Methods: Zircona discs (Bruxzir, Glidewell, Frankfurt, Germany) having 15mm diameter, 1mm thickness, and shade A3 were milled and divided according to the sintering temperatures (1460°C, 1530°C, and 1600°C) into three groups (n=30). Each group was later divided into three subgroups (n=10) according to the sintering holding time (1, 2, and 4 hours). CIELAB coordinates were measured for each specimen using Easyshade spectrophotometer (Vita, Bad Säckingen, Germany) and ΔE was obtained after comparing to the standard Vita coordinates of the shade A3. Contrast ratio (CR) was obtained after comparing the reflectance of light through the specimens over black and white background. Biaxial flexural strength was tested using the piston on three balls technique according to ISO 6872 standards in a universal testing machine (Zwick, Ulm, Germany).
Results: Two-way ANOVA and Tukey’s multiple comparison tests showed statistically significant decrease in the ΔE as the sintering temperature increased (4.1±0.3, 2.9±0.2, 2.2±0.1). No significant differences were observed between the 2 and 4 hours holding times (2.9±0.4, 2.9±0.2) however a significant increase in ΔE was detected with the 1 hour holding time (3.3±0.4). As for the CR a statistically significant decrease was observed with the 4 hours sintering holding time(0.69±0.01) when compared to 1 and 2 hours (0.72±0.01, 0.72±0.02) and with the 1530°C and 1600°C sintering temperatures(0.70±0.01, 0.70±0.02) when compared to 1460°C (0.73±0.02). No significant change in the biaxial flexural strength with the evaluated sintering parameters was observed.
Conclusions: Sintering zirconia ceramic using long cycles and high temperatures will result in deduction of ΔE and enhanced translucency. Biaxial flexural strength is not affected by changes in the evaluated sintering parameters.
Supported by Bruxzir, Glidewell, Frankfurt, Germany