Effect of the cooling rate on the properties of veneer porcelain for zirconia dental prosthesis

Abstract

To solve chipping of veneer porcelain in zirconia dental prosthesis it has been studied different cooling rates in firing schedules to control residual stresses generated by differences in thermal properties between the ceramics. However, cooling rates may affect the porcelain, irrespectively of interaction with zirconia. This study analyzed the effect of cooling rate on the three-point flexural strength (FS), residual stress, Vickers hardness (VH) and V-notch fracture toughness (SEVNB) on a feldspathic porcelain (Vita VM9) indicated for covering zirconia. Bar-shaped specimens were sintered according manufacturer's schedule firing and cooled at different rates (slow, normal or fast). Specimens were tested for FS (n=30), VH (n=15) and SEVNB tests (n=15). Weibull analysis were performed from FS values to measure reliability and probability of failure (PF). One-way ANOVA was significant for VH (p<0.001), and Tukey's test showed that slow cooling led to higher values. There was no significant difference for FS (p=0.654) and SEVNB (p=0.734). Fast cooling led to the lowest PF, but cooling rate factor was not significant for Weibull modulus. All cooling group showed residual tensile stress (MPa): Slow = 34.81; Normal = 27.04; Fast = 18,98. It was concluded that cooling rate produce minor changes in the porcelain properties.