Color Characteristic Alteration of Different Yttrium Oxide-Containing Multilayer Partially Stabilized Zirconia at Different Sintering Rates

Abstract Sintering influences the optical properties of zirconia. This study examined the effect of altering sintering rates on color characteristics of 3, 4, and 5 mol% yttria (Y)-containing multilayer zirconia. A total of 135 specimens (width × length × thickness = 11.2 × 20 × 1.5 mm) were prepared from multilayer (cervical [C], middle [M], and incisal [I]) 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 (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 microscopy (SEM), X-ray diffraction (XRD), and 3D-digital topography. ANOVA and Bonferroni comparisons were determined for significant differences (p < 0.05). Significant differences in TP00, CR, OP, and ∆E00 of zirconia types, layers, sintering rates, and their interactions were indicated (p < 0.05). Significant increases in TP00 and ∆E00, but decreases in CR and OP, upon rising the amount of Y (5Y > 4Y > 3Y), region (I > M > C), and speed sintering (SS > FS > RS) (p < 0.05) were observed. Nevertheless, the color alteration was within an acceptable threshold (∆E00 ≤ 1.8); Ra values: 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 and color alteration, and decreasing contrast and opalescence were influenced by Y content (5Y > 4Y > 3Y), region (I > M > C), and sintering rate (SS > FS > RS). Nonetheless, color alterations were within acceptable limits, suggesting the speed sintering rate to produce better color characteristics of restoration.