Effect of nitric and hydrofluoric acid solution and silica-coating on surface roughness and adhesion of luting cements to yttrium-stabilized zirconia

This study investigated the effect of varying acid etching durations using nitric and hydrofluoric acid solution on the surface roughness and adhesion of luting cements to 4-mol yttria-stabilized zirconia (4YSZ) and compared the results with those of silica coating. Zirconia specimens (IPS e.max ZirCAD MT) (N = 192, n = 12 per group) were randomly assigned to two surface conditioning methods: a) silica coating (CoJet) (20 s, 2.5 MPa) (SC) and b) acid etching (Zircos E, ZE). Etching was performed for 30 min, 1 h, and 2 h. After elemental analysis (energy-dispersive spectroscopy, EDS), surface roughness (Ra, Rz, Rv, and Rc, RzJIS, Rp) measurements (3D microscopy), two adhesive luting systems (PV: Panavia 21 and ML: Multilink Hybrid Abutment) were bonded onto the conditioned substrates. Half of the specimens were tested dry, and the other half were tested after thermocycling (x6000 cycles, 5–55 °C) conditions. Shear bond strength (SBS) was measured in a universal testing machine (1 mm/min). Failure modes were analyzed using 3D digital microscopy. Data were analyzed using 2-way ANOVA, Tukey's post-hoc, and Pearson's correlation tests (α = 0.05). Only ZE for 30 min produced surface roughness similar to that of SC for the Ra, Rz, Rv, and Rc, but showed higher values for the RzJIS and Rp parameters. SBS results were not significant in Dry conditions in all groups (19.38 + 5.35–14.5 + 7.39) (p > 0.05). While for ML, roughness parameters showed mostly weak or negative correlations for SC and ZE (30 min, 1 h, and 2 h) under both Dry and TC conditions, for PV, a moderate to strong positive correlation was observed in SC and ZE (2h) after TC, with weaker or mixed trends in ZE (30 min) and ZE (1h). After aging, SBS decreased by 12–66 % across all groups, and failure types were more frequently mixed types of failures (50–100 %) for all conditions and luting agents. Etching with a nitric and hydrofluoric acid solution for 30 min can be considered an alternative to silica coating to improve adhesion to 4YSZ ceramic with the tested resin cements, provided that after aging, a decrease in adhesion was observed in all combinations.