Current speed sintering and high-speed sintering protocols compromise the translucency but not strength of yttria-stabilized zirconia

To investigate the impacts of speed and high-speed sintering on the densification, microstructure, phase composition, translucency, and flexural strength of yttria-stabilized zirconia (YSZ). A total of 162 disc-shaped specimens (n = 18) were cold-isostatically pressed from 3YSZ (Zpex), 4YSZ (Zpex 4), and 5YSZ (Zpex Smile) powders (Tosoh Corporation) and sintered according to the following protocols: conventional (control, ∼12 h), speed (∼28 min for 3YSZ; ∼60 min for 4YSZ and 5YSZ), and high-speed (∼18 min) sintering. Dimensions of zirconia specimens after sintering and polishing (1-μm diamond grit finish) were Ø13.75 × 1 mm. Density, microstructure, phase content, translucency parameter, and biaxial flexural strength were evaluated using Archimedes’, SEM, XRD, spectrophotometric, and piston-on-3-ball methods, respectively. Data were analyzed with either one-way ANOVA and Tukey's test or Kruskal–Wallis with Dunn’s test (α = 0.05). For all YSZ compositions, conventional sintering yielded the highest density followed by speed then high-speed sintering. All sintering protocols resulted in similar strength values; however, speed and high-speed sintering protocols afforded significantly lower translucency relative to conventional sintering. XRD analysis revealed similar spectra for YSZs sintered by various protocols. The speed sintered specimens had the smallest grain size whereas the high-speed sintered 5YSZ possessed the largest grain size among all groups. SEM examination of all YSZ compositions revealed that the average pore size was an order of magnitude smaller than the average grain size. Speed and high-speed sintering of YSZs yield similar strength but diminished density and translucency relative to their conventionally sintered counterparts.