Evaluation of Mechanical Strength, Translucency, and Microstructure in Graded Zirconia Layers Before and After Hydrothermal Aging

Abstract

This study aimed to evaluate the monotonic and fatigue strength, translucency, topography, and grain size of different layers in two types of graded zirconia, both before and after hydrothermal aging. A total of 180 bar-shaped specimens (n = 15 per group) were prepared from each layer (cervical, transitional, and incisal) of two graded zirconias (IPS e.max ZirCAD MT Multi—4 mol% yttrium stabilized at the cervical region and 5 mol% at the incisal region; IPS e.max ZirCAD Prime, Ivoclar AG—3 mol% yttrium stabilized at the cervical region and 5 mol% at the incisal region) with dimensions of 14.0 × 4.0 × 3.0 mm, following ISO 6872 standards for a three-point-bending test. These specimens were divided into two groups: baseline and hydrothermal aging (134°C under 2 bars of pressure for 20 h). Monotonic and fatigue three-point bending tests (initial stress: 250 MPa/5000 cycles at 20 Hz; increments: 50 MPa/step) were performed until fracture. The translucency parameter (TP00) was calculated using the CIEDE2000 formula, and L*, a*, b* color coordinates were measured on disc-shaped specimens (n = 6 per layer; Ø = 10 mm, 1 mm thickness) before and after aging. Additional analyses included topography, fractography, and grain size. Data were analyzed using three-way ANOVA, Kaplan–Meier, and Mantel-Cox post hoc tests (log-rank). Hydrothermal aging decreased fatigue flexural strength across all layers and types of graded zirconia. After aging, the fatigue flexural strength of Prime zirconia was higher than that of MT Multi in the cervical layer. The cervical layers exhibited the highest flexural strength and smallest grain size, while incisal layers showed higher translucency compared to other layers. The induced aging protocol impacted the mechanical properties of the graded zirconia layers. Each layer within the multilayer zirconia systems displayed distinct mechanical, microstructural, and optical properties, with variations depending on the specific zirconia material. Overall, all layers demonstrated satisfactory performance, supporting the use of multilayer zirconia systems.