Leucite and cooling rate effect on porcelain–zirconia mechanical behavior

Objective: This study investigated the influence of the cooling protocol on the mechanical behavior of Y-TZP veneered with porcelain with different compositions. The tested hypotheses were: (1) Y-TZP infrastructures veneered with porcelain containing leucite in its composition presents higher flexural strength (σ) and reliability (m), and (2) slow cooling protocol results in greater σ and m. 

Methods: A total of 120 bilayer porcelain-Y-TZP bar-shaped specimens were prepared with the dimensions of 1.8 mm (0.8 mm Y-TZP ± 1.0 mm porcelain) × 4.0 mm × 16.0 mm. Specimens were divided into four groups (n = 30) according to the porcelain composition (containing or not leucite) and cooling protocol. Fast cooling was performed by opening the furnace chamber at sintering temperature. For the slow cooling, the chamber was maintained closed until it reached the room temperature. Specimens were tested in three-point bending with the porcelain surface under tension using a universal testing machine, in 37 °C water, at 0.5 mm/min crosshead speed. Data were analyzed by two-way ANOVA, Tukey post-hoc test (α = 0.05) and Weibull. 

Results :Y-TZP veneered with porcelains with different microstructural composition presented similar σ and m values (p = 0.718). The cooling protocol had no influence on the σ and m values of the experimental groups (p = 0.718). Cracking represented 95% of failures, whereas the initial flaw propagated from the porcelain surface towards the interface. Significance Y-TZP veneered with porcelain containing or not leucite present similar mechanical behavior and, at 1-mm thickness, is not sensitive to the cooling protocol.