Objectives. The mechanical strength and the surface hardness of commercially available yttrium doped zirconia were investigated, together with the study of the effect of a coloring procedure, by means of commercially available dyes, and of the surface roughness on the flexural strength. Furthermore, a comparative study of eight different ceramic veneers, to be used for the production of two-layered all-ceramic restorative systems, was carried out by means of bending tests on bi-material specimens and Vickers micro-hardness measurements.

Methods. Four types of zirconia specimens were prepared and analyzed by three-point bending tests, according to a standard ISO procedure (ISO 6872), in order to determine the effect of a coloring procedure and of the surface roughness on the flexural strength.
In particular, colored and non-colored specimens were analyzed, each of them with two types of surface finishing conditions. Besides, two-layered zirconia/veneer specimens were prepared for three-point bending tests, and a comparative analysis of eight selected commercial veneering ceramics was carried out. Finally, Vickers micro-hardness measurements were carried out on zirconia core materials and on the selected ceramic veneers.

Results. A strong effect of the surface roughness on the mechanical strength of zirconia specimens was observed. Furthermore, no significant effects of the coloring process on the mechanical behaviors were measured. Finally, a comparative study of eight commercially available veneering ceramics shows different mechanical behaviors and different modes of failure between the selected veneers.

Conclusion. The results indicate that close attention should be paid to the preparation of zirconia based crowns and bridges by CAD/CAM process, with the aim of obtaining smooth surfaces, because surface roughness has an important effect on the mechanical 3/28 strength of the material; the results also indicate that the coloring process is not of critical concern, as no differences.

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