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

Abstract

OBJECTIVE: This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia. MATERIALS AND METHODS: 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: 1,300°C, 1,350°C, 1,400°C, 1,450°C, 1,500°C, 1,550°C, 1,600°C, 1,650°C, or 1,700°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 2471ISO : 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 Scheffé test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures. RESULTS: The highest flexural strength was observed in groups sintered between 1,400°C and 1,550°C. The highest Weibull moduli were obtained for zirconia sintered at 1,400°C and the lowest at 1,700°C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650°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). CONCLUSIONS: 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. CLINICAL RELEVANCE: Considering the flexural strength values and Weibull moduli, the sintering temperature for the zirconia tested in this study should not exceed 1,550°C. 20]. In fact, the stability of the complete system consisting both the zirconia framework and the veneering ceramic is of clinical importance.In order to decrease the costs and at the same time overcome the chipping problem, it is possible today to produce zirconia FDPs without veneering ceramic.Such monolithic zirconia FDPs require higher translucency with superior mechanical properties especially when they are to be used in the anterior region. Framework translucency is therefore one of the primary factors with respect to aesthetic properties in the selection of dental materials [21,22]. The translucency of dental ceramic is highly dependent on light scattering [23]. When the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material would appear opaque. However, if only part of the light is scattered and most of it is diffusely transmitted, the material would appear translucent where Y B was the value with a black background, and Y w was the value with a white background. In all calculations, 0 was considered as totally transparent and 1 as

Comparison of four monolithic zirconia materials with conventional ones: Contrast ratio, grain size, four-point flexural strength and two-body wear

“…Previous studies observed an impact of sintering parameters of zirconia materials on contrast ratio values and reported that higher sintering temperatures/times led to decreased contrast ratio values (Stawarczyk et al, 2013a;Stawarczyk et al 2015). This observation cannot be confirmed by the results of the present study.…”

“…Conforming to the present study, prior investigations showed that a higher flexural strength of ceramic materials resulted in a higher contrast ratio (Stawarczyk et al, 2013a;Baldissara et al 2010). In this study, the measured flexural strength values for the monolithic zirconia materials ranged between 611 -784 MPa, depending on material type and aging regime.…”

“…The first steps to get translucent zirconia went through the optimization of sintering parameters (first generation). By increasing the sintering temperature and the sintering time, the zirconia became translucent and showed good esthetic results (Janney et al, 1992;Stawarczyk et al, 2013a;Stawarczyk et al, 2015). The translucency of zirconia can be improved by reducing 4 residual porosity and through the generation of a nanometric microstructure because the in-line transmittance of 50% at the visible wavelength range is expected for grain sizes <40 nm (Klimke et al, 2011;Zhang et al, 2011).…”

“…The translucency of zirconia can be improved by reducing 4 residual porosity and through the generation of a nanometric microstructure because the in-line transmittance of 50% at the visible wavelength range is expected for grain sizes <40 nm (Klimke et al, 2011;Zhang et al, 2011). The characteristics of porosity and nanometric microstructure can be manipulated as mentioned by sintering parameters, and through the resulting enlargement of the zirconia grains size (Jiang et al, 2011;Stawarczyk et al, 2013a;Stawarczyk et al, 2015, Zhang et al, 2012.…”

“…However, recent studies showed that the increase of grain size decreases the flexural strength and the reliability of the conventional zirconia materials (Stawarczyk et al, 2013a). Even worse, the higher sintering temperature affected negatively the low-temperature degradation of zirconia materials (Hallmann et al, 2012).…”