Optical and Mechanical Properties of Speed Sintered Yttria Stabilized Zirconia

Objectives: 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).
Methods: A total of 90 disc-shaped specimens (n = 30) were cold-isostatically pressed from 3YSZ (Zpex), 4YSZ (Zpex 4), and 5YSZ (Zpex Smile) powders (Tosoh Corporation, Japan). Ten specimens from each composition were sintered according to the following protocols: conventional (~12 h, control group), speed (~28 min for 3YSZ; ~60 min for 4Y and 5YSZ), and high-speed (~18 min) sintering. Dimensions of zirconia specimens after sintering and polishing (1-mm diamond finish) were Ø13.75 x (1 ± 0.02) mm. The theoretical density of these compositions was calculated, and relative density/apparent porosity estimated. Translucency was assessed using a spectrophotometer against a black and white background. Biaxial flexural strength was measured using the piston-on-3-ball method. Zirconia specimens were thermally etched for microstructural observations using scanning electron microscopy (SEM). X-ray diffraction (XRD) was used to determine the phase assemblies. Data were analyzed with either one-way ANOVA and Tukey's test or Kruskal–Wallis with Dunn’s test (α = 0.05).
Results: 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 among all groups whereas the high-speed sintered 5YSZ possessed the largest grain size. Finally, SEM examination of all YSZ compositions revealed that the average pore size was an order of magnitude smaller than the average grain size.
Conclusions: Speed sintering of YSZs yielded similar strength but diminished density and translucency relative to their conventionally sintered counterparts due to the presence of pores.