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Article

Effects of Surface Treatment on Adhesive Performance of Composite-to-Composite and Composite-to-Metal Joints

by
Nikhil Paranjpe
1,
Md. Nizam Uddin
2,
Akm Samsur Rahman
3 and
Ramazan Asmatulu
1,*
1
Department of Mechanical Engineering, Wichita State University, 1845 Fairmount, Wichita, KS 67260, USA
2
Division of Engineering, Texas A & M University-Texarkana, 7101 University Ave., Texarkana, TX 75503, USA
3
Department of Mechanical Engineering Technology, New York City College of Technology, City University of New York, New York, NY 11201, USA
*
Author to whom correspondence should be addressed.
Processes 2024, 12(12), 2623; https://doi.org/10.3390/pr12122623
Submission received: 30 October 2024 / Revised: 15 November 2024 / Accepted: 19 November 2024 / Published: 21 November 2024
(This article belongs to the Special Issue Development and Characterization of Advanced Polymer Nanocomposites)
Browse Figures
Versions Notes

Abstract

:
This study deals with the long-running challenge of joining similar and dissimilar materials using composite-to-composite and composite-to-metal joints. This research was conducted to evaluate the effects of surface morphology and surface treatments on the mechanical performance of adhesively bonded joints used for the aircraft industry. A two-segment, commercially available, toughened epoxy was chosen as the adhesive. Unidirectional carbon fiber prepreg and aluminum 2021-T3 alloys were chosen for the composite and metal panels, respectively. Surface treatment of the metal included corrosion elimination followed by a passive surface coating of Alodine®. A combination of surface treatment methods was used for the composite and metal specimens, including detergent cleaning, plasma exposure, and sandblasting. The shear strength of the single-lap adhesive joint was evaluated according to the ASTM D1002. Ultraviolet (UV) and plasma exposure effects were studied by measuring the water contact angles. The test results showed that the aluminum adherent treated with sandblasting, detergent, and UV irradiation resulted in the strongest adhesive bonding of the composite-to-composite panels, while the composite-to-metal sample cleaned only with detergent resulted in the least bonding strength. The failure strain of the composite-to-composite bonding was reduced by approximately 50% with only sandblasting. However, extended treatment did not introduce additional brittleness in the adhesive joint. The bonding strength of the composite-to-composite panel improved by approximately 35% with plasma treatment alone because of the better surface functionalization and bonding strength. In the composite-to-aluminum bonding process, exposing the aluminum surface to UV resulted in 30% more joint strength compared to the Alodine® coating, which suggests the origination of higher orders of magnitude of covalent groups from the surface. A comparison with published results found that the joint strengths in both similar and dissimilar specimens are higher than most other results. Detailed observations and surface analysis studies showed that the composite-to-composite bonding mainly failed due to adhesive and cohesive failures; however, failure of the composite-to-aluminum bonding was heterogeneous, where adhesive failure occurred on the aluminum side and substrate failure occurred on the composite side.

