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Enhancing PEEK bond strength: the impact of chemical and mechanical surface modifications on surface characteristics and phase transformation

BMC Oral Health volume 25, Article number: 511 (2025) Cite this article

Abstract

Objectives

This study investigated the effects of different surface modifications on surface roughness, phase transformation, and bond strength of polyetheretherketone (PEEK) to composite resin.

Materials and Methods

A total of 140 specimens measuring 10 mm × 10 mm × 2 mm were fabricated from PEEK blocks (BioHPP; Bredent GmbH & Co KG, Germany). Seven groups were randomly formed from the specimens: C) untreated control group, (SE) sulfuric acid application (Merck KGa, Darmstadt, Germany), (PE) piranha solution application (Albar Chemistry, Kocaeli, Turkey), (S) sandblasting with Al2O3 particle (Zhermack, Rovigo, Italy), (T) tribochemical silica coating (3 M ESPE, Seefeld, Germany), (L) Er-YAG laser treatment (LightWalker AT, Fotona, Slovenia), and (P) plasma treatment (Kinpen, Neoplas, Germany) (n = 20). Surface topography was examined using a profilometer device (Taylor Hobson, Leicester, England). Surface images of the specimens were captured using Scanning Electron Microscopy (SEM) (JSM- 6610, Jeol, USA). Phase change analysis was conducted on each group using an X-Ray Diffractometer device (Rigaku SmartLab Diffraktometer, Tokyo, Japan). After thermal aging, shear bond strength was tested using a Universal Test Machine (Model 3340, Instron Corp., Wycombe, UK). Data were analyzed with One-way analysis of variance (ANOVA) test; comparisons were made with Tukey's multiple comparison test, with a significance level of 0.05.

Results

Group S (3.09 ± 0.40 μm) exhibited significantly higher surface roughness values compared to the other groups (p < 0.05). Group SE (13.28 ± 1.69 MPa) exhibited significantly higher shear bond strength values than the other groups (p < 0.05). No statistically significant correlation was found between surface roughness and bond strength data (p > 0.05). According to XRD results, Group S and Group L differ from the other groups in having a slightly higher intensity and broader peaks.

Conclusions

All surface modification methods enhanced the bond strength between PEEK to composite resin. However, Group L and Group S revealed different XRD patterns from the control group. Tribochemical silica coating can be a reliable alternative method for acid applications due to its stable phase structure and high bond strength.

Peer Review reports

Introduction

Polyetheretherketone (PEEK), a member of the polyaryletherketone family, is one such polymer [1,2,3,4,5]. PEEK is a semi-crystalline thermoplastic polymer and has been widely used as a bone graft in medicine, especially in orthopedic and cranio-maxillofacial surgery. It is preferred in dentistry for dental implants, abutments, fixed crowns, and bridges due to its excellent biocompatibility, lightweight and corrosion resistance [6,7,8]. Nonetheless, PEEK’s low surface energy and chemically resistant structure create difficulties in bonding composite resin to its surfaces [2, 3]. This bonding challenge remains a significant hurdle in the clinical application of PEEK [4, 9].

There are two methods to achieve an improved bonding performance of PEEK: modifying the surface topography and conditioning it with an adhesive system to enable chemical interactions [8]. Researchers have employed various pretreatment methods to examine the bond strength between resin and PEEK materials. These techniques include sandblasting [4, 10,11,12], tribochemical bonding [11, 13, 14], acid etching [13], diverse plasma systems [3, 15], and laser etching [16,17,18,19]. Acid etching is commonly recommended for altering the PEEK surface due to its chemically resistant properties [16, 17, 20]. Sandblasting is a method that enhances micromechanical interlocking, increases the surface roughness, and creates an active surface layer for bonding [4, 10,11,12]. Plasma treatment, an innovative approach for PEEK surfaces, enhances surface energy by conferring hydrophilic properties to the material [2, 21]. Studies have shown that spherulite size, processing temperature, and PEEK molecular weight significantly affect aging, failure properties, and fracture mechanisms in compact tension tests [3, 22]. With tribochemical silica coating, micromechanical adhesion is achieved with Al2O3 while chemical bonding occurs with silica tribochemical bonding [11, 13, 14]. Although it is known that micro-retentive areas enhancing adhesion [5, 20, 23, 24], other studies have indicated that chemical interactions with adhesive systems are equally crucial for ensuring effective chemical bonding between PEEK and composite resin [3, 25].

