The effect of repetitive firing cycles on physical and optical properties of zirconia reinforced lithium silicate ceramics

This study’s objective was to evaluate repetitive firing cycles’ effects on the translucency, light’s absorption coefficient, and flexural strength of zirconia-reinforced lithium silicate ceramics. Two zirconia-reinforced lithium silicate ceramics and one lithium disilicate glass-ceramic were tested. Blocks of all materials were sectioned into tiles with different thicknesses and subjected to up to five firing cycles using the firing schedule indicated in the manufacturer’s user instructions. Light transmission ratio (T) and absorption coefficients were determined using a spectrophotometer. Further, bars were sectioned from blocks of all materials and tested for three-point-bend flexural strength using a Universal Testing Machine (Instron), and flexural strength was calculated from load at failure. Factorial ANOVA and Tukey’s HSD tests were conducted to analyze light transmission and flexural strength, while regression was used to analyze the absorption coefficient. Weibull parameters and fractographic analysis also were investigated. The results showed that repetitive firing cycles reduced e.max® and Vita Suprinity’s® translucency, but not that of Celtra® Duo, which showed no significant difference. All materials of greater thickness exhibited less translucency, and e.max® CAD had the highest mean light transmission; however, it was not significantly different than Celtra® Duo. Repetitive firing cycles showed more absorption coefficient of light with Vita Suprinity® and e.max®, except for Celtra® Duo, which showed no difference. Vita Suprinity® showed the highest absorption coefficient; however, it was not significantly different than e.max® CAD. Repetitive firing cycles had no significant effect on flexural strength. High and low flexural strength samples for all materials showed similar characteristics with respect to crack propagation patterns, and fracture origins. In conclusion, repetitive firing cycles decreased both e.max® and Vita Suprinity’s® translucency significantly. Repetitive firing cycles increased e.max® and Vita Suprinity’s® absorption coefficient significantly, particularly at shorter wavelengths. Repetitive firing cycles did not increase flexural strength statistically significantly. Vita Suprinity® showed an inherent and more homogeneous flaw-distribution in the first two firing cycles compared to the distribution of flaws in the other two materials