Analyzing Interfaces of Various Classes of Multi-Layer Zirconia

Objectives: The use of multi-layered CAD/CAM zirconia is rapidly increasing in dentistry. Thus, a comprehensive investigation of the effect of multi-layering fabrication techniques and variation in strength between different layers is necessary. Each of the multi-layer zirconia tested is unique in its composition and properties.
Methods: Four multi-layer zirconia materials with different compositions were investigated (n = 120): IPS ZirCAD MULTI (MULTI), Katana Ultra Translucent Multi-Layer (UTML), Super Translucent Multi-Layer (STML), and Multi-Layer (ML). Bar-shaped specimens (16x2x1.5 mm) were obtained from the enamel, dentin and cross-section layers of pre-sintered CAD-CAM zirconia pucks. Samples were sintered, and polished to a 1-µm finish. To measure the flexural strength, a four-point bending test was performed using the universal testing machine (1.0 mm/min). Fractographic-features were analyzed using optical microscope and SEM.
Results: The mean flexural strength significantly varied between layers of the MULTI material dentin layer had the highest strength, while enamel and cross-sectional layers exhibited lower but similar strength. There was no difference between the enamel and dentin layers of UTML, STML and ML; however, the strengths of cross-sectional samples were ∼30% lower than enamel and dentin layers. The fracture site for cross-sectional samples were observed mainly in the enamel/transition-1 interface for all groups tested.
Conclusions: The layered manufacturing technique compromises the bulk strength of zirconia. This is consistent with the fact that fracture site in cross-sectional multi-layer beams was predominantly located at the interface of enamel/transition-1.