Enhancing PEEK surfaces to improve resin bonding is currently a major research focus, as the luting process is vital for the clinical success of fixed dental prostheses [8, 26]. However, the effect of the applied surface treatments on the phase change on PEEK material was not investigated. PEEK is a semi-crystalline thermoplastic characterized by elevated transition temperatures, namely a glass transition temperature of around 148 °C and a melting temperature of around 340 °C [27]. PEEK crystalline morphology consists of ordered lamellae with amorphous regions, affecting mechanical properties like viscoelastic behavior, fracture toughness, and strain-rate sensitivity. Studies have shown that crystal morphology and spherical size affect the mechanical properties of semi-crystalline thermoplastics [22].

Several surface modifications of PEEK were evaluated in the literature but information regarding the comparison of the effective treatments and the effect of these treatments on phase changes in the PEEK structure is still insufficient and unclear. While studies indicate that the structure of PEEK material is stronger with the addition of nanoparticles, the effect on surface properties, bond strength, and phase change after surface treatment of the material with these properties is not given [7, 28, 29]. Furthermore, the effect of phase changes in PEEK material on bond strength is unknown [21, 30].

Consequently, this study aimed to assess the impact of different mechanical and chemical surface pretreatment on the surface topography and phase transformation of PEEK specimens and bond strength to composite resin. The null hypothesis of the present study proposes that surface modifications would have no effect on phase change, surface roughness, or bond strength of PEEK to composite resin.

Materials and methods

Preparation of Specimens

G*Power v3.1.9.2 (www.psychologie.hhu.de) was used to assess power and specimen size for"repeated measures, within–between interaction."To detect a medium effect size with an f of 0.25 (alpha = 0.05, power = 0.95, correlation between repeated measures = 0.5, nonsphericity correction ε = 1, seven groups, and two measurements), 20 specimens per group were required for 7 groups. A total of 154 rectangular PEEK specimens, measuring 10 × 10 × 2 mm, were milled from a single blank containing 20% nanoceramic filler (BioHPP; Bredent GmbH & Co KG, Germany).

Surface modifications

All specimens were randomly separated into seven groups (n = 22 each). 2 specimens of each group were reserved for Scanning Electron Microscopy (SEM) and X-Ray Diffractometer (XRD) analysis after surface modifications without a bonding process. Surface treatments were performed by a single operator to ensure standardization.

  • Group C: Consisted of untreated specimens that served as the control group.

  • Group (SE): Specimens were exposed to 98% sulfuric acid (Merck KGa, Darmstadt, Germany) for 60 s [31]. A water drops with a volume of 100 µl was placed on the sample surface. The acid was then rinsed with distilled water for 1 min. Finally, the specimens were dried using oil-free compressed air

  • Group (PE): Specimen surfaces were treated with 100 μL of piranha solution (a 10:3 ratio of 98% sulfuric acid to 30% hydrogen peroxide) (Albar Chemistry, Kocaeli, Turkey) for 30 s. Finally, the specimens were dried using oil-free compressed air [32].

  • Group (S): PEEK surfaces underwent alumina particle abrasion with 110 µm Al2O3 particles for 30 s at 2.8 atmospheres. The particles were applied perpendicularly from a distance of 10 mm (Zhermack, Rovigo, Italy) [33]. The tip of the blasting device scanned the entire surface.

  • Group (T): PEEK surfaces were coated with 30 µm tribochemical silica-coated Al2O3 particles for 15 s at 2 atm pressure, applied perpendicularly from a 10 mm distance (3 M ESPE, Seefeld, Germany) [34]. The tip of the blasting device scanned the entire surface.

