Effect of different aging methods on the mechanical behavior of multi-layered ceramic structures

Objective: To evaluate the effect of two aging methods (mechanical cycling and autoclave) on the mechanical behavior of veneer and framework ceramic specimens with different configurations (monolithic, two and three-layers). 

Methods: Three ceramics used as framework for fixed dental prostheses (YZ—Vita In-Ceram YZ; IZ—Vita In-Ceram Zirconia; AL—Vita In-Ceram AL) and two veneering porcelains (VM7 and VM9) were studied. Bar-shaped specimens were produced in three different designs: monolithic, two layers (porcelain–framework) and three layers (porcelain–framework–porcelain). Specimens were tested for three-point flexural strength at 1 MPa/s in 37 °C artificial saliva. Three different experimental conditions were evaluated (n = 10): control; mechanical cycling (2 Hz, 37 °C artificial saliva); and autoclave aging (134 °C, 2 bars, 5 h). Bi-layered specimens were tested in both conditions: with porcelain or framework ceramic under tension. Fracture surfaces were analyzed using stereomicroscope and scanning electron microscopy. Results were statistically analyzed using Kruskal–Wallis and Student-Newman–Keuls tests. 

Results: Only for AL group, mechanical cycling and autoclave aging significantly decreased the flexural strength values in comparison to the control (p < 0.01). YZ, AL, VM7 and VM9 monolithic groups showed no strength degradation. For multi-layered specimens, when the porcelain layer was tested in tension (bi and tri-layers), the aging methods evaluated also had no effect on strength (p ≥ 0.05). Total and partial failure modes were identified. 

Significance: Mechanical cycling and autoclave aging protocols had no effect on the flexural strength values and failure behavior of YZ and IZ ceramic structures. Yet, AL monolithic structures showed a significant decrease in flexural strength with any of the aging methods.