Micro-Chemical Changes and Micro-Tensile Bond Strength of Zirconia After Advanced Surface Treatments

Introduction

CLINICAL CHALLENGES IN ZIRCONIA-RESIN BONDING:

Surface treatments play a critical role in modifying the properties of dental materials, particularly in optimizing their bonding to resin cements [5]. Effective bonding between resin cements and zirconia ceramics remains a central challenge in prosthodontics. Despite advancements in materials science, the inherently inert surface chemistry of zirconia creates obstacles to achieving durable adhesion, often leading to restoration failures at the cementation interface. Overcoming this limitation is vital because clinical demand increasingly favors minimally invasive, long-lasting esthetic restorations. Improved bonding techniques not only extend the functional lifespan of restorations but also reduce the risk of debonding and mechanical failure.

In zirconia ceramics, surface treatments are commonly utilized to enhance bonding to resin cement, thus improving the performance and longevity of dental restorations [6]. The long-term success of these restorations heavily depends on the adhesion of luting cements to ceramic surfaces. Failures originating from cementation interfaces, whether at marginal or internal regions, underscore the importance of effective surface conditioning methods to reinforce these critical areas [7]. Previous studies have indicated that, even when impressions of implants [8,9] or natural abutments and other clinical procedures are performed correctly, most clinical failures arise from cementation or internal surfaces. Although surface treatments are widely applied in clinical practice, their influence on the micro-chemical composition of GLZR has not been sufficiently explored.

SURFACE TREATMENT MECHANISMS AND EFFECTS:

Understanding the micro-chemical changes induced by surface treatments is essential for efforts to tailor the properties of GLZR to meet specific clinical requirements. Various surface treatments, including airborne particle abrasion (sandblasting), acid etching, laser etching, and application of silane coupling agents, have been proposed to modify the surface characteristics of dental materials. Conditioning zirconia In-Ceram ceramic surfaces with silica coating and silanization, performed with either chairside or laboratory devices, has been shown to provide higher bond strength of resin cement relative to airborne particle abrasion using 110 μm Al2O3 [10]. These treatments can modify the topography, surface energy, and chemical composition of GLZR, thereby influencing its adhesion to resin cements, interaction with oral tissues, and overall performance in the oral environment [11].

Sandblasting, among the most commonly used surface treatments, involves projecting abrasive particles onto the material surface to generate micro-roughness and enhance mechanical interlocking with resin cement [12]. Silica coating constitutes an approach in which silicic acid-modified Al2O3 particles are blasted onto the ceramic surface. This process results in the embedding of silica particles, rendering the surface more chemically reactive to resin via silane coupling agents.

Acid etching utilizes acidic solutions to selectively dissolve the glass phase within GLZR. This process exposes zirconia grains, increases surface energy, and promotes improved bonding capacity [13]. Vila-Nova et al. evaluated the effects of 2 surface conditioning methods, hydrofluoric acid versus self-etching primer, along with the application of adhesive, on the bond strength of resin cement to computer-aided design/computer-aided manufacturing (CAD/CAM) glass ceramics. They reported that additional adhesive application after surface treatments did not improve bond strength [13]. Similarly, Şeker and Okutan assessed the effects of air abrasion, hydrofluoric acid, and self-etching primer (Monobond Etch & Prime) on the flexural strength of resin-based CAD/CAM materials. Their findings indicated that extended self-etching primer application may be the recommended surface treatment, whereas air abrasion can negatively affect mechanical properties [14].

Laser etching is a precise and controlled method of surface modification that enables the formation of micro- and nano-scale structures to promote adhesive bonding and cell attachment [15]. Silane coupling agents, although less frequently used with zirconia ceramics, have shown potential for improving the wetting properties of GLZR surfaces and enhancing bond strength to resin-based materials [16].

Although surface treatments offer several potential advantages, their effects on the micro-chemical composition of GLZR are not fully understood. To elucidate the underlying mechanisms and refine clinical procedures, it is important to thoroughly examine the elemental distribution, nature of chemical bonds, and surface texture after various treatments [17]. Advanced spectroscopic techniques, including energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, provide valuable insights into the chemical composition and structural modifications induced by surface treatments. Energy-dispersive X-ray analysis (EDAX), performed with scanning electron microscopy (SEM), is used to measure nanoparticles. In this method, nanoparticles are activated and analyzed with an EDS X-ray spectrophotometer, which is typically integrated into modern SEM systems [18,19].