  • Group (L): Specimens underwent irradiation using an Er:YAG laser (LightWalker AT, Fotona, Slovenia) operating in quantum-square-pulse mode. The laser emitted at a wavelength of 2,940 nm with 150 mJ energy and 1.5 W output power, utilizing a pulse duration of 5 × 50 µs. Irradiation was applied for 20 s under air–water cooling, employing a noncontact handpiece positioned at a 10 mm working distance[35]. The tip of the laser device shot over the entire surface.

  • Group (P): An atmospheric-pressure plasma system (Kinpen, Neoplas, Germany) was employed to apply argon gas to PEEK specimens. The gas pressure was set at 30 Pa, and the treatment lasted for 2 min. A working distance of 10 mm was maintained between the applicator tip and the specimen surface [36]. The tip of the plasma device shot over the entire surface.

Surface roughness measurement and scanning electron microscopy evaluations

The surface roughness (SR) of each specimen was measured using a profilometer (Taylor Hobson, Leicester, England), with three single individual measurements per specimen. By taking the arithmetic mean of the three measurements made for each specimen, the mean surface roughness (µm) of the specimen was calculated. A scanning electron microscope (JSM- 6610, Jeol, USA) was used to analyze gold-coated specimens at magnifications of 500 × and 5,000 × at 20 kV.

X-Ray diffractometer analysis

The specimens were evaluated using an X-Ray Diffractometer (Rigaku SmartLab Diffraktometer, Tokyo, Japan) operating at 40 kV and 30 mA. The device settings included a scanning range of 10°–35° 2θ, a scanning speed of 2°/min, and a thin film incidence angle of 1°[29, 37, 38].

Adhesive system and resin cement application

An adhesive system (Visio.link, Bredent GmbH & Co KG) containing methyl methacrylate (MMA) and pentaerythritol triacrylate was applied to the PEEK surface using a single microbrush. The adhesive was polymerized for 90 s at 220 mW/cm2 and a wavelength of 370–400 nm using a Bre.Lux Power Unit (Bredent GmbH & Co KG). Resin cement (Panavia SA Cement Plus, Kuraray, Japan) was then applied to the center of the PEEK surface using a custom mold with a diameter and height of 3 mm each. Thus, samples of resin cement with a diameter of 3 mm and a height of 3 mm were prepared on the PEEK surface. The resin was cured for 40 s using the same curing apparatus [39].

Thermal Aging Process and Shear Bond Strength (SBS) Test

The specimens were immersed in distilled water for 24 h prior to thermal aging. Each group underwent 5,000 thermocycles between 5 °C and 55 °C, with 30-s dwell times in each water bath, using an automated thermocycling machine (Gökceler Machines, Turkey). In the oral cavity, the 5,000 thermocycles replicated a time frame of 4–5 years [24]. After thermal aging, SBS testing was performed using a universal testing machine (Model 3,340, Instron Corp., Wycombe, UK) at a crosshead speed of 1 mm/min. The machine’s knife-edge tip applied force parallel to the PEEK and composite interface, measuring the maximum shear load before debonding. SBS was calculated using the following formula:

$$SBS = breaking \;load/bond \;area = N/mm^2 = MPa$$

Examination of failure modes of specimens under stereomicroscope

Each specimen was examined under a stereomicroscope (SZx10 Olympus, Tokyo, Japan) at 25 × magnification) to determine the fracture type. The specimens are divided according to type as adhesive, cohesive, and mixed. The failure mode is classified as adhesive along the bonding surface between PEEK and composite resin, and there is no residual composite resin on the bonding surface; cohesive: entirely within the PEEK or within the composite resin; or mixed: the bonding surface contains less than 50% of composite resin.

Statistical analysis

Data analysis was conducted using SPSS version 24.0 (Chicago, IL, USA). In order to assess the homogeneity of the distributions of the variances of the surface treatments for each group (n = 20), the Shapiro–Wilk test was applied, and normal distributions were found. One-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison test was employed to assess the SR and SBS data, with statistical significance established at p < 0.05. Spearman’s rho (ρ) was used to examine the correlations between bond strength and SR.

Results

Table 1 shows the mean SR values and standard deviations for the specimens. Statistical analysis using ANOVA revealed significant differences in SR values (p < 0.05). Group S displayed the highest SR values (3.09 ± 0.40 μm) (p < 0.001). The other groups (SE, PE, T, P, and L) showed significantly higher SR values compared to group C. However, no statistically significant differences were found among these groups.

Table 1 Mean and standard deviation values of surface roughness (SR) values (µm)
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SEM evaluations revealed variations in surface treatments and morphological properties. Group C (Fig. 1) and Group L (Fig. 2) exhibited regular surface scratches. While scratches in Group C were observed in the form of regular milling cutters, scratches in Group L were observed with pits due to shooting. Sulfuric acid treatment eliminated the regular surface scratches, resulting in a homogeneous porous structure (Fig. 3). The Group PE displayed a heterogeneous rough surface, distinct from the porous structure observed in the Group SE (Fig. 4). Group S surfaces demonstrated rougher, irregular, fissured surfaces compared to the other groups (Fig. 5). Group T showed more uniform surface imperfections than the sandblasted group (Fig. 6). While Group P appeared similar to Group C at × 500 magnification, higher magnification revealed a homogeneous roughness structure (Fig. 7).

Fig. 1
figure 1

SEM images of group C (× 5,000, × 500 magnification)

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Fig. 2
figure 2

SEM images of group L (× 5000, × 500 magnification)

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Fig. 3
figure 3

SEM images of group SE (× 5000, × 500 magnification)

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Fig. 4
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SEM images of group PE (× 5000, × 500 magnification)

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Fig. 5
figure 5

SEM images of group S (× 5000, × 500 magnification)

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Fig. 6
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SEM images of group T (× 5,000, × 500 magnification)

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Fig. 7
figure 7

SEM images of group P (× 5000, × 500 magnification)

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X-ray diffraction patterns of one specimen from each group are shown in Fig. 8. The XRD patterns display the semi-crystalline structure of PEEK material. The characteristic peaks of the control group were observed in the range 2θ = 25–30°. It was observed in the figure that the intensities of the peaks changed with the phase transformation size of the PEEK specimens. Different peak areas were observed in the XRD pattern of Group S and Group L compared to the control group. However, the other groups exhibited very similar XRD patterns, with the characteristic peaks being of the same intensity and width, revealing an almost identical material structure. In group L, the intensity of the major peak was decreased (Fig. 8G). Sandblasting with Al2O3 particles (Group S) appears to shift the 2θ toward the higher angles than the control group XRD pattern. (Fig. 8D).

Fig. 8
figure 8

X-Ray diffraction patterns of PEEK specimens. A. Group C, B. Group SE, C. Group PE, D. Group S, E. Group T, F. Group L, G. Group P

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Table 2 shows the mean and standard deviation of SBS values for various surface treatments. Statistical analysis using ANOVA revealed significant differences in SBS values (p < 0.05). Group SE exhibited the highest SBS values (13.28 ± 1.69 MPa). Group L (10.29 ± 2.59 MPa), Group T (11.01 ± 3.79 MPa), and Group PE (10.50 ± 4.05 MPa) also demonstrated significantly higher SBS values compared to Group C (p < 0.05). However, no significant differences were observed among these three groups (p > 0.05). While Group S (7.77 ± 2.47 MPa) and Group P (8.40 ± 1.69 MPa) showed higher SBS values than Group C, these differences were not statistically significant.

Table 2 Mean and standard deviation values ​​of bond strength values ​​(MPa)
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The predominant failure mode was adhesive. However, Group T, Group SE, Group PE, Group P, Group L also showed mixed failures. Group C and group S only showed adhesive failures. (Table 3). No cohesive failure was observed in the tested specimens (Fig. 9).

Table 3 Modes of failure in each group of specimens
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Fig. 9
figure 9

Failure between veneering composite resin and PEEK specimen. a Mixed failure, b Adhesive failure

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Table 4 presents the findings on the relationship between SR and SBS. The correlation between SR and SBS was analyzed using Spearman’s rho (ρ). Results indicate no statistically significant correlation between SR and SBS (p > 0.05).