1. Introduction

Fiber-reinforced polymeric composites find extensive use in the aerospace, automotive, energy, and marine industries due to their diverse advantages, including lightweight construction, long-term cost-effectiveness, corrosion and creep resistance, and prolonged durability during harsh service conditions [1,2,3]. The necessity for joining processes in the manufacturing of composite laminate components, especially those with intricate geometries and large dimensions, is unavoidable. Aerospace structures often demand the joining of lightweight, high-performance materials, such as carbon fiber-reinforced polymers (CFRPs) and aluminum and titanium alloys [4,5]. Adhesive bonding stands out as a well-established technique for joining such materials. In comparison to alternative methods, adhesive bonding circumvents stress concentrations induced by hole drilling and avoids heat-affected zones associated with high-temperature welding and other joining processes [6,7]. In hybrid joining involving dissimilar materials, the inclusion of a polymeric adhesive layer can effectively delay or even prevent crevice and galvanic corrosion [8]. The mechanical performance of structures bonded with adhesives is highly sensitive to the surface properties of the two adherents before bonding. To ensure optimal joint performance, suitable surface pre-treatment is essential in the aviation, automotive, and many other industries. This pre-treatment serves not only to cleanse the adherend surfaces and remove contaminants or weak boundary layers but also to enhance the interaction between the adhesive and adherend surfaces [9]. For instance, modification of the surfaces’ topography and chemistry improves mechanical interlocking, van der Waals interactions, and chemical and electrostatic bonding. Additionally, surface pre-treatment may enhance the wettability of the adhesive surfaces [10].
Wahab explains that adhesive bonding is advantageous over mechanical fastening for several reasons, including reduced local stress, weight, and bimetallic corrosion; higher fatigue stress; and longer fatigue life [11]. As structural adhesives have evolved, the manufacturing and assembly of components have been made possible with complexity and sophistication. The premature failure of adhesive joints is caused by concentrations of transverse shear and normal stresses along the edges of the adhesive bond line. Barbosa et al. investigated the behavior and strength of an adhesive joint, which is influenced by various factors, including the type of adhesive (brittle or ductile, strong, or weak) and joint geometry [12]. Among these factors, the overlap length stands out as a crucial parameter affecting joint strength. To determine the most suitable adhesive for specific joint geometries, a comparative study encompassing various joint types and utilizing adhesives with diverse characteristics was conducted. The selected joint configurations for testing included single-lap joints, double-lap joints, stepped-lap joints, and scarf joints, each subjected to evaluation with three different adhesives. The findings of the study indicate that the optimal joint type is significantly dependent on the type of adhesive employed. Less strong and ductile adhesives are found to be more suitable for joint geometries displaying substantial stress variations. Conversely, stronger but more brittle adhesives are recommended for joint geometries characterized by more uniform stresses. This insight underscores the importance of aligning the adhesive characteristics with the specific stress distribution in the joint geometry for optimal performance. Sun et al. found that, due to the presence of environmental effects on the available adhesive joining methods and materials, adhesive joints also lack confidence in long-term reliability [13]. Therefore, the aerospace and automotive industries have yet to improve the performance of adhesive joints, which depends on several factors, including surface preparation, mechanical interlocking, and material selection. The strength of adhesive bonding is affected by various factors, including the type of adherent, operating temperature, and surface pre-treatment. Surface preparation/pre-treatment has been demonstrated by several techniques, including ultraviolet (UV) laser, chemical treatment, plasma treatment, sandblasting, and conversion coating [14]. Several studies have presented the effects of surface treatment on the strength of adhesive joints. Prolongo and Ureña examined the effects of the alloying elements of 2024 aluminum alloys (copper and magnesium) on the adhesive bonding strength and found that these alloy elements positively influence the adhesive strength by creating intermetallic compounds that result in higher electrochemical potential [15].
Aradhana et al. investigated the mechanical strength of a single-lap bonded joint between two unidirectional composite panels [16]. This bonded joint was created using three different techniques: co-curing without adhesive joints, co-curing with the use of adhesives, and secondary bonding using adhesives. It was found that the co-cured specimens without adhesives had the highest failure strength, while the co-cured specimens with adhesive film suffered from delamination failure and, therefore, lower strength. Studies have suggested that, in a single-lap shear test, the strength of the epoxy adhesive increased by ~52% with a formulation using multi-walled carbon nanotubes and nanoclay. Complexities in structural design have also led to an increase in composite-to-metal joining in various industries, including aerospace and automobiles. A few things must be considered when maintaining effective bonding between composites: bonding strength, failure process, and mode and surface preparation. A clean and corrosion-free aluminum surface is essential in a strong bonding process. Several commercially available coating agents are used in aerospace industries to inhibit corrosion through the chromate conversion process.
Plasma treatment has been adopted in the composite-to-composite and composite-to-aluminum bonding processes. Hasirci et al. explained that the plasma treatment chemically modifies the outermost surface of the composite substrate and creates selective functional groups on the surface that are suitable for chemical bonding with adhesive [17]. Various types of gases have been utilized to achieve different surface polarity and chemistry. Among them, heliox (He/O2), air plasma, and vacuum plasma are prominent. Williams et al. concluded that the plasma treatment resulted in significant changes in the surface chemical configuration, including a 50–220% increase in oxygenated surface, which is a measure of the functional groups [18]. However, the comparison between UV irradiation and plasma treatment on a select polymer surface suggested that oxidation to a higher state, resulting in the formation of carbonyl/carboxyl and hydroxyl species, was observed with plasma treatment but not with UV irradiation. This difference was also reflected in contact angle measurements. On the other hand, UV irradiation was found to be effective in modifying aluminum surfaces.
Joining dissimilar materials includes a multitude of variabilities in mechanical and thermal performances. Dissimilar single-lap joints were studied by Stuparu and Apostol using carbon fiber–aluminum and Araldite® 2015 with a constant overlap length and adhesive thickness but variations in adherent thickness [19]. That study proved that dissimilar and thinner adherents achieve the desired stiffness, while carbon–carbon bonding with a similar thickness suffers from interfacial failure. Therefore, it is essential to optimize the adhesive joint from delamination failure. A new combination of surface treatments can resolve the delamination issue in composite-to-aluminum adhesive joints.
Unlike prior studies, which often focused on single-surface treatments or homogeneous material joints, our work systematically explores the synergistic effects of plasma and UV treatments on both composite-to-composite and composite-to-metal joints. This novel, dual-material approach addresses the complexities of achieving optimal adhesion across dissimilar substrates, offering a more advanced perspective for enhancing joint performance. The current study was aimed at investigating the effect of various surface treatments on the bonding strengths of adhesive joints between composite-and-composite and metal-and-composite surfaces. In this study, dissimilar materials were exposed to various treatment methods, including conversion coating, sandpapering, oxygen plasma, and UV treatments on aluminum surfaces. The novelty of this study is that, for the first time, the surface treatment effects were evaluated on similar and dissimilar composite and metal surfaces in terms of wettability as well as surface tension and interactions for improved mechanical performance. Also, the bonding strengths of the pre-treated joints were evaluated using destructive testing and morphology changes. Several published studies were compared with the test results, which may be useful for aircraft, drones, automobiles, ships, and wind turbines. Our findings provide valuable insights that can potentially redefine surface treatment protocols in high-demand fields like aerospace and automotive engineering.

