Selective Infiltration Etching as a surface modification for high-translucency zirconia: Comparative analysis of roughness, composition, and bond strength

Study design

This comparative in vitro experimental study was conducted at the Department of Fixed Prosthodontics, Faculty of Dental Medicine, Damascus University, between October 2024 and March 2025. Ethical approval was obtained from the Ethics Committee at Damascus University (Approval No. 2023/3664).

The sample size was determined using G*Power 3.1.9, based on the study by Kwon et al.¹² (Effect size f = 1.575, α error probability = 0.05, power (1 − β error probability) = 0.95, number of groups = 3). Although thirty zirconium cubes were initially required, the study was conducted on thirty-six zirconium cubes to enhance accuracy.

Samples preparation

Thirty-six highly translucent cubic zirconia blocks (10 × 10 × 10 mm³) were used in this study. The specimens were ultrasonically cleaned in distilled water for 3 minutes, then sequentially polished with silicon carbide papers to achieve a smooth, glossy surface. All specimens were randomly allocated into three equal groups (n = 12), each subjected to a distinct surface treatment protocol.

Surface treatment protocols

Group 1 (Air-abrasion), specimens were surface-treated by blasting with 50 μm aluminum oxide particles articles at a pressure of 2.5 bar for 20 seconds, from a distance of 10 mm.

Group 2 (CoJet System) specimens were treated with 30 μm silica-coated alumina particles (CoJet, 3 M ESPE) under a pressure of 2.8 bar.

Group 3 (selective infiltration etching, SIE) specimens were coated with a laboratory-prepared paste consisting of 65% silica, 15% alumina, 10% sodium oxide, 5% potassium oxide, and 5% titanium oxide, mixed with distilled water. The coated specimens underwent thermal infiltration at 750 °C for 3 minutes, followed by cooling and etching with 9.5% hydrofluoric acid for 20 minutes.

The Ra value, measured in micrometres (μm), was used as the surface roughness indicator in this study, before and after surface treatment with a profilometer (TR200, Time Group, USA). Measurements were conducted at the Faculty of Mechanical and Electrical Engineering, University of Damascus. The stylus scanned a cutoff length of 0.8 mm three consecutive times, and the average Ra value was recorded for each specimen.

All zirconia blocks were bonded to standardized composite resin cylinders using a dual-cure MDP-containing resin cement (Theracem BISCO, USA) according to the manufacturer’s instructions. Cementation was performed under a constant load using a standardized metal device. Subsequently, all specimens were subjected to 5000 thermocycles between 5 °C and 55 °C with a dwell time of 30 seconds.

Microstructural analysis

To further evaluate of surface morphology, atomic force microscopy (AFM) measurements were performed in tapping mode under ambient conditions (26–28 °C). Silicon cantilevers (Tap190 Al-G, NanoSensors™, Neuchâtel, Switzerland) with 30-nm-thick aluminum reflex coating were used. According to the manufacturer’s datasheet, the cantilever spring constant ranged from 1.5–15 N/m, and the tip radius was less than 10 nm. The scan rate was set at 1 Hz, and the scanning size was 5 × 5 μm². Three-dimensional images were reconstructed using Nanosurf Digital Surf software (NanoSurf^®, Liestal, Switzerland), and the area roughness parameter (Sa) was recorded to assess the average surface roughness.

Two specimens from each group were examined using scanning electron microscopy (SEM) (VEGA II, XMMU, TESCAN, Czech Republic). SEM analysis was performed on the treated zirconia surfaces at accelerating voltage of 30 kV and a scan speed of 6–7 ms/pixel. Images were acquired using both backscattered electron (BSE) and secondary electron (SE) detectors at various magnifications to evaluate surface morphology. The same specimens were subsequently analyzed using energy-dispersive X-ray spectroscopy (EDX) to assess elemental composition.

Shear Bond Strength (SBS) testing was performed using a universal testing machine at a crosshead speed of 1 mm/min until failure occurred. The maximum load (N) at failure was recorded and divided by the bonding area to calculate bond strength in megapascals (MPa).

Failure modes were examined under SEM and categorized as adhesive failure (at the zirconia–cement interface), cohesive failure (within the resin cement), or mixed failure (combination of adhesive and cohesive failures).