Despite increasing interest in optimization of GLZR surface properties, a substantial knowledge gap persists regarding the micro-chemical changes induced by surface treatments. Whereas surface treatments are widely utilized to improve bonding between resin cements and zirconia ceramics, their effectiveness remains inconsistent. Bond strength can be substantially affected by factors such as treatment type, application method, and resulting chemical modifications, but standardized guidelines have not been established. Furthermore, conflicting reports regarding the efficacy of treatments such as hydrofluoric acid etching, silica coating, and airborne particle abrasion underscore the need for a better understanding of their micro-chemical effects. To clarify the mechanisms involved and to predict the long-term durability and effectiveness of GLZR restorations, detailed investigation of these micro-chemical changes is essential [4].

STUDY RATIONALE AND OBJECTIVES:

The present study was undertaken to address this knowledge gap by investigating the micro-chemical changes in GLZR (Vita Zahnfabrik, Bad Saeckingen, Germany) surfaces subjected to various surface treatments. By systematically analyzing the elemental composition, chemical bonding, and surface characteristics of treated GLZR specimens, the study aimed to determine the effects of laboratory grit-blasting, laboratory silica coating, and hydrofluoric acid etching on the microstructural and chemical properties of GLZR. The study included assessing the influence of these conditioning methods on the micro-tensile bond strength (MTBS) of resin cement (3M/ESPE, St. Paul, MN, USA) to GLZR ceramics, as well as analyzing surface changes in topography, roughness, and elemental properties using EDAX. This investigation sought to link surface treatments to elemental composition and bonding performance. By elucidating the micro-chemical changes induced through common conditioning methods, the findings contribute to materials science while also providing clinicians with practical strategies to enhance restoration longevity. This focused approach has the potential to inform future protocols and improve clinical outcomes. The findings may support the development of optimized surface treatment protocols for GLZR in dental applications, ultimately enhancing the performance and durability of GLZR restorations. The null hypothesis was that no difference would be observed in the elemental properties of the conditioned surfaces and no change would occur in the bond strength of resin cement to GLZR ceramics.

Material and Methods

SAMPLE FABRICATION:

Thirty blocks of zirconia In-Ceram (Vita Zahnfabrik) were milled in accordance with the manufacturer’s instructions. Each block was then duplicated in composite resin (3M/ESPE) (Figure 1). A total sample size of 30 was selected based on the work of Amaral et al [10]. Accordingly, 30 zirconia blocks and 30 composite resin blocks were fabricated.

FABRICATION OF ZIRCONIA BLOCK SAMPLES:

The block design was created using Exocad software [DentalCAD software, version 2.4, exocad GmbH, Plovdiv (https://dentona.de/en/optimill/software/cad-software-exocad/40763)]. The block dimensions were standardized at 5×5×4 mm, in accordance with recommendations by Amaral et al [10]. The STL file of the block was transferred to CAD/CAM software [3Shape A/S, Holmens Kanal 7, Copenhagen, Denmark, Splint Studio 2020.3 (https://www.3shape.com/en/software/splint-studio)] for the design and fabrication of zirconia blocks using a 5-axis milling machine (Ceramill Matik, Amann Girrbach, Koblach, Austria). The milled zirconia specimens were polished and cleaned in an ultrasonic bath before air-drying. Preparation included the use of silicon carbide abrasives (3M), a polishing machine (Labpol 8–12, Extec, Enfield, CT, USA), and an ultrasonic bath (Sidilu Ultrasonics, Bengaluru, India).

FABRICATION OF COMPOSITE RESIN BLOCK SAMPLES:

Composite resin blocks were fabricated by duplicating zirconia blocks using silicon putty impression material (Express, 3M/ESPE). For duplication, a circular plastic cup was used, leaving 1–2 cm of space around the zirconia blocks when positioned at the center. Impressions of the zirconia blocks were made in the putty-filled cups. After complete setting, the zirconia blocks were retrieved; composite resin material was incrementally layered and condensed into the mold to ensure complete filling. Each layer was polymerized with a light-curing unit (SELECTOR LA500, Apoza Enterprise Co.). Each zirconia block had a corresponding composite resin block (Figure 2).

GROUP DIVISION AND SURFACE TREATMENT OF ZIRCONIA SAMPLES:

Each zirconia block was subjected to 1 of 3 predetermined surface treatments: airborne particle abrasion with Al2O3, silica coating, or hydrofluoric acid etching (Figure 3). The 30 specimens were randomly divided into 3 groups (n=10 per group) according to the treatment applied.

Group I – Laboratory Grit-Blasting: Airborne particle abrasion was conducted using an air-abrasive device (3M ESPE AG, Seefeld, Germany) with 110 μm Al₂O₃ particles at a pressure of 2.8 bar for 20 s in circular movements.