Table 4 Correlation coefficients between surface roughness and bond strength results (r)
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Discussion

This in vitro study assessed the impact of different surface modifications on the phase transition, surface roughness, and bond strength of PEEK to composite resin. The results indicate that all surface treatments significantly enhanced SR and SBS values. However, the correlation between SR and SBS values showed limited statistical significance. Group L and Group S exhibited distinct peak areas compared to Group C. Therefore, the null hypothesis that surface modifications would have no effect on phase change, surface roughness, or bond strength of PEEK to composite resin was rejected.

Various aging methods are employed to simulate the oral environment, with thermocycling being a prevalent technique for in vitro aging of specimens prior to bond strength testing [40]. Uhrenbacher et al. [24] indicated that 5,000 thermal cycles equate to roughly 4–5 years of clinical use. Although there is no consensus about subjecting materials to thermocycling regimens, 5,000 thermal cycles were utilized to replicate the oral environment for standardizing the thermal stress in the present study.

Dental adhesives are frequently used to prepare inert PEEK surfaces for adhesion. The composition and solvents in the adhesive system affect the bonding success between PEEK and resin [41]. Adhesives containing MMA strengthen the bond between PEEK and composite resin by enhancing the functional groups of the polymers [10]. The Visio.link adhesive system is effective in improving the bond strength of PEEK surfaces [13, 42]. Pentaerythritol triacrylate, a component of Visio.link, dissolves the PEEK surface, the dimethacrylate monomers to be linked to the two carboxyl groups, which are the connective sites in the composite [43]. Huang et al. [44] stated that both MMA and urethane dimethacrylate (UDMA) were found to provide high bond strength on PEEK surfaces, reaching the ISO 10477 standard of 5 MPa. Consequently, in the present study, an MMA-containing adhesive system was selected to enhance and standardize the PEEK surface.

Various surface modifications are employed to enhance the micro-roughness of dental materials’ bonding areas, as surface topography plays a crucial role in adhesive processes [5]. Studies reported that etched PEEK surfaces demonstrate higher bond strength values compared to untreated ones [16, 17, 20]. While sulfuric acid is considered the most effective acid for PEEK, other combinations, such as piranha solution (a mixture of 98% sulfuric acid and 2% hydrogen peroxide in a 10:3 ratio), are also used in the present study. In the present study, Group SE (1.41 ± 0.24 µm) and Group PE (1.20 ± 0.58 µm) showed statistically higher SR values than Group C (0.65 ± 0.23 µm), with no significant differences between the two treatment groups. Group SE (13.28 ± 1.69 MPa) demonstrated statistically higher SBS values compared to the other groups, despite having similar SR values to Group PE. Although similar roughness values ​​are shown between these groups, the reason for higher bond strength of group SE can be explained by the fact that the acids used cause different changes in the PEEK structure.

Sulfuric acid destroys the carbonyl groups and functional ethers between the benzene rings on the PEEK surface, while the benzene ring of PEEK reacts directly with the atomic oxygen produced by the interaction of sulfuric acid and hydrogen peroxide in the piranha solution [4, 11]. The differing effects of these acids on the PEEK surface were also evident in the SEM results. SEM images of the Group SE revealed a porous and permeable surface with large pores, likely enhancing the bond between PEEK and composite resin [3, 32, 43, 45]. In contrast, the Group PE produced a nonhomogeneous rough surface without pores on the PEEK surface [16, 32, 43]. However, when the phase change values were analyzed, both surface treatments showed similar XRD values. As a result, acid applications do not cause phase change while changing the surface properties to improve bond strength.

