Chipping Resistance of Zirconia/Veneering Ceramic Bilayers

Objectives: To evaluate the effect of the processing method and ceramic microstructure on the edge chipping resistance (Re) of zirconia/veneering ceramic bilayers.
Methods: Bilayered bars (n=10) were produced (25mm-long/ 4mm-wide/2mm-thick) according to the following groups: powder/liquid application/stratification (PL), injection (IJ), rapid layer technology (RLT), and CAD-on (CO). One layer of the specimen (1mm-thick) was composed of yttria-stabilized tetragonal zirconia polycrystals (either IPS e.max ZirCAD/Ivoclar for groups PL, IJ, and CO, or In-Ceram-2000-YZ-Cubes/Vita for group RLT). The veneering layers (1mm-thick) were composed of one of the following materials: IPS emax Ceram (PL), IPS e.max ZirPress (IJ), Vitablocs Mark II (RLT) or IPS emax CAD (CO). The veneering layers of PL and IJ were sintered and injected, respectively, following manufacturer’s recommendations and slow cooling was performed for both groups by keeping the furnace closed until reaching the glass-transition temperature. The veneering layers of RLT and CO were obtained by slicing CAD-CAM blocks, which were processed strictly according to manufacturers’ instructions. Re was obtained with Vickers indenter coupled to a universal testing machine (0.1mm/min). Ten indentations were performed varying distance (d) between the edge of the specimen and indenter (0.1 to 1.0mm). Chipping forces (F/Newtons) were recorded and Re was calculated by averaging F/d values. Data were analyzed by one-way ANOVA (alpha=0.05).
Results: CO obtained significantly higher Re values compared to those calculated for all other groups. RLT resulted in statistically higher Re than those obtained for PL and IJ, which showed similar Re values.
Conclusions: Injection did not improve Re of bilayers compared to powder/liquid application. Processing methods using CAD-CAM blocks as veneering material (RLT/CO) resulted in higher Re compared to powder/liquid application or injection.