The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio

This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia. Zirconia specimens (Ceramill ZI, Amann Girrbach) were prepared in partially sintered state. Subsequently, the specimens were randomly divided into nine groups and sintered with different final sintering temperatures: 1,300A degrees C, 1,350A degrees C, 1,400A degrees C, 1,450A degrees C, 1,500A degrees C, 1,550A degrees C, 1,600A degrees C, 1,650A degrees C, or 1,700A degrees C with 120 min holding time. Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008. The contrast ratio (N = 90; n = 10 per group) was measured according to ISO 2471: 2008. Grain sizes and microstructure of different groups were investigated (N = 9, n = 1 per group) with scanning electron microscope. Data were analyzed using one-way ANOVA with Scheff, test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures. The highest flexural strength was observed in groups sintered between 1,400A degrees C and 1,550A degrees C. The highest Weibull moduli were obtained for zirconia sintered at 1,400A degrees C and the lowest at 1,700A degrees C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650A degrees C exhibited defects. Sintering temperatures showed a significant negative correlation with both the flexural strength (r = -0.313, p < 0.001) and the contrast ratio values (r = -0.96, p < 0.001). The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength.