A novel etching technique for surface treatment of zirconia ceramics to improve adhesion of resin-based luting cements

Abstract

Objectives: Bonding of zirconia crowns and bridges to abutments is important, not only bonding of the thin resin layer to the abutment, but also bonding to the zirconia ceramic is crucial. Both mechanical and chemical adhesion are desired. Mechanical retention of dental porcelain achieved by etching with moderately concentrated hydrofluoric acid is not possible with zirconia ceramics. The purpose of this study was to show that etching is possible with relative low melting fluoride compounds such as ammonium hydrogen difluoride and potassium hydrogen difluoride. Materials and methods: Before melting, the fluorides can be introduced as powders or as aqueous slurries to the contact surfaces of the zirconia. After melting, the yttria-stabilized zirconia surface revealed a surface similar to an HF-etched dental feldspathic porcelain surface. Shear bond testing (n = 10) was performed with zirconia attached to zirconia with the Duo-Link composite luting cement (Bisco) after treatment of the etched zirconia surfaces with Bis-Silane (Bisco) and the Porcelain Bonding Resin (Bisco). Results: Values for adhesive strength (mean ± standard deviation) after melt etching of the surfaces with initially dry powders were for K[FHF], (31.2 ± 7.5) MPa and for NH₄[FHF] (31.0 ± 11.8) MPa. When initially aqueous slurries were applied, the values were for K[FHF] (42.7 ± 12.7) MPa and for NH₄[FHF] (40.3 ± 10.0) MPa. Conclusion: Good adhesion to zirconia can be achieved by a procedure including etching with selected melted fluoride compounds.

Keywords: Difluoride melt etching; FTIR spectroscopy; SEM; XRD analysis.

Figure 1.

Schematic view of etching process. Diffusion of active etching agents in the melt and to the solid zirconia surface with simultaneous diffusion of etching products, fluorozirconates, from the zirconia surface and into the melt.

Figure 2.

Schematic illustration of combination of materials (zirconia bonded to zirconia ceramic).

Figure 3.

SEM images of the zirconia disc specimens (reference) after different surface treatments (a) Sandblasted; (b) K[FHF] powder, etched; (c) K[FHF] slurry, etched; (d) NH₄[FHF] powder etched; (e) NH₄[FHF] slurry etched.

Figure 4.

SEM images of the zirconia disc specimens after shear bond measurements (a) EKPS (b) EKPU and (c) EKSU.

Figure 5.

SEM images of the fracture surfaces of zirconia disc and rod specimens after shear bond testing measurements. Light areas are thin layers and darker areas are thicker layers of bonding resin cement. (a) SBS zirconia disc, contact area at the upper part; (b) SBS zirconia rod, contact area at the upper part; (c) EASS zirconia disc, contact area; (d) EASS zirconia rod, contact area.

Figure 6.

Rietveld fit to XRD data for Y-TZP specimens after different surface treatments. (a) Polished; (b) Sandblasted; (c) K[FHF] slurry etched; (d) NH₄[FHF] slurry etched. Upper plots showing observed and calculated intensities, lower plot shows the difference for the samples. *Monoclinic peak.

Figure 7.

FT-IR spectra of (a) etched surface product and (b) K₂[ZrF₆] in the region 510–460 cm⁻¹. The figures on the absorption peaks represent wavenumbers and peak areas.

Figure 8.

FT-IR spectra for (a) etched surface product and (b) K₂[ZrF₆] in the region 500–200 cm⁻¹.