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Application of 1% hydrofluoric acid (HFA) to improve the repair bond strength of a microhybrid (Z250; 3 M/ESPE) and a nanofilled dental composite (Supreme XT, 3 M/ESPE) was investigated. Aged composites were etched using 1% HFA for 10, 30, or 60 s. Negative (non-etched) and positive (10% HFA for 60 s) control groups were tested. The surfaces received a layer of a silane and adhesive (Single Bond 2; 3 M/ESPE). Cylinders of fresh composite (n = 15 per group) were built up and a shear bond strength test was carried out after water storage for 24 hr or 6 months. Surface topography was assessed using atomic force microscopy (AFM). For the nanofill at 24 hr, all etching regimens yielded higher bond strengths than the negative control. 

After 6 months, the negative and positive controls showed lower bond strength than 1% hydrofluoric acid(HFA) for 30 s. For the microhybrid, the negative control generally showed lower bond strength at 24 hr, while all groups had similar results after 6 months. Only the positive controls showed lower bond strengths after 6 months. AFM analysis showed that the nanofill was more susceptible to acid-etch roughening. In conclusion, exposure to 1% HFA may improve the repair bond strength of dental composites.

Study the ceramic surface and cross section morphology in different concentration of hydrofluoric acid etching  by scanning electron microscopic, measuring the porosity and surface etching depth section were also studied. The results show that: with the increasing concentration and time the etching depth increased, the etching depth with increasing concentration and time there is an increasing trend, but when the HF acid concentration of more than 5.0%, the etching time above 7.5min, the etching depth did not change significantly. Porosity and HF acid concentration in a time-dependent relationship.