Effect of aging process on some properties of conventional and multilayered translucent zirconia for monolithic restorations

The purpose of this study was to evaluate the surface characteristics and optical and mechanical properties of materials through hydrothermal aging using different types of translucent monolithic zirconia. Three conventional materials (LT, KT, and ET) and three multilayered materials (LM, KM, and EM) of translucent monolithic zirconia were investigated. 45 bar-shaped specimens in each group were fabricated for the measurement of surface characteristics and mechanical properties and were randomly divided into 3 sub-groups. 10 plate-shaped specimens in each group were fabricated for the measurement of optical properties. All specimens were autoclaved for 0, 5, and 10 h, respectively. The various properties of translucent monolithic zirconia, such as phase distribution, surface roughness, surface texture, elemental analysis, total transmittance (Tt), contrast ratio (CR), three point flexural strength (TFS), nanoindentation hardness (NH), and Young's modulus (YM), were measured through different tests. Statistical analyses were performed with one-way ANOVA, Tukey HSD post hoc test, independent sample t-test, paired sample t-test, and two-way ANOVA (alpha = 0.05). After aging, the monoclinic phase of all groups increased relative to the tetragonal phase. All groups exhibited an increase in the surface roughness and oxygen levels, but exhibited a decrease in the zirconium and yttrium levels (P < .05). All groups had increased microcracks and grain push-out. Tt increased and CR decreased in the LT, KT, and ET groups. However, Tt decreased and CR increased in the LM, KM, and EM groups (P < .05). The TFS, NH, and YM showed a decreasing tendency, but there were no significant difference in several materials (P > .05). Alterations in the surface characteristics were found on the surfaces of translucent monolithic zirconia after aging. The aging process caused conventional materials to become more translucent, but multilayered materials were more opaque. The various properties of translucent monolithic zirconia were affected by the materials fabricated with different types after aging.