Group II – Laboratory Silica Coating: A laboratory air-abrasion device (3M ESPE AG) was used. Specimens were first abraded at 2.8 bar with 110 μm Al₂O₃ particles. They were then abraded with 110 μm SiO₂ particles at 2.8 bar, at a distance of approximately 10 mm, in circular motions for 20 s.

Group III – Hydrofluoric Acid Etching: Ceramic samples were etched for 90 s with 9.5% hydrofluoric acid gel (Ultradent), in accordance with the manufacturer’s instructions. The procedure was performed under proper ventilation using acid-resistant protective goggles and gloves. After etching, the gel was rinsed into a polyethylene cup, and the diluted solution was neutralized for 5 min with calcium carbonate (CaCO₃) and sodium carbonate (Na₂CO₃) powder. The substrates were thoroughly rinsed and washed to remove residual acid, then subjected to air-drying.

All groups were subsequently treated with silane and adhesive; they were bonded to composite blocks using resin cement. No separate untreated control group was included, given that comparative evaluation across the 3 clinically relevant surface treatments formed the core of the experimental design.

BONDING PROCEDURE:

After conditioning procedures, sand particles were gently air-blown from the surfaces; a silane coupling agent (3M ESPE AG) was applied and allowed to evaporate for 5 min. All surface-treated discs were coated with silane coupling agent (Silano, Angelus, Londrina, Brazil) using a microbrush and then gently air-dried. Two coats of adhesive (Single Bond Universal Adhesive, 3M/ESPE) were applied, gently agitated, and dried with a stream of air to allow solvent evaporation. Following the manufacturer’s guidelines, the adhesive was light-cured for 10 s.

Zirconia and composite blocks were joined with a metallic clamp after application of resin cement (3M RelyX) to maintain uniform pressure. After cementation of the ceramic and composite blocks with resin cement, the samples were stored for 7 days at 37°C in distilled water before MTBS testing. This combined block, consisting of ceramic, resin cement, composite, and adhesive extending from ceramic to composite, is represented in Figure 4.

SAMPLE PREPARATION FOR TESTING:

For MTBS testing, cyanoacrylate was used to fix the assembled blocks to a metal base attached to a cutting device. Slices were made with a slow-speed diamond wheel saw under running cold water (Figure 5). Marginal slices were excluded when the amount of resin cement at the interface was either excessive or insufficient, which could have affected the outcome. Initially, 3 slices were obtained from each block. The slices were then rotated 90° and reattached to the metal foundation. Marginal bar specimens were also excluded for the same reasons. The blocks were subsequently sectioned into bar specimens with a bonding area of approximately 0.6 mm2.

After sectioning, the samples were gently air-dried, ultrasonically cleaned for 30 min in a deionized water bath, and rinsed with tap water for 1 min. Six additional zirconia block samples were fabricated in a similar manner for micro-chemical analysis.

ASSESSMENT OF MICRO-CHEMICAL CHANGES (SURFACE MORPHOLOGY, SURFACE ROUGHNESS, SURFACE ELEMENTAL COMPOSITION ANALYSIS):

Micro-chemical changes in GLZR surfaces were assessed by evaluating alterations in surface morphology, surface roughness, and surface elemental composition after surface treatment procedures. To characterize these changes, advanced analytical methods were utilized.

Surface microstructure analysis was performed using via SEM. Two discs were randomly selected from each group for microstructural examination. Each sample was sputter-coated with a 10-nm-thick layer of gold. In all 3 groups, surface analysis was conducted using a scanning electron microscope (Zeiss EVO MA 18; Carl Zeiss, Jena, Germany) operated at 15 kV with ×2000 magnification.

EDAX (EDS X-act, Oxford Instruments, Abingdon, UK) was used in conjunction with SEM to evaluate the elemental composition of discs subjected to various surface treatments. At high magnifications, SEM was also used to examine the surface morphology and topography of conditioned zirconia specimens.

Quantitative analysis of the elemental composition of zirconia specimen surfaces was carried out by EDS, which provided essential information regarding the chemical modifications induced by surface treatments.

ASSESSMENT OF MICRO-TENSILE BOND STRENGTH: The MTBS test was performed using a universal testing machine (LR 50K: Lloyd Instrument Ltd., Bognor Regis, UK). The bonded zirconia and composite resin blocks were embedded in clear acrylic resin blocks to secure them to the testing machine. For identification, blocks were color-coded according to group by embedding them in acrylic blocks of different colors: Group I samples in pink acrylic, Group II samples in green acrylic, and Group III samples in white acrylic (Figure 5).

