Effect of Varying Sintering Speed on Optical Characteristics Alteration of Different Yttria-Doped Monochromatic...

Sintering influences the color of zirconia. This study assessed the influence of varying sintering rates on optical characteristics of 3, 4, and 5 mol% yttria (Y) containing monochromatic zirconia.

A total of 135 bar specimens (width × length × thickness = 11.2 × 20 × 1.5 mm) were prepared from monochromatic 3Y, 4Y, and 5Y zirconia, and randomly sintered at regular (RS: 10°C/min), fast (FS: 35°C/min), and speed (SS: 70°C/min) sintering rate (n = 15/group). Translucency parameter (TP₀₀), contrast ratio (CR), opalescence parameter (OP), and color difference (∆E₀₀) were evaluated with the CIEL*a*b* system. Microstructure, crystalline [monoclinic (m), tetragonal (t), and cubic (c)] phases, and surface roughness (Ra) were evaluated by scanning electron microscope (SEM), X-ray diffraction (XRD), and three-dimensional digital microscopy.

Analysis of variance and Bonferroni comparisons were determined for significant differences (α = 0.05).

Significant differences in TP₀₀, CR, OP, and ∆E₀₀ upon zirconia types and sintering rates were indicated (p < 0.05). Significant increasing TP₀₀, whereas decreasing CR and OP was shown upon increasing the Y content (5Y > 4Y > 3Y), and speeding sintering (SS ≅ FS > RS). Significant increasing ∆E₀₀ upon increasing the Y content (5Y > 4Y > 3Y) was shown but was within an acceptable threshold (∆E₀₀ ≤ 1.8). Ra was higher for 3Y > 4Y > 5Y. SEM indicated a larger grain for 5Y > 4Y > 3Y. XRD indicated higher t-phase in 3Y, whereas higher c-phase in 5Y.

Increasing translucency, whereas reducing contrast and opalescence were affected by the amount of Y content (5Y > 4Y > 3Y) and speeding sintering rate (SS ≅ FS > RS). Increasing color alteration was affected by the amount of Y content (5Y > 4Y > 3Y), but was within acceptable limits, suggesting rapid sintering rate to achieve better optical characteristics.