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

## TL;DR

This study shows how changing sintering rates affects the color and optical properties of zirconia with different yttrium contents.

## Contribution

The study reveals how varying sintering rates impact the optical characteristics of yttrium oxide-containing zirconia.

## Key findings

- Higher yttrium content (5Y) increased translucency and color difference but decreased contrast and opalescence.
- Speed sintering (70 °C/min) produced the most significant optical changes but within acceptable thresholds.
- Surface roughness decreased with increasing yttrium content, and grain size increased with higher yttrium.

## 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 (TP
00
), contrast ratio (CR), opalescence parameter (OP), and color difference (∆E
00
) 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 TP
00
, CR, OP, and ∆E
00
of zirconia types, layers, sintering rates, and their interactions were indicated (
p <
 0.05). Significant increases in TP
00
and ∆E
00
, 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 (∆E
00
≤ 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.