Fracture strength and microstructure of Y-TZP zirconia after different surface treatments.

STATEMENT OF PROBLEM: Airborne-particle abrasion of the inner and outer surfaces of an yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) core is used in an attempt to enhance the bond strength between the core and the veneering porcelain and to increase the surface area for cementation. However, airborne-particle abrasion introduces surface flaws that act as stress concentrators that may compromise the mechanical strength of the ceramic. PURPOSE: The purpose of this study was to investigate the effect of airborne-particle abrasion and heat treatment on the microstructure, biaxial flexural strength, and reliability of Y-TZP zirconia ceramics before veneering and cementation. MATERIAL AND METHODS: Forty-eight disks (15 mm in diameter, 0.5 mm in thickness) of Y-TZP were divided into 6 groups. Three treatments (untreated, airborne-particle abrasion, and heat treatment after airborne-particle abrasion) were applied to the upper surfaces, and 2 treatments (untreated and airborne-particle abrasion) were applied to the lower surfaces to mimic the preparation for veneering and cementation. For airborne-particle abrasion, 110 mum Al2O3 particles were used. The maximum load at fracture was calculated with a biaxial flexural strength test. The upper surfaces were facing the loading piston, and the lower surfaces were facing the supporting jig during testing. Results were analyzed with 2-way ANOVA (alpha=.05). The treated and fractured surfaces were observed with a scanning electron microscope. The relative content of the monoclinic phase was quantified with an x-ray diffraction analysis. RESULTS: The group with airborne-particle abraded lower surfaces showed significantly higher flexural strength than the untreated group (P<.001). The SEM images of the airborne-particle abraded zirconia specimens showed rough and irregular surfaces. The fracture initiated from the tension side, which was opposite to the applied load. CONCLUSIONS: Within the limits of this in vitro study, the results showed that airborne-particle abrasion of the lower surfaces increases the flexural strength of Y-TZP zirconia.