2. Experiment

2.1. Materials

A unidirectional carbon fiber prepreg roll (Cytec Engineered Materials, Greenville, TX, USA) lot #301910285, sheet thickness 0.15 mm) was used to prepare the composite flat panels. It consists of T300 3K carbon fibers and a CYCOM® 970 epoxy resin, with a fiber areal weight of 193 gsm and a resin content of 38% by weight. It cures at 350 °F (177 °C) and has a service temperature up to 300 °F (149 °C). Mechanically, it provides high tensile strength (231–255 ksi) and modulus (17.4–17.8 Msi) along with strong compressive strength (192–212 ksi), making it well-suited for high-strength structural applications, particularly in aerospace [20]. An aluminum 2024-T3 plate (purchased from Online Metals, Dallas, TX, USA) with dimensions of 30 × 30 × 0.16 cm was used as the parent structural aerospace component to be investigated for the adhesive bonding tests. Magnolia 6380 (Atlanta, GA, USA), a two-part epoxy adhesive, was used as the bonding material. The quick-setting characteristics of this epoxy saved processing time and facilitated sufficient strength within a relatively shorter period. Alumiprep 33, an acid-based cleaning agent, was used to clean the aluminum surfaces and remove surface corrosion and oxidation. Alodine® 1201, an acid-based non-flammable conversion coating agent, was used to create a thin protective chromate coating. This coating creates corrosion resistance and chemical bonding with the adhesives for better surface-to-surface bonding. The Alodine® chemical has a few benefits like providing a visible conversion coating that protects the surface from corrosion and enhancing paint adhesion. All materials and chemicals were used in the experiments without any further modification.

2.2. Methods

2.2.1. Surface Preparation

A widely used traditional industrial method for treating the surfaces of aluminum alloys involves employing chemical treatments, like chromic acid etching and anodizing [21]. This combined process effectively eliminates surface contamination and weak oxide layers, replacing them with a sophisticated surface topography featuring microscale roughness. These alterations significantly enhance adhesive bonding for better chemical and mechanical interactions. However, the use of certain acids poses a hazard to both operators and the environment [22]. Consequently, alternative surface pre-treatment methods have been explored. Before adhesive bonding between the composite-to-composite and composite-to-aluminum plate in this study, the surfaces were pre-treated to reveal the functionalization effects. Table 1 shows the pre-treatment schemes on the adherent surfaces of the prepared composite–composite and metal–composite samples.

2.2.2. Composite Panel Fabrication Process

The composite panel was prepared using carbon fiber prepreg stacked into layers in a sequence of [0,90,0,90] and then, a roller was used to properly stack them uniformly. The stacked layers were placed under a plastic sheet, where a vacuum was applied using the vacuum bagging technique. For approximately 16 h, the prepreg panels were debulked under the vacuum to remove air pockets in the prepregs and then transferred to an oven for curing in three stages. In stage 1, the temperature in the oven was raised from 25 °C to 125 °C in 45 min. In stage 2, the temperature was maintained at 125 °C for 2 h. In stage 3, the temperature was gradually brought down to 25 °C within 45 min. The cure time for the entire process was 3 h and 30 min.

2.2.3. Aluminum Surface Cleaning and Coating Process

Surface preparation of the aluminum 2024-T3 sample was performed using sandpaper with P120, P400, and P600 grit sizes, followed by deionized (DI) water and acetone cleaning. Metal substrates were also sonicated in DI water and acetone solution to remove the oily surface contaminants and lose particles.

2.2.4. Plasma Treatment

Plasma treatment was performed on both the composite and aluminum plates using a Harrick Basic Plasma Cleaner PDC-32G with a variation in treatment duration. In this process, the sample surfaces were placed in a chamber where a reactive gas (oxygen) was ionized by a high-voltage electric field, producing plasma. The plasma’s reactive species interacted with the surface, removing contaminants and adding functional groups like hydroxyl and carboxyl that improve surface energy and bonding properties [23]. The individual aluminum and composite specimens were separately treated for periods of 4, 8, and 12 min. In the next step, these plasma-exposed and clean areas were used for applying the selected adhesives (Figure 1).

2.2.5. Sandpaper Abrasion

Sandpaper was used to abrade the aluminum surface and increase the surface area for improved wettability and mechanical interlocking for better adhesion. Starting with a P120 grit size, the sanding treatment was gradually accomplished using grit sizes of P400 and P600. During this process, caution was used to ensure the removal of debris, which may be detrimental to the bond strength, possibly causing delamination and premature failure.

2.2.6. Alodine® Coating on Aluminum Surface

Alodine® coating was used on the aluminum surface to simultaneously improve adhesion bonding and corrosion resistance. Before applying this coating, Turco Alumiprep 33 was used for pre-treatment of the aluminum surface to remove the oxide layer. Once treated with Alumiprep 33, the aluminum sample was air-dried and placed in