Probing Interfacial Strength and Crack Resistance of Novel Composition-Gradient Zirconia.

Objectives: The use of multi-layered CAD/CAM zirconia is rapidly increasing in dentistry due to their superior esthetics. Thus, a comprehensive investigation of the effect of multi-layering fabrication techniques and variations in strength and crack resistance between different layers is necessary. The novel zirconia tested is unique in its composition and properties and is developed with an innovative gradient technology by intermixing two variants of zirconia: 3Y-PSZ and 5Y-PSZ.
Methods: Bar-shaped specimens (16 x 2 x 1.2 mm) were obtained from the enamel, dentin, and cross-section layers of pre-sintered IPS e.max ZirCAD Prime. Samples were cut, sintered, and polished to a 1-μm finish. To measure the flexural strength, a four-point bending test was performed using a universal testing machine (1.0 mm/min). To measure the crack resistance, a Vickers hardness test was performed following a step-wise pattern (0.25 mm at a 40° less than horizontal) across a sintered sample (17 x 17 mm). 20 N of force were applied for 15 seconds at each location. Fractographic features were analyzed using an optical microscope. Chemical composition of the sample tested was examined by conducting a 0.5-mm step-wise line mapping using energy-dispersive X-ray spectroscopy (EDS).
Results: There were significant differences among mean flexural strength within groups (p < 0.001). Dentin group had the highest strength (773 ± 171 MPa). Enamel (411 ± 107 MPa) and cross-section (460 ± 136 MPa) groups showed lower but statistically similar mean flexural strength. The yttria content decreased from 5 to 3 mol.%. The indentation crack length decreased as the material transitioned from the enamel layer to dentin layer, indicative of an increasing crack resistance.
Conclusions: Different layers within the multilayer material showed different flexural strength and crack resistance. The enamel layer with a higher yttria content exhibited a higher translucency but lower flexural strength and crack resistance.