The bonded zirconia–resin cement–composite resin blocks had 3 zones: (1) zirconia–resin cement, (2) resin cement, and (3) resin cement–composite resin. For MTBS evaluation in the Lloyd universal testing machine, specimens were positioned horizontally in the lower component of the shear chuck. The inner surface of the chuck was designed to hold the specimens securely once positioned and tightened. A shearing chisel with a knife edge was used to de-bond the prepared specimens. The chisel was first fixed to the upper component of the shear chuck. At the zirconia–resin cement interface (Figure 3), the cutting edge of the chisel was engaged, and force was applied perpendicular to the specimen’s long axis. The maximum force required to de-bond the specimens was measured in Newtons. The apparatus was operated at a crosshead speed of 1 min−1. Shear bond strength was calculated in megapascals through division of the maximum load in Newtons by the cross-sectional area of the bonded interface in square millimeters (Figure 6).

To minimize the influence of potential confounding variables, several measures were implemented during study design and execution. All zirconia blocks were fabricated from the same commercial batch and milled under identical CAD/CAM settings to ensure material uniformity. Experimental procedures, including surface treatments, bonding, and testing, were conducted under controlled laboratory conditions at a consistent room temperature. Materials were handled in accordance with the manufacturers’ instructions, and standardized protocols were followed throughout to minimize variability. Composite resin blocks were duplicated from a standardized mold to maintain uniform dimensions and surface contact. Surface treatments were applied with calibrated equipment under fixed parameters (particle size, pressure, duration, and distance) to standardize treatment intensity. Equipment such as the air-abrasion unit, curing light, and universal testing machine was calibrated prior to experimentation. A single operator performed all procedures – including surface treatment, silane application, bonding, and slicing – to eliminate inter-operator variability. Environmental conditions, including temperature and humidity, were maintained uniformly; all specimens were stored under identical conditions before testing. These measures collectively reduced variability and enhanced the reliability of comparisons between treatment groups.

STATISTICAL ANALYSIS:

Data obtained from the study were analyzed; comparisons were conducted within and between Groups I, II, and III. Mean bond strength, standard deviation, coefficient of variation, and significance levels were calculated for shear bond strength within and between the groups. Analysis of variance (ANOVA) was performed to determine the significance of study parameters, followed by pairwise comparisons using Tukey’s multiple post hoc test. P-values < 0.05 were considered statistically significant. All analyses were performed using SPSS (Statistical Package for the Social Sciences), version 20.0 (IBM Corp., Armonk, NY, USA) (https://www.ibm.com/products/spss-statistics).

Results

Part I

ASSESSMENT OF MICRO-CHEMICAL CHANGES (SURFACE MORPHOLOGY, SURFACE ROUGHNESS, SURFACE ELEMENTAL COMPOSITION ANALYSIS): Analyses of Surface Morphology and Surface Roughness: Table 2 presents the results of surface morphology analyses, indicating the average surface roughness (Ra) values for GLZR specimens subjected to various surface treatments. The mean surface roughness values were highest for laboratory grit-blasting with Al2O3 compared with other groups. Laboratory silica coating resulted in a noticeable decrease in surface roughness, whereas chemical etching with concentrated hydrofluoric acid produced the most pronounced reduction in Ra.

SEM examination at 15 kV and ×2000 magnification revealed that sand particles penetrated the ceramic surfaces and remained embedded; abundant particles were present even after air blowing. High craters were observed in samples treated with laboratory grit-blasting, followed by those treated with laboratory silica coating and chemical etching (Figure 7A–7C). The morphologically roughened surfaces of sandblasted specimens exhibited irregular craters of varying sizes and shapes. In contrast, specimens etched with hydrofluoric acid displayed irregular gaps and micropores in addition to altered surface texture. The roughness parameter was calculated from SEM images using proprietary software (Topography Package; Zeiss) [20,21], which conforms to relevant standards.

Elemental Composition Analysis: Figure 8 presents the results of elemental composition analysis obtained from EDAX measurements. The table lists the weight percentages of key elements present on the surfaces of GLZR specimens after various surface treatments. Laboratory grit-blasting primarily resulted in a reduction in oxygen content, and zirconia weight percentage was comparatively lower than in the other groups. Silica was detected only in the laboratory silica coating group, whereas hydrofluoric acid etching produced high oxygen weight percentages and showed complete absence of aluminum.

Overall, these findings provide comprehensive insights into the micro-chemical changes induced by various surface treatments on GLZR surfaces, contributing valuable information to optimize the clinical performance and longevity of GLZR restorations.

