Layer characteristics in strength-gradient multilayered yttria-stabilized zirconia

Objectives: To investigate crystallography, translucency, phase content, microstructure and flexural strength of two commercial strength-gradient multilayered dental zirconia grades.Methods: Two zirconia grades, i.e., KATANA Zirconia YML (Kuraray Noritake; referred to as YML; composed of four layers: enamel, body 1-3) and IPS e.max ZirCAD Prime (Ivoclar Vivadent; referred to as Prime; composed of three layers: enamel, transition, body) were investigated. Fully sintered square-shaped zirconia specimens from each layer were prepared. Microstructure, chemical composition, translucency parameter and zirconiaphase composition of each layer were characterized. Four-point and biaxial flexural strength of each layer was measured using fully sintered bar- and square-shaped specimens. Square-shaped samples were used to measure strength across the layers.Results: For both multilayer zirconia grades, the 'enamel' layer contains a higher amount of c-ZrO2, which resulted in higher translucency but lower flexural strength than the 'body' layers. The characteristic 4-point flexural strength of the YML 'body 2 ' (923 MPa) and 'body 3 ' (911 MPa) layers, and of the Prime 'body' (989 MPa) layer were comparable and higher than for the YML 'enamel' (634 MPa), Prime 'transition' (693 MPa) and 'enamel' (535 MPa) layers. The biaxial strength of specimens sectioned across the layers was in-between that of the 'enamel' and 'body' layers for both YML and Prime, implying the interfaces did not form a weak link. Significance: The difference in yttria content affects the phase composition and mechanical properties of each layer of the multi-layer zirconia. The strength-gradient approach allowed to integrate monoliths with irreconcilable properties.

Automated summary

This study investigated crystallography, translucency, phase content, microstructure and flexural strength of two commercial strength-gradient multilayered dental zirconia grades. The results showed that the 'enamel' layer contains a higher amount of c-ZrO2, resulting in higher translucency but lower flexural strength. The strength-gradient approach allowed for integration of monoliths with irreconcilable properties.