulus and nanoindentation hardness values decreased significantly in all groups with aging. Vickers microhardness values of Group MB and MP decreased significantly, but those of Group CE and CL did not.Both monolithic and conventional zirconia exhibited similarities in terms of their elastic modulus, nanoindentation hardness, m-phases, and oxygen levels. However, monolithic zirconia was more prone to aging in terms of surface roughness.