The use of acid applications in clinical settings poses a risk to living tissues. Consequently, several studies have explored alternative techniques in both clinical and laboratory settings [11, 17, 18]. Hallmann et al. [11] reported that sandblasting with Al2O3 particle alters the surface morphology of PEEK, enhancing its micromechanical retention. Schmidlin et al. [5] suggested that silica coating improves SBS values by creating a rough surface with silica particles. In the present study, Group T demonstrated higher SBS values than Group S, despite the latter having significantly higher SR values. Ates et al.[1] observed no significant differences were found between sandblasting and tribochemical silica coating on SR and SBS values of PEEK. Discrepancies in results could be attributed to variations in resin composites and sandblasting conditions across studies. The 110 μm Al2O3 particles employed in the present study produced a more rugged surface compared to the 50 μm Al2O3 particles used in the previous study. When SEM images were analyzed, the bonding surface of Group S exhibited rougher than Group T. Sandblasting with Al2O3 particle increased the roughness by creating deep grooves on the surface and decreased the bond strength by preventing the flow of the composite on the PEEK surface. The difference in surface roughness between tribochemical silica coating and sandblasting with Al2O3 particle can be attributed to the sand particle size [46]. However, the higher bond strength resistance in tribochemical silica coating may be due to the silica in its structure. The presence of silica in the tribochemical structure provides benefits in both chemical and mechanical bonding [26]. Upon comparison of XRD data between the two groups, Group S exhibited distinct peak values. The thermal alteration seen following the surface treatment may have induced changes in the PEEK material structure. When the failure modes were analyzed, Group S showed only adhesive failure. This may be an indication that the composite is not fully spread on the surface. While adhesive mode was observed in groups with low bond strength values, mixed failure modes were observed as the bond strength value increased [13].

Several studies have investigated the effectiveness of the Er:YAG laser in improving the bonding between dental ceramic [1, 18, 35]. Jahandideh et al. [18] suggested that SBS values of composite resin to PEEK can be enhanced with Er:YAG laser irradiation. Similarly, in this study, the group L (10.29 ± 2.59 MPa) showed higher SBS values than the group C (3.15 ± 0.79 MPa). When SEM images were compared, it was shown that the scratches in Group C disappeared by the application of laser in Group L. Although no treatment was applied in group C, the scratches may be due to the milling process. Olawumi et al. [47] stated that laser application smoothened the PEEK surface according to the control group. Although there is a decrease in scratches on the PEEK surface in Group L, laser energy creates a more homogeneous surface roughness value (1.45 ± 0.57 μm) for the bonding in the present study. In results, there is a decrease in scratches on the PEEK surface. In addition, if the photons from the laser beam are sufficiently energetic, they can alter the surface chemistry of the material by breaking chemical bonds, which may have an effect on the bond strength [48].

In the results of the X-ray diffraction, there is no peak created or destroyed in the groups T, SE, S, and P compared to the control group. It is meant that there is no significant interaction between PEEK molecules after these surface modifications. However, an X-ray diffraction analysis of PEEK surfaces revealed distinct peak areas in Group L and Group S. It is known that thermal changes in PEEK cause oxidative degradation and change the crystalline structure of the polymer [49]. The reaction of the material to laser irradiation and alumina particle abrasion may be due to temperature rise, which explains these variations in the XRD pattern. Although Group S and Group L exhibited significantly higher bond strength values than the control group, the changing XRD pattern may have caused deterioration within the structure. Xie et al. [38] analyzed the XRD results graphically by randomly selecting samples between groups in their study. Within the limitations of the study, with reference to previous studies, the XRD phase change values of PEEK material were analyzed graphically with reference to the points where the PEEK material showed peak change, not as a percentage [29, 37]. Analyzing the percentage of phase change in PEEK material will give an idea for future studies.

The difference in SEM images between Group C and Group L may be due to the phase change seen in Group L. At low temperatures or under controlled heating conditions, some parts of PEEK can crystallize [22]. The material's ability to absorb significant laser radiation and provide alumina particle abrasion surface reactions that cause temperature rise may explain these variations [49]. The reason for the difference between Group L and Group S in terms of surface roughness may be due to the fact that sand particles make the surface extra rough. These results need to be supported by mechanical testing in future studies.

Plasma treatment of PEEK specimens has been recommended to enhance surface hydrophilicity and bond strength by generating various functional groups [14]. In the present study, the group P exhibited higher SR values (1.27 ± 0.34 μm) and SBS values (8.40 ± 1.69 MPa) compared to the group C (3.15 ± 0.79 MPa). Turkkal et al. [50], reported that plasma treatment reduced the surface roughness of PEEK while enhanced the bond strength. SEM images of the plasma group showed that the plasma treatment had no significant effect on the PEEK surface and an almost smooth surface morphology was observed. Unlike the previous study [50], surface roughness values are higher than group C in the present study. The reason for the difference in the results can be attributed to the plasma time applied. It is thought that the surface roughness might have decreased owing to the abrasive or thermal effect of the long-term plasma treatment. The authors explained the increasing in bond strength despite the low surface roughness by the low contact angle. Same to the previous study, plasma applications (8.40 ± 1.69 MPa) caused higher SBS values than control group. In a study comparing surface roughness and bond strength values after PEEK surface treatments, laser, plasma, sulfuric acid was applied as surface treatments and compared with the control group. The results obtained were in parallel with the results of our study. Although the plasma group was higher than the control group, its effect on bond strength was not as high as sulfuric acid and laser [50].

