Effect of changes in sintering parameters and coloring on mechanical and optical properties of translucent zirconia
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OBJECTIVES: To evaluate the effect of sintering time and temperature on the optical property, grain size, and mechanical property of various translucent zirconias. The specific aims are to compare the contrast ratio, grain size, and biaxial flexural strength of four different monolithic translucent zirconia materials using different coloring liquids. MATERIALS AND METHODS: For the test of the effect of sintering parameters on translucent zirconia materials, commercial TOSOH Y-TZP powders, Zpex and Zpex Smile were uniaxial dry-pressed in a mold, yielding 1.3 mm thickness discs. They were presintered using either a short or a long duration and then sintered at different final temperatures (1300 °C, 1450 °C, 1600 °C). Biaxial flexural strength was measured using an Instron Universal Testing machine. Contrast ratio was measured with a spectrophotometer. Grain size was measured on scanning electron microscopic (SEM) micrographs using ImageJ software. For the test of the effect of coloring liquids on contrast ratio, four different monolithic translucent zirconia blocks were sectioned into 1.3 mm thickness tiles. Coloring liquids of two shades (A2, D3) for each corresponding brand and Tanaka Zircolor dipping liquids were used for coloring the zirconia. For the same brand coloring liquid, Bruxzir HT was colored by BruxZir Coloring Liquid, Crystal Diamond Zirconia by Crystal color, Zenostar Zr translucent by Zenostar color Zr, and Zirkonzhan translucent by Zirkonzahn prettau aquarelle. A spectrophotometer was used to measure contrast ratio. For the test of the effect of coloring liquids on flexural strength, four different monolithic translucent zirconia milling blocks were sectioned into bars 1.5mm x 2 mm x15 mm, and colored into shade A2 with their manufacturers coloring liquid and Tanaka Zircolor liquid. A three-point bend test was performed using an Instron machine. Data was analyzed by JMP Pro 13 using multifactorial linear regression and ANOVA post hoc Tukey HSD test. The effects of difference factors and differences between groups were compared. The P-values less than 0.05 were considered statistically significant. RESULTS AND CONCLUSIONS: The contrast ratio and biaxial flexural strength of Zpex is significantly greater than Zpex Smile. There is a significant difference in contrast ratio and biaxial flexural strength between different presintering cycles and final sintering temperatures. The grain size of Zpex Smile discs was significantly greater than Zpex discs. The final sintering temperature, material and their interaction have significant effects on the grain size while there is no significant effect on the grain size by different presintering cycles. The contrast ratio of Bruxzir HT, Crystal diamond Zirconia and Zirkonzahn translucent is significantly greater than Zenostar Zr tanslucent. There is significant difference in contrast ratio with different shades and coloring liquid brands. The highest contrast ratio was seen when the material was colored with Tanaka shade D3. The flexural strength of Bruxzir HT and Crystal Diamond Zirconia was significantly greater than Zenostar Zr translucent and Zirkonzahn translucent. There was a significant difference in flexural strength between unaltered and colored Zirconia with Tanaka coloring liquid. The highest flexural strength was seen when the unaltered material was sintered.