Introduction.Themechanical strength and the surface hardness of commercially available yttrium-doped zirconiawere investigated.\r\nFurthermore, a comparative study of eight different ceramic veneers, to be used for the production of two-layered all-ceramic\r\nrestorative systems,was carried out. Materials and Methods. Four types of zirconia specimenswere analyzed, according to a standard\r\nISO procedure (ISO 6872). Besides, two-layered zirconia-veneer specimens were prepared for three-point bending tests. Results. A\r\nstrong effect of the surface roughness on the mechanical strength of zirconia specimens was observed. Finally, a comparative study\r\nof eight commercially available veneering ceramics shows different modes of failure between the selected veneers. Conclusion. The\r\nresults indicate that close attention should be paid to the preparation of zirconia-based crowns and bridges by CAD/CAM process,\r\nbecause surface roughness has an important effect on the mechanical strength of the material. Finally, the results of the mechanical\r\ntests on two-layered specimens represent an important support to the choice of the veneering ceramic.
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