Improved Resin-Zirconia Bonding by Room Temperature Hydrofluoric Acid Etching

Abstract

This in vitro study was conducted to evaluate the shear bond strength of "non-self-adhesive" resin to dental zirconia etched with hydrofluoric acid (HF) at room temperature and to compare it to that of air-abraded zirconia. Sintered zirconia plates were air-abraded (control) or etched with 10%, 20%, or 30% HF for either 5 or 30 min. After cleaning, the surfaces were characterized using various analytical techniques. Three resin cylinders (Duo-Link) were bonded to each treated plate. All bonded specimens were stored in water at 37 °C for 24 h, and then half of them were additionally thermocycled 5000 times prior to the shear bond-strength tests (n = 12). The formation of micro- and nano-porosities on the etched surfaces increased with increasing concentration and application time of the HF solution. The surface wettability of zirconia also increased with increasing surface roughness. Higher concentrations and longer application times of the HF solution produced higher bond-strength values. Infiltration of the resin into the micro- and nano-porosities was observed by scanning electron microscopy. This in vitro study suggests that HF slowly etches zirconia ceramic surfaces at room temperature, thereby improving the resin-zirconia bond strength by the formation of retentive sites.

Keywords: etching; hydrofluoric acid; resin–zirconia bonding; zirconia ceramic.

Figure 1

Scanning electron microscopy (SEM) surface images of the zirconia ceramic specimens after different surface treatments (20,000× magnification, bar = 1 μm): (a) APA; (b) 10F5; (c) 10F30; (d) 20F5; (e) 20F30; (f) 30F5; (g) 30F30.

Figure 2

Scanning electron microscopy (SEM) images at high magnification showing nanoscale structures formed on 30-min etched surfaces (100,000× magnification, bar = 100 nm): (a) 10F30; (b) 20F30; (c) 30F30. Such nanostructures were not evident on the 5-min etched surfaces.

Figure 3

Atomic force microscopy (AFM) images of the zirconia ceramic specimens after different surface treatments (5 μm × 5 μm): (a) APA; (b) 10F5; (c) 10F30; (d) 20F5; (e) 20F30; (f) 30F5; (g) 30F30. Note that more retentive surface morphologies were produced with increasing concentration and application time of the etching solutions.

Figure 4

Contact angles of water droplets on the specimen surfaces. Black squares denote mean values, boxes represent standard deviations, and whiskers define the minimum and maximum values. Identical lower-case letters indicate statistically equivalent values (p > 0.05).

Figure 5

Wide-scan X-ray photoelectron spectroscopy (XPS) spectra in the range from 0 to 800 eV: (a) APA; (b) 10F30; (c) 20F30; (d) 30F30. Al: aluminum; C: carbon; F: fluorine; N: nitrogen; O: oxygen; Y: yttrium; Zr: zirconium.

Figure 6

Far-infrared (FIR) spectra of zirconia polished and etched with 30% HF for 30 min (30F30).

Figure 7

Ion chromatograms of (a) 0.1 ppm fluoride standard solution; (b) five-day storage water of the 30F30 specimen; and (c) five-day storage water of the resin (Duo-Link)-covered 30F30 specimen. Note that no distinct peaks corresponding to fluoride appeared at the characteristic retention time of 3.88 min in the specimens.

Figure 8

Scanning electron micrographs (SEM) of 30F30 specimens etched (left) and debonded (right): (a,b) surface images (20,000× magnification, bar = 1 μm), (c,d) overview of cross-sections prepared by FIB (15,000× magnification, bar = 1 μm), (e,f) enlarged cross-sections (50,000× magnification, bar = 500 nm). In Figure 8f, the pointers indicate incomplete resin infiltration into the micropores.