The acceptable SBS value between the framework and resin-based material is 5 MPa, as per ISO 10,477 standards. In clinical oral environments, however, the minimum SBS value for resin-based material ranges from 10 to 12 MPa [30]. In this study, all tested specimens except Group C (3.15 ± 0.79 MPa) fell within the acceptable SBS range. Nevertheless, Group P (8.40 ± 1.69 MPa) and Group S (7.77 ± 2.47 MPa) failed to meet the clinical SBS requirement. Group SE demonstrated the highest SBS values (13.28 ± 1.69 MPa). Likewise, Çulhaoğlu et al. [23] found that Group S had the highest SR values (3.09 ± 0.40 µm), while Group SE showed the highest SBS values (13.28 ± 1.69 MPa).

Nevertheless, the present study has several limitations. Only one PEEK blank was used, which limits the generalizability of the results to other PEEK brands. Moreover, only resin cement was assessed for SBS values. The study’s findings might not entirely reflect clinical settings, as the mechanical testing and specimen production procedures were performed in vitro.

Conclusion

In conclusion, within the constraints of this in vitro study, all surface modification methods enhanced the surface roughness and shear bond strength of PEEK. The study found that no significant correlation between SR and SBS, which means both topographical effects and chemical alterations on the surface were as crucial as SR for achieving optimal bond strength. Sulfuric acid emerged as the most effective method for enhancing the bond strength between PEEK and composite resin. Considering the potential health hazards of sulfuric acid and the phase transformation with laser irradiation and alumina particle abrasion applications, tribochemical silica coating can be suggested as an alternative method to increase bond strength in clinical and laboratory conditions. Although all surface modifications enhanced the SBS values ​​of PEEK to composite, XRD analysis shows that all surface modifications may not be reliable for the PEEK structure. A future study can be planned in which the XRD patterns obtained from the applied surfaces can be correlated with the fracture strength of the PEEK material.

Data availability

The datasets generated and/or analysed during the current study are not publicly available due [The data is not publicly available as additional analyses are currently being carried out] but are available from the corresponding author on reasonable request.

Abbreviations

PEEK:

Polyetheretherketone

UDMA:

Urethane dimethacrylate

SEM:

Scanning Electron Microscopy

Al2O3 :

Alumina oxide

XRD:

X-ray Diffraction

SBS:

Shear Bond Strength

SR:

Surface Roughness

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Acknowledgements

We would like to thank Recep Tayyip Erdoğan University Research Fund for providing financial support for this study.

Funding

This work was supported by the Research Fund of the Recep Tayyip Erdogan University, Rize, Turkey (Project no: TDH- 2020–1095).

Recep Tayyip Erdogan University

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Authors and Affiliations

  1. Department of Prosthodontic, Faculty of Dentistry, Bolu Abant Izzet Baysal University, Bolu, 14030, Turkey

    Emel Arslan

  2. Department of Prosthodontic, Faculty of Dentistry, Recep Tayyip Erdogan University, Rize, 53020, Turkey

    Ipek Caglar

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Conceived and designed the analysis: EA, IC Data collection or data entry: EA Contributed data/analysis tools: EA, IC Performed the analysisEA, IC Writing: EA, IC. All authors read and approved the final version of the manuscript.

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Correspondence to Emel Arslan.

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Arslan, E., Caglar, I. Enhancing PEEK bond strength: the impact of chemical and mechanical surface modifications on surface characteristics and phase transformation. BMC Oral Health 25, 511 (2025). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12903-025-05933-3

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BMC Oral Health

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