2.1

Specimens preparation

Blocks of Aadva Zr pre-sintered zirconia (GC Europe, Leuven, Belgium) were cut with a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA). Dimensional changes occurring during sintering were taken into consideration in specimen cut. Specimens were sintered by the zirconia manufacturer (GC Europe, Leuven, Belgium) and wet-finished with silicon-carbide papers of 600, 1200 and 2400 grit, until dimensions of 16 ± 0.2 mm length, 4 ± 0.2 mm width, and 0.8 ± 0.2 mm thickness were achieved. The obtained 45 specimens were randomly assigned to three groups ( n = 15), numbered, and then measured in thickness by means of a digital caliper with an accuracy of ±0.01 mm. A thin layer of modifier (Initial Zr-Fs Frame Modifier, GC Europe, Leuven, Belgium) was applied and fired for all the specimens in each group. Then, specimens were treated as follow:

2.1.1

Group 1 (1 firing cycle)

Veneering ceramic (Initial Zr-Fs Ceramic Dentin, GC Europe, Leuven, Belgium and Initial Zr-Fs Ceramic Enamel, GC Europe, Leuven, Belgium) was layered positioning the zirconia bar within a silicone mold that allowed placing/adding the veneering ceramic to a total height of 2 mm. Then the specimen was gently removed from the mold and fired.

2.1.2

Group 2 (3 firing cycles)

Each zirconia bar was positioned into a silicone mold that allowed to apply approximately 1/3 of the veneering ceramic space (about 0.4 mm). For the first layer the Initial Zr-Fs Ceramic Dentin was applied, while for the second and third layer the Initial Zr-Fs Ceramic Enamel was added. Each layer was individually fired.

2.1.3

Group 3 (5 firing cycles)

The samples were treated as in the Group 2 but the thickness of the silicon mold was approximately 17% of the veneering ceramic space (about 2.5 mm). For the first two layers the Initial Zr-Fs Ceramic Dentin was applied, while for the third, fourth and fifth layers the Initial Zr-Fs Ceramic Enamel was used. Each layer was individually fired. No adjustment procedure by grinding was performed on each single layer.

The Initial Zr-Fs Ceramic was always mixed with the proprietary liquid Zr-Fs modeling liquid, reported as plastifying agent-free from the manufacturer. For all the groups, at the end of the ceramic firings, an auto-glaze firing was performed. The firing cycles are reported in Table 1 .

After the firing procedures, all the specimens were again wet-finished using silicon-carbide papers of 600, 1200 and 2400 grit, to obtain the total height of 2 mm, with approximately 0.8 mm of zirconia core and 1.2 mm of veneering ceramic. After the finishing procedure, each specimen was again measured with the same procedure as for the initial measurement of the zirconia layer, so that it was possible to calculate by difference the thickness of the veneering ceramic.

2.2

Test methods

For flexural strength evaluation, a three-point bending test (3PBT) appliance was used. The tip and the supports were made in Cobalt-HSS (high speed steel), using polished rollers 2.0 mm in diameter. The remaining part of the rig was milled from a stainless steel (A.I.S.I. type 316) block. The span was set at 13.0 mm. Specimens were tested dry and at room temperature. Tests were performed in a universal testing machine (Triax 50, Controls, Milano, Italy) with a cross-head speed of 0.5 mm/min. Specimens were positioned with the veneering ceramic in tensile stress (opposite to force load), and tested until fracture of the veneering ceramic occurred, as assessed by a load peak .

The flexural strength ( σ ) was calculated in MPa using the following equation :

where:

M is the moment of the load, calculated with the following equation:

where:

P is the fracture load in N;

L is the distance between the center of the supports in mm;

K is a constant calculated within the following equation:

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