Bond strength between layering ceramic and zirconia core: influence of various surface conditioning of zirconia

It is recommended to apply various surface modification techniques to improve the mechanical and chemical interaction between zirconia core and veneering ceramic. Therefore, the objective of this in-vitro study was to evaluate and compare the effects of various surface treatments over the zirconia core on the shear bond strength (SBS) between layering ceramic and zirconia core.

A total of 90 zirconium oxide (Cercon, Degudent GmbH, Hanau-Wolfgang, Germany) specimens were prepared with standardized dimensions (10 mm × 10 mm × 5 mm). The specimens were randomly divided into six groups of 15 specimens each (Control group-no surface treatment; Sandblasting with 25 µm Al₂O₃; Sandblasting with 50 µm Al₂O₃; Sandblasting with 110 µm Al₂O₃; Etching with hydrofluoric acid (9.5%); Surface treatment using Zirface (nano layer etching technology for enhanced bonding strength of zirconia). All specimens were veneered with fluorapatite glass-ceramic (5 mm × 5 mm × 5 mm) using a custom silicone mold, sintered in a calibrated porcelain furnace, and verified with a digital caliper. Following veneering, the samples were embedded in self-curing acrylic resin within polyvinyl chloride molds and immersed in distilled water at 37 °C for 24 hours to simulate intraoral conditions. Using universal testing machine, specimens were tested for SBS. Analysis of variance (ANOVA) and Tukey's-test were used for statistical analysis (P ≤ 0.05). Surface topography of the surfaces treated and after debonding were examined using scanning electron microscope (SEM) and failure mode of different groups was also noted.

The application of Zirface to the zirconia core had the highest mean SBS value (26.56 ± 1.25 MPa), followed by the hydrofluoric acid only group (23.42 ± 0.94 MPa). The control group with no surface treatment showed the least SBS values. While the groups with surface treatments of 25 and 50 aluminum oxide (Al₂O₃) sandblasting showed SBS values of 17.16 ± 0.48 and 18.06 ± 0.89, respectively, with no significant difference (p > 0.05) between the SBS values of the two groups. However, the sandblasting with 110 Al₂O₃ particles over the zirconia core surface showed higher SBS values (20.53 ± 1.14) as compared to the 25 and 50 Al₂O₃ particles.

The study found that the shear bond strength between zirconia and veneering ceramic is significantly influenced by the surface treatment used, with nano-layer etching and hydrofluoric acid etching enhancing adhesion, and larger alumina particles enhancing bonding durability.