Effect of Zirconia Core Thickness and Veneer Firing Cycle on the Biaxial Flexural Strength of Veneering Ceramic

Purpose To evaluate the influence of various Y-TZP thicknesses and veneer firing cycles on the strength of two ceramic veneers. Materials and Methods 180 Y-TZP cores of 0.5, 1.0, and 5.0 mm thickness were prepared followed by sintering in a high temperature furnace; 180 presintered veneering ceramic discs (Vita VM9 porcelain and e.max Ceram) were also prepared using a mold. The discs were placed on zirconia plates (zirconia cores) of different thickness (0.5, 1.0, and 5.0 mm) and exposed to different firing cycles (Vita VM9 porcelain-910, 930, and 950 degrees C; e.max Ceram-750, 770, and 790 degrees C). Ball-on-three-balls flexural strength test was performed (universal testing machine) at a crosshead speed of 0.5 mm/min. Scanning electron microscopy of fractured specimens was performed. Means and standard deviations of flexural strength were analyzed using Tukey-Kramer HSD test for multiple comparisons. Results Specimens within material groups showed no significant difference (p > 0.05) for flexural strength with respect to Y-TZP core thickness (0.5, 1.0, and 5.0 mm) (VM9 [117.30 +/- 14.328, 117.75 +/- 13.66, 113.75 +/- 20.10], e.max Ceram [94.79 +/- 17.5, 100.02 +/- 14.7, 95.23 +/- 15.4]). Flexural strength within material groups with respect to different firing cycles ([VM9-910, 930, 950 degrees C], e.max Ceram [750, 770, 790 degrees C]), for VM9 (111.49 +/- 15.7, 120.86 +/- 13.2, 116.46 +/- 18.4), and e.max Ceram (94.64 +/- 15.2, 101.6 +/- 16.69, 93.8 +/- 15.20) showed no significant difference (p > 0.05). Conclusions Different zirconia thicknesses (0.5, 1.0, and 5.0 mm) and veneer firing cycles for Vita VM9 and e.max ceramics failed to show any significant influence on their biaxial flexural strengths.