Impact of high-speed sintering on translucency, phase content, grain sizes, and flexural strength of 3Y-TZP and 4Y-TZP zirconia materials

Abstract

STATEMENT OF PROBLEM The lengthy sintering time of zirconia is costly and limits applications. The consequences of shortening the sintering time are mainly unknown. PURPOSE The purpose of this in vitro study was to test and compare 2 high-speed sintering protocols and 1 conventional sintering protocol on the translucency, phase content, grain sizes, and flexural strength of 3 zirconia materials. MATERIAL AND METHODS In total, 450 specimens of 3 zirconia materials-two were 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZPs), Ceramill ZI and Zolid (ZD), and a 4 mol% yttria-stabilized tetragonal zirconia polycrystal (4Y-TZP), Zolid HT+ (n=150)-and 5 thicknesses (1.0, 1.5, 2.0, 2.5, and 3.0 mm; n=30) were sintered according to 2 high-speed sintering protocols (final temperature 1570°C and 1590°C; n=10) and a reference sintering protocol (1450°C; n=10). After measuring the monoclinic phase content with Raman spectrometry (n=3), the specimens were polished, and translucency was determined. The biaxial flexural strength of specimens with a thickness of 1.0 mm and 1.5 mm was tested (n=20). Statistical evaluation included 1-way ANOVA, the Kolmogorov-Smirnov, Kruskal-Wallis, Mann-Whitney-U, and Spearman-Rho tests and the Bonferroni correction (=.0011). RESULTS For ZI, the sintering protocols did not affect the translucency or biaxial flexural strength. ZD and HT+ showed significantly lower translucency for high-speed sintering protocols (P.001), but the biaxial flexural strength remained the same after the high-speed sintering protocol at 1590°C. Grain sizes increased with increasing final sintering temperature for ZI and HT+, whereas translucency generally decreased with increasing material thickness. No monoclinic phase was detected in any group. CONCLUSIONS The flexural strength was maintained with high-speed sintering but led to a decrease in translucency for ZD and HT+.

Strength and translucency of zirconia after high‐speed sintering

“…Theoretically, the larger grains achieved with Prettau Anterior and Zpex Smile with the high‐speed sintering protocol should have increased their translucency due to the reduction in grain boundaries . A previous study also reported decreased translucency when a 5 mol% yttria‐containing zirconia was sintered with a high‐speed protocol . No explanation was given for the observation in the previous study.…”

“…Prettau Anterior and Zpex Smile experienced a 30% and 40% reduction in flexural strength, respectively, following high‐speed sintering. A previous study of 5 mol% yttria‐containing zirconia reported no effect on strength with a high‐speed sintering program . This result is consistent with the strength achieved with Katana STML Block with the high‐speed sintering protocols, but not the other zirconia materials tested in this study.…”

“…Previous studies have reported that increasing sintering time and temperature both increased grain sizes of 3 mol% and 5 mol% yttria‐containing zirconia . Increasing temperature (ranging 1350°C–1550°C) has a bigger effect on grain size than sintering time (ranging 1–3 hours) .…”

“…Several studies have reported that increasing the sintering temperature of 3 mol% yttria‐containing zirconia has increased its translucency . Regarding 5 mol% yttria‐containing zirconia, there are reports that an increased sintering temperature increases and decreases translucency.…”

“…Several studies have reported that increasing sintering temperature of 3 mol% yttria‐containing zirconia increases its strength . Studies of 5 mol% yttria‐containing zirconia have reported no effect of sintering temperature on flexural strength …”