Effect of shading the zirconia framework on biaxial strength and surface microhardness

Zirconia ceramics for frameworks of fixed partial dentures can be color shaded to better match the shade of veneering porcelain. The aim of this study was to evaluate the effect of color shading the green-stage zirconia ceramic on some mechanical properties.

Zirconia disks (diameter 19 mm, thickness 0.8 mm) were divided into 10 groups of 10 disks each according to the color shade of the zirconia ceramic. The disks were shaded with the same color liquid using either the recommended shading time (3 s) or prolonged shading time (60 s). Nine control disks were tested without color. Composition of the color liquids was determined with EDX analysis. All the disks were sintered at 1500 degrees C and their biaxial flexural strength was measured dry and at room temperature. Surface Vickers microhardness was measured. Data were evaluated using ANOVA analysis.

Disks of shade D4 (Vita Classic-scale) revealed the highest strength (1007 MPa) of the shaded disks, but the strength did not differ statistically from that of the control group (1132 MPa). Other color shades had values of A3: 898 MPa, B1: 918 MPa, C4: 885 MPa, and D2: 897 MPa. Prolonging the shading time from 3 s to 60 s lowered the strength even more. Shading time had no effect on microhardness, but there were some differences among the different color shade groups (p<0.05). EDX analysis showed slight differences between compositions of different color liquids.

Some variation was found in strength and surface hardness of zirconia that had been color shaded to different shades.