ASSESSMENT OF MICRO-TENSILE BOND STRENGTH:

Thirty blocks of zirconia In-Ceram were fabricated and duplicated in resin composite; each ceramic block was assigned to 1 of 3 treatment conditions (n=10). After conditioning, the blocks were coated with a silane coupling agent and cemented using resin cement.

The dataset was checked for normality. According to the Levene and Shapiro-Wilk tests, P-values were not statistically significant, indicating normally distributed data with homogeneous variance. The mean bond strength values with standard deviations for each group are presented in Figure 9. Group II (laboratory silica coating) exhibited the highest mean bond strength (28.23±1.53 MPa), followed by Group I (laboratory grit-blasting) with a mean bond strength of 20.27±2.33 MPa. Group III (hydrofluoric acid etching) displayed the lowest mean bond strength (10.41±1.46 MPa). The coefficient of variation was highest for Group III and lowest for Group II (Table 3). One-way ANOVA revealed a statistically significant difference among the groups (P<0.05), and intergroup comparisons also showed significant differences (P<0.05) (Table 3). Pairwise comparisons of mean bond strength across Groups I, II, and III using Tukey’s multiple post hoc test indicated significant differences among all pairs (Group I vs Group II, Group II vs Group III, and Group III vs Group I) (Table 4).

The results presented in both parts of the study directly address the central research question and the null hypothesis. Micro-chemical analysis confirmed that different surface treatments produced distinct morphological and compositional changes in GLZR. Surface morphology and roughness were highest in specimens subjected to airborne particle abrasion, whereas elemental analysis revealed unique silica deposition only in the laboratory silica coating group. These findings were linked to bond strength outcomes – silica-coated specimens exhibited the highest MTBS values – confirming that enhanced micro-retention and chemical bonding improve adhesion. By emphasizing statistically significant intergroup differences and omitting redundant metrics, this research highlights the clinical implications of selecting optimal surface treatments for zirconia restorations.

Discussion

COMPARATIVE ANALYSIS OF SURFACE TREATMENT METHODS:

In the present study, 3 surface conditioning methods commonly used for GLZR – laboratory grit-blasting, laboratory silica coating, and hydrofluoric acid etching – were assessed. Previous studies evaluated these treatments individually; however, in the present work, all 3 methods were compared, and in addition to MTBS, surface topography, roughness, and elemental composition were analyzed. Mechanical bonding relies on surface roughness to promote micro-retentive interlocking. High-velocity particles create micro-craters and irregularities on the zirconia surface, improving resin penetration and physical anchorage. SEM analysis confirmed the presence of deep crater formations that supported micro-mechanical interlocking. Chemical bonding occurs through molecular interactions between functional groups on the conditioned ceramic surface and the resin cement. When followed by silane application, a siloxane network forms through hydrolysis and condensation reactions between silane methoxy groups and surface hydroxyls [5,22,23].

In grit-blasting, abrasive particles roughen the ceramic surface, generating micro-retentions that enhance mechanical interlocking between cement and ceramic. Previous studies demonstrated that grit-blasting significantly increases bond strength, particularly when combined with silane coupling agents [1,2,4]. Sandblasting, or airborne particle abrasion, is frequently used for this purpose. By projecting abrasive particles at high velocity, the technique produces micro-roughness on the ceramic surface. This micro-roughness increases the surface area, facilitating improved interlocking between cement and ceramic and thus strengthening the mechanical bond. Nevertheless, airborne particle abrasion may introduce impurities and alter the ceramic’s composition, potentially reducing long-term bond durability [13,15,16]. In zirconia, studies have shown that airborne particle abrasion with Al2O3 particles of varying sizes, including 110 μm, effectively increases surface roughness and enhances bond strength with resin cements [22,24,25]. Ruales-Carrera et al [24] reported that airborne particle abrasion is instrumental in removing contaminants and creating a roughened surface, which provides sufficient surface area and energy for effective bonding. This result was supported by the findings of Moon et al [26], who demonstrated that both smaller (50 μm) and larger (110 μm) particles can improve the shear bond strength of zirconia surfaces. Similarly, Sulaiman et al [22] confirmed that particle sizes of 50 and 110 μm both increased shear bond strength, indicating that larger particles enhance adhesion.

