Objectives: To compare the mechanical properties of different layers of multi-layered zirconia materials.
Methods: 720 cylindric test plates were fabricated from four defined layers of three multi-layered zirconia ceramics (IPS e.max ZirCAD Prime, Optimill Multilayer 3D; Ceramill zolid fx multilayer) and divided into two equal groups. One group underwent thermal cycling (5-55 °C, 10 000 cycles; "TC") and one did not ("no TC"), before density, flexural strength, Weibull modulus, and Vickers hardness were evaluated. EDX analysis was conducted using an additional cylinder of each material. Data were analyzed using Kruskal-Wallis and Mann-Whitney U tests. Statistical analysis was performed with Bonferroni correction (α < 0.001).
Results: After aging, ZirCAD layer 4 showed the overall highest density (6.04 ± 0.02 g/cm3), which was significantly higher than density of layer 4 of Optimill (6.02 ± 0.06 g/cm3) and Ceramill (5.80 ± 1.08 g/cm3) (both p < 0.001). Flexural strength of ZirCAD and Optimill increased consecutively after thermal aging. ZirCAD layer 4 had the overall highest flexural strength before and after artificial aging. After thermal cycling, the Weibull modulus ranged between 4.32 (ZirCAD layer 1) and 13.58 (Ceramill layer 4). ZirCAD had the overall highest Vickers hardness: in layer 1 (1579.18 ± 47.14 HV) before aging, and in layer 2 (1607.1 ± 149.71 HV) after aging. Flexural strength and Vickers hardness differed significantly between the four ZirCAD layers (p < 0.001). Thermal ageing had no significant impact on mechanical properties (p > 0.001).
Significance: Mechanical properties were affected by plate position within the blank. When nesting a restoration within a multi-layered zirconia blank, the mechanical properties required should be considered.
Keywords: Density; EDX analysis; Flexural strength; Mechanical properties; Multilayer zirconia; Thermal cycling; Vickers hardness; Weibull modulus.
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