Statistical analysis

Statistical analysis was conducted using SPSS software (version 21.0; IBM Corp., Armonk, NY, USA). Data were first assessed for normality using the Kolmogorov–Smirnov test. As the results indicated a normal distribution (p > 0.05), parametric tests were applied. Mean and standard deviation values were calculated for all variables. One-way analysis of variance (ANOVA) was used to compare mean shear bond strength (SBS) across groups. Tukey’s post hoc test was employed to evaluate pairwise differences in surface roughness and SBS values. A significance level of p < 0.05 was considered statistically significant.

1- Morphologic results

AFM was used to evaluate surface morphology, revealing distinct topographical variations among the treated zirconia surfaces. The sandblasted group exhibited irregular granular elevations interspersed with large, compact clusters. In contrast, the CoJet group showed nodular agglomerates with uneven spatial distribution. The SIE group demonstrated well-defined, sub-parallel ripple-like grooves and the greatest height variation, indicative of increased surface roughness. Following etching, the SIE specimens presented a uniformly textured surface characterized by dense, fine spherical granules and numerous micro- to nano-scale pores. Quantitative analysis confirmed that the SIE group exhibited the highest roughness parameters (Ra and Sa), aligning with profilometer measurements and underscoring the method’s effectiveness in producing a highly retentive surface suitable for durable resin bonding ( Figure 1).

Figure 1. Typical three-dimensional images of the zirconia surface after treatments.

(a) sandblasted group, (b) Cojet, (c) SIE and (d) SIE after etching.

Following the surface treatments, the SEM examination showed that sandblasted specimen exhibited an irregular topography characterized by deep grooves and a pitted morphology with sharp edges and angular peaks resulting from the impact of alumina particles. This roughened surface increased the available area for micromechanical interlocking; however, it also revealed localized microcracks and stress concentration zones that may compromise structural integrity. In contrast, CoJet-treated surfaces displayed isolated clusters of silica-rich nodules and “islands” adhered to the zirconia substrate.

These alumina deposits, coated with silica, imparted moderate surface reactivity and roughness, potentially enhancing silane coupling. Although their distribution across the surface was non-uniform. The specimens treated with SIE before etching exhibited a dense, glazed-like surface composed of molten, spherical glassy beads formed through thermal infiltration of the silica-alumina-alkali glass layer into grain boundaries. This process yielded a chemically modified, compact surface free from mechanical defects. Following hydrofluoric acid etching, the SIE-treated specimens developed a distinct honeycomb-like micro/nanoporous structure resulting from the selective dissolution of the infiltrated glassy phase. This topography featured an interconnected network of pores and channels, enhancing micromechanical retention and resin infiltration, thereby promoting stronger adhesion between the resin cement and zirconia ( Figure 2).

Figure 2. SEM images of zirconia surface after treatments (a) sandblasted, (b) CoJet, (c) SIE and (d) SIE after etching.

The grey arrow indicates sharp edges in group a, while it points to clusters of silica islands in group b. fused round beads were observed in group c, and group d appeared honeycomb –like due to the etching.

EDX analysis revealed pronounced compositional variations across the treated zirconia surfaces. In the sandblasted group, oxygen (50.14 wt%) and zirconium (40.78 wt%) were the predominant elements, accompanied by detectable traces of aluminum (7.59 wt%) originating from alumina abrasion particles. In the CoJet group, silica (Si: 2.5 wt%) was identified, and the aluminum signal decreased, indicating the deposition of SiO₂-coated alumina particles onto the zirconia surface. The SIE-treated samples exhibited a markedly different elemental profile characterized by elevated levels of silicon (39.69 wt%), sodium (8.77 wt%), and potassium (6.86 wt%), while zirconium content was significantly reduced (0.09 wt%). This suggests that the zirconia surface was entirely covered by a glassy infiltration layer. Following HF etching (group d), the glassy phase partially dissolved, resulting in a surface enriched with oxygen (52.6 wt%), fluorine (19.7 wt%), and sodium (14.39 wt%), while trace amounts of zirconium (0.17 wt%) remained Compared to the previous group, zirconium content decreased significantly, while silicon, sodium, and potassium increased after SIE treatment but declined following HF etching. The presence of fluorine in this group confirms a chemical interaction between HF and the glass phase. Collectively, these findings indicate that SIE treatment effectively modifies zirconia surface chemistry through glass infiltration and subsequent etching, producing a compositionally active surface that enhances the durability of resin bonding ( Figure 3).

Figure 3. EDX analysis showed considerable compositional variations in the treated zirconia surfaces.

SEM analysis revealed distinct failure modes among the experimental groups. In the CoJet and sandblasted groups, predominantly adhesive failures were observed, characterized by clean zirconia surfaces with minimal traces of resin. This result suggests weak interfacial bonding. In contrast, the specimens treated with SIE primarily exhibited cohesive failures, with fractures occurring within the resin cement and the zirconia surfaces fully covered by resin residues. This observation indicates a strong micromechanical and chemical bond. Additionally, some mixed failures were observed, marked by partial resin residues exhibiting intermediate adhesion strength between the two extremes. These findings align with the quantitative data, confirming that the SIE surface treatment achieved the most reliable and strongest bond between resin and when zirconia compared to conventional methods ( Figure 4).

Figure 4. SEM images of failure types at a magnification of 17×.

(a) Adhesive failure, (b) mixed failure, and (c) cohesive failure.

SEM micrographs revealed distinct morphological differences at the zirconia–resin interfaces across the surface treatment group. In the sandblasted specimens (a), which used 50 μm Al₂O₃ particles, the interface appeared relatively smooth, exhibiting few micro-retentive features and limited mechanical interlocking. A similar pattern was observed in the CoJet group (b), where only surface irregularities were present, offering minimal enhancement in surface roughness and resin penetration. In contrast, the samples treated with the SIE technique (c) exhibited longer micro- and nanoscale porosity on the zirconia surface, with clear resin infiltration within the three-dimensional interconnected nanopores. This indicated robust micromechanical interlocking and close interfacial contact. These observations underscore the superior adhesive performance of the SIE technique, as increased surface roughness facilitates both mechanical retention and chemical interaction with MDP-containing resin cements ( Figure 5).

Figure 5. SEM Image (1000X) of the zirconia/resin interface of samples treated with 50 μm particles (a), showing lack of surface irregularity and retention properties and this is similar to CoJet group (b), while SEM Image (2000X) of the zirconia/resin interface of SIE-treated samples demonstrates resin infiltration of the 3D nanopores (c).

2- Statistical results

Surface roughness: Among the study groups, Group 3 exhibited the highest surface roughness with a mean value of 1.53 μm, followed by Group 1 at 0.24 μm, and Group 2, which recorded the lowest value at 0.16 μm. Consistency was observed between the surface roughness values obtained using the TR200 and AFM methods ( Table 1). An ANOVA test was conducted to compare surface roughness among the groups (F = 496.096, p < 0.001). No statistically significant difference between Group 1 and Group 2 (p > 0.05). However, Group 3 differed significantly from both Group 1 and Group 2 (p < 0.001), as detailed in Table 2. Tukey’s post-hoc comparisons further confirmed significant differences in the average surface roughness across the groups ( Table 2).

Shear bond strength: In this study, SBS was highest for Group 3, at 19.36 MPa, followed by Group 1 at 7.32 MPa and Group 2 at 6.89 MPa ( Table 1). The ANOVA test revealed statistically significant differences among the groups (F = 2344, p < 0.001). Specifically, significant differences were observed between Group 3 and Group 2 (p < 0.001) and between Group 3 and Group 1 (p < 0.001). However, no statistically significant difference was observed between Group 2 and Group 1 (p > 0.05). Tukey’s post-hoc comparisons were conducted to identify significant differences in the average shear bond strength between the study groups ( Table 3).

Failure modes: The results indicated statistically significant differences in the distribution of failure modes after surface treatments, as assessed by the chi-square test. In the sandblasted group, adhesive failures were predominant, occurring in 83.3% of specimens, indicating the weakest interfacial bonding. Similarly, The CoJet group exhibited an adhesive failure rate of 66.7%. In contrast, the SIE group demonstrated a substantial improvement in bonding, with no instances of adhesive failure. Instead, this group exhibited failure modes of 75% cohesive and 25% mixed, reflecting a significantly stronger, more durable bond. Overall, these findings highlight the superior bonding performance achieved with the SIE surface treatment method. ( Table 4).