Diferentes concentrações de ácido fluorídrico: efeito na adesão e comportamento em fadiga de uma cerâmica de dissilicato de lítio

Abstract

The studies compound the present thesis. Firstly, the effect of different hydrofluoric acid (HF) concentrations (1%, 3%, 5% and 10%) on the microshear bond strength between a lithium disilicate ceramic and a resin cement was evaluated. Samples (12×14×2mm) of lithium disilicate were etched with the respective HF concentrations, silanized, and resin cement cylinders (Ø=0.72mm) were built up over the surfaces. A half of the samples was tested after 24h, and the other half was submitted to aging (150 days + 12,000 thermocycles – 5° and 55°C) previously to testing. Contact angle, atomic force and surface topography analysis were performed. On the “baseline” condition, HF3=HF5=HF10 (13.9–15.9MPa) (p>.05), and HF1<HF5. After aging, the group HF1 presented the lowest bond strength (HF3=HF5=HF10>HF1). The groups HF3, HF5, and HF10 presented lower contact angle values (7.8–10.4°). Higher HF concentrations promoted higher topographic alterations and consequently, rougher surfaces. In terms of adhesion, concentrations of 3%, 5% and 10% seem to be adequate for etching lithium disilicate. Secondly, the effect of different HF concentrations on the cyclic load-to-failure of lithium disilicate discs luted to a dentin analogue material was investigated. Ceramic discs (Ø=10mm; thickness=1.5mm) and of a dentin analogue material (G10; Ø=10 mm, thickness=2mm) were produced. With exception of the control group (CRTL), the ceramic discs were etched (HF3), (HF5) or (HF10) and later silanized, meanwhile the G10 discs received an adhesive primer application. After the adhesive luting, the cyclic load-to-failure (500,000 cycles, 20Hz, initial load=720N, step size=70N) was obtained by the staircase method, under water. A stainless-steel piston (Ø=40mm) applied the load in the center of the samples. Fractographic and topographic analysis were performed. Intermediate HF concentrations (3% and 5%) presented higher cyclic load-to-failure values, and the control group (non-etched) presented the worst behavior under fatigue [HF3(1355.0) = HF5(1335.0) > HF10(1175.0) > CTRL(965.0)]. All failures observed were radial cracks starting from the cementation surface. Lastly, the effect of different HF concentrations was evaluated on the cyclic load-to-failure of monolithic lithium disilicate restorations machined by CAD-CAM adhesively luted to simplified prosthetic preparations (G10). The staircase method was used for the tests following the same test assembly and experimental design of the previous study. Topographic, fractographic and fractal dimension analyses were performed. There was no statistical difference among the tested groups [CTRL(805.00) = HF3(781.25) = HF5(755.00) = HF10(833.75)]. Despite the fractal dimension and surface topography analyses have shown complex surfaces to HF3 and HF10, the topographic path created by machining was overweight on the lithium disilicate crowns. All failures found were radial cracks started from the luted surface. Based on the findings of the present thesis, in terms of adhesion and fatigue behavior, the ceramic tested can be etched with 3 and 5% HF.