MECHANISMS OF ENHANCED BONDING WITH ZIRCONIA:

The pressure applied during the grit-blasting process is a critical factor. Evidence indicates that pressures around 2 bar are effective for achieving optimal surface modifications without compromising the integrity of the zirconia substrate [27]. Le et al [27] reported that sandblasting with 110 μm Al2O3 particles at 2 bar significantly improved surface roughness, which is essential to enhance mechanical interlocking between zirconia and adhesive materials. Although pressures of 2.4 to 2.8 bar are commonly used, bond strength outcomes may vary based on specific parameters of the grit-blasting process [28]. In conclusion, the use of airborne particle abrasion with 110 μm Al2O3 particles at 2 bar for 20 s in circular movements has extensive literature support as an effective method for enhancing zirconia surface properties. This technique improves surface roughness and facilitates stronger adhesion of resin cements, contributing to the longevity and performance of dental restorations [27,28]. The implementation of circular motions during laboratory grit-blasting is considered an effective strategy to ensure uniform surface treatment and to increase the efficiency of the abrasive process. This approach promotes more even distribution of abrasive particles across the treated surface, which is key to achieving the desired surface roughness and enhancing adhesion characteristics. Such considerations are particularly important for zirconia, where preservation of substrate integrity is essential for optimal bonding with resin cements [29,30]. Studies have demonstrated that uniform surface treatment significantly enhances mechanical interlocking between the substrate and adhesive, thereby improving bond strength [29]. Additionally, circular motion facilitates more effective cleaning by enabling abrasive particles to interact with the surface from multiple angles. This multidirectional action aids in the removal of contaminants and debris, which is critical for achieving a clean substrate prior to bonding [31].

Similar findings were observed in the present study. SEM analysis of surface topography in samples treated with laboratory grit-blasting and laboratory silica coating revealed the presence of residual sand particles on the surfaces. Laboratory silica coating, another technique for surface conditioning assessed in this study, involves silanization after the application of a silica-based substance to the ceramic surface. The silane coupling agent uses the silica layer as a chemical bonding substrate, improving the chemical interaction between cement and ceramic surfaces. This technique has been shown to effectively increase bond strength, particularly in high-strength ceramics such as zirconia [5,6,10,32–34].

Recent studies have reinforced the importance of comprehensive surface characterization for optimizing zirconia–resin bonding. Pereira et al [35] evaluated the effects of various surface treatments, including airborne particle abrasion and silica coating, on translucent zirconia using both roughness and chemical analyses. They reported that silica-coated zirconia demonstrated superior surface chemistry favorable for silane interaction, strongly linked to higher bond strength values – an observation consistent with the present findings. Their application of multiple characterization techniques, including profilometry and EDS, supports the integrated approach adopted in the present study and affirms the importance of combining mechanical and chemical treatment modalities for optimal bonding performance.

In the current investigation, hydrofluoric acid etching was also assessed as a surface conditioning technique. Hydrofluoric acid (9.5%) dissolves the glassy matrix of ceramics in a targeted manner, roughening the surface and increasing surface area for bonding. However, hydrofluoric acid etching has several weaknesses, including potential cytotoxicity and the requirement for careful handling due to its corrosive nature [6,10,11].

Among the surface conditioning techniques examined in this study, silica coating followed by silanization produced the strongest bond. This result aligns with previous studies showing that silica coating enhances the connection between ceramics and resin cement. Strong and durable bonds are achieved when silane coupling agents act on silica-coated surfaces, which provide a stable and chemically reactive substrate [4–6,10–12]. In the present study, silica coating yielded higher bond strengths than hydrofluoric acid etching or grit-blasting. Although grit-blasting enhances mechanical interlocking, silica coating may achieve a greater degree of chemical bonding. In contrast, surface roughening via hydrofluoric acid etching may result in less effective bonding due to factors such as surface contamination and incomplete silanization [13,15,16].

Air abrasion is commonly performed with Al2O3 particles ranging from 25 to 250 μm in size to create undercuts or rough surfaces that promote strong adhesion of resin cement. Subasi and Inan evaluated the effects of various surface conditioning methods on the roughness of ZrO2; they found that all tested methods increased Ra values compared with untreated surfaces. They also concluded that air abrasion was the most effective conditioning method. Another study also indicated that the air abrasion group exhibited the highest surface roughness values. Previous investigations assessing the effects of surface conditioning on ZrO2 have consistently shown that sandblasting improves surface roughness values [36]. Furthermore, Casucci et al [37] reported that sandblasting significantly altered roughness compared with untreated ceramic surfaces (7.31 Ra vs 7.27 Ra, respectively).

These results were aligned with the findings of the present study, where zirconia surface roughness increased and was highest among all groups when treated with laboratory grit-blasting. GLZR ceramics are fused at high temperatures to form a ceramic composite. Strong covalent interactions with hydroxyl groups at the ceramic surface bind the chemical components of the ceramics (traces of Li2O, Na2O, K2O, CaO, and MgO) to each other [38]. Application of a silane coupling agent increases the number of hydroxyl groups on the surface, enhancing micro-mechanical retention after air abrasion. Additionally, the methoxy groups in silane interact with water to form silanol groups, which react with hydroxyl groups on the surface to form a siloxane network. Furthermore, amphoteric alumina in the ceramic matrix can chemically bond with hydrolyzed silanol groups via covalent bridges established by surface hydroxyl groups. This mechanism substantially improves the bond strength between resin cement and zirconia. The present study also demonstrated this mechanism: the mean MTBS for Group II, which utilized laboratory grit-blasting, was 20.27 MPa, surpassing the 10.41 MPa observed in Group III. These findings are consistent with the results of Ozcan and Vallittu [39], although their study utilized a different experimental design involving a bisphenol A-glycidyl methacrylate-based resin cement and a shear bond test.

Treatment with 110 μm SiO₂ at 2.8 bar produced statistically significantly higher mean MTBS values relative to Group I, which underwent laboratory grit-blasting with 110 μm Al₂O₃ particles. The highest mean bond strength (28.23 MPa) was reported in Group II with laboratory silica coating. This outcome can be attributed to the fact that air abrasion increased surface roughness, whereas subsequent silica particle coating further enhanced bond strength.

The vitreous phases of GLZR ceramics demonstrated good silica particle adherence, as confirmed by the present study and several prior investigations [40,41]. In a similar previous study, samples blasted only with Rocatec-Pre (Al2O3 particles) were compared with samples that had been blasted with Rocatec-Plus (SiO2). A considerable increase in silica content (15.8–19.7 wt%) was observed on the surface of In-Ceram ceramics, indicating that a higher silica concentration and its interaction with silane agents enhance the bond strength between resin cement and In-Ceram ceramics.

The lowest MTBS values were recorded in Group III specimens. Hydrofluoric acid dissolves the glassy matrix of ceramics and is frequently used for etching [23]. However, due to the low concentration of silica-based phases in zirconia, acid application has limited effectiveness and is not recommended for etching of high-strength zirconia cores. It also failed to increase average surface roughness or induce morphological changes [42]. Similarly, Papadopoulos et al. reported that hydrofluoric acid produced only small gaps and micropores; it had minimal effect on dissolving the glassy phase of the material surface [43]. The present results differ from those of Sales et al [44], who found that etching alone could improve bonding between opaque zirconia and resin cement without requiring prior air abrasion. This discrepancy may be attributed to differences in zirconia type and the use of a nitric acid–hydrofluoric acid solution for surface treatment. In such mixtures, nitric acid acts as an oxidizing agent that modifies zirconia surface chemistry by removing impurities and promoting oxide layer formation, whereas hydrofluoric acid selectively dissolves surface grains to create micromechanical retention. The combined chemical and morphological alterations achieved by the mixed solution therefore differ from those produced by hydrofluoric acid etching alone.

To further improve bond strength, chemical primers and adhesion boosters are often utilized in combination with surface conditioning techniques. These products typically contain silane coupling agents or functional monomers that form chemical bonds with both the resin cement and ceramic surface, promoting adhesion [45–47]. However, several factors – including surface roughness, application technique, and ceramic composition – can influence the effectiveness of such primers. The MTBS findings of the present study are supported by surface morphology analysis, surface roughness evaluation, and elemental composition analysis of the treated samples. Ra is the parameter most commonly used to quantitatively describe surface roughness [48,49] because it provides practical and interpretable values. The Ra values followed the order Group I >Group II >Group III, corresponding to 9.34 >7.79 >6.19, respectively.

Examination of SEM images showed that hydrofluoric acid did not induce sufficient surface modification in the ceramic specimens. Groups I and II exhibited significantly rougher zirconia surfaces compared with Group III. Although SEM data indicated increased roughness in all groups after conditioning, pits formed by dissolution of surface particles in Group III were smaller than those produced by laboratory grit-blasting and laboratory silica coating. The present findings were consistent with studies conducted by Sismanoglu et al. and Chuenjit et al [50,51]. Ansari et al [52] investigated the effects of Zircos-E Etchant on 2 types of zirconia. Samples were categorized into 4 zirconia groups: unetched anterior, etched anterior, unetched posterior, and etched posterior. Transparent zirconia was classified as anterior zirconia. The study identified stronger bond strength in etched groups than in unetched groups; statistical significance was observed only in the anterior zirconia group. All specimens were air-abraded before this division, leaving the direct effect of etching on tensile bond strength unclear. Elemental analysis in the present study revealed that the high silica content in Group II enhanced bonding between resin and zirconia when a silane coupling agent was applied. In contrast, the high zirconia content in Group III reflected reduced surface roughness resulting from hydrofluoric acid treatment; the absence of silica and aluminum negatively affected bonding. These observations were consistent with prior research examining the effects of surface treatments on zirconia characteristics and micro-shear bond strength.

STUDY LIMITATIONS AND STRENGTHS:

It is essential to acknowledge the limitations of the present research. Although its in vitro design provides valuable information regarding material performance, it cannot fully replicate the complex oral environment. Thus, further investigations are recommended to evaluate these findings in clinical settings. Bond strength between resin cement and dental ceramics can also be influenced by parameters beyond surface conditioning techniques. These include the resin cement composition, ceramic material type, bondable surface area, and curing protocol used. Achieving consistent and durable bond strength in clinical practice requires a clear understanding of how these factors interact. In the present study, roughness levels for each group were assessed using only 2 specimens. The analysis of a larger number of samples would provide a more reliable understanding of the relationship between tensile bond strength and roughness. A deeper understanding of the effects of zirconia surface treatment on bond strength could also be achieved through investigations that vary etching time, etching solution temperature, cement type, air-abrasion particle size, thermocycling, and the use of zirconia primers. It is also recommended that future studies compare these outcomes with standard treatment protocols for zirconia surface conditioning, including primer application, tribochemical silica coating, Er: YAG laser treatment, air abrasion involving Al₂O₃ particles, and silica nanofilm deposition.

The present study contributes to the evolving understanding of zirconia–resin bonding by integrating micro-chemical surface analysis with MTBS testing – an approach that few studies have fully utilized. The significantly higher bond strength observed with silica coating is consistent with the findings of Pereira et al. and others, reinforcing the role of chemical interaction via silane coupling agents on silica-enriched surfaces. In contrast, the results diverge from earlier reports advocating hydrofluoric acid etching, likely due to the limited glassy phase in GLZR ceramics. Although this divergence underscores the specificity of material-dependent treatment responses, it also highlights a critical limitation—treatment variability and challenges in clinical translation. Future research should focus on developing standardized protocols and conducting broader clinical simulations to bridge in vitro evidence with clinical applicability. These insights emphasize the practical importance of the present findings in guiding adhesive strategies for high-strength ceramics, particularly in enhancing restoration longevity.

CLINICAL IMPLICATIONS AND FUTURE DIRECTIONS:

The findings of the present study hold substantial clinical relevance for practitioners, underscoring the importance of selecting appropriate surface conditioning techniques based on the specific characteristics of ceramic materials and the clinical context of treatment. The combination of silanization and silica coating appears to be an effective strategy for establishing strong and durable bonds in GLZR ceramics. Additionally, surface conditioning methods influence therapeutic outcomes beyond bond strength. Factors such as marginal integrity, color stability, and the long-term success of dental restorations are affected by variations in conditioning techniques. Therefore, it is essential that practitioners consider the individual requirements of each patient when selecting surface conditioning approaches. These methods remain integral to modern adhesive dentistry.

Surface conditioning innovation continues to be driven by advances in materials science and technology. Emerging techniques such as nano-coatings, bioactive ceramics, and smart adhesives have the potential to further enhance bond strength and extend the lifespan of dental restorations. However, comprehensive analyses via rigorous research and clinical trials is necessary to determine their long-term performance and clinical utility. The present findings imply that modern conditioning techniques can improve the longevity and performance of high-strength ceramic restorations. By optimizing adhesion, clinicians can achieve more reliable and durable outcomes, thus increasing patient satisfaction and reducing the need for costly replacements.

Conclusions

This in vitro study demonstrated that surface treatment significantly affects the MTBS between resin cement and GLZR. Among the 3 techniques evaluated, laboratory silica coating followed by silane application produced the highest bond strength, significantly outperforming grit-blasting and hydrofluoric acid etching. These results were supported by corresponding surface roughness and elemental composition findings obtained through SEM and EDAX analyses. The data confirm the superior efficacy of silica-based conditioning in promoting durable chemical adhesion to GLZR. Given the in vitro design of this study, clinical extrapolation should be approached with caution. In vivo studies are recommended to validate the present results across diverse clinical conditions. These insights provide practical guidance for optimizing adhesive protocols in zirconia-based restorations, ultimately improving their longevity and reliability. Continued refinement of surface conditioning methods will further enhance the quality and durability of dental restorations, benefitting both patients and practitioners.