Effects of thickness, processing technique, and cooling rate protocol on the flexural strength of a bilayer ceramic system

ObjectiveTo determine whether the thickness, processing technique, and cooling protocol of veneer ceramic influence the flexural strength of a bilayer ceramic system.Materials and methodsSixty-four bar-shaped specimens (20 mm × 4 mm × 1 mm) of yttria-stabilized tetragonal zirconia (Vita In-Ceram YZ, Vita) were fabricated (ISO 6872) and randomly divided into 8 groups (n = 8) according to the factors “processing technique” (P – PM9 and V – VM9), “thickness” (1 mm and 3 mm), and “cooling protocol” (S – slow and F – fast). The veneer ceramics were applied only over one side of the bar-shaped specimens. All specimens were mechanically cycled (2 × 106 cycles, 84 N, 3.4 Hz, in water), with the veneer ceramic under tension. Then, the specimens were tested in 4-point bending (1 mm/min, load 100 kgf, in water), also with the veneer ceramic under tension, and the maximum load was recorded at first sign of fracture. The flexural strength (σ) was calculated, and the mode of failure was determined by stereomicroscopy (30×). The data (MPa) were analyzed statistically by 3-way ANOVA and Tukey's test (α = 0.05).ResultsANOVA revealed that the factor “thickness” (p = 0.0001) was statistically significant, unlike the factors “processing technique” (p = 0.6025) and “cooling protocol” (p = 0.4199). The predominant mode of failure was cracking.SignificanceThe thickness of the veneer ceramic has an influence on the mechanical strength of the bilayer ceramic system, regardless of processing technique and cooling protocol of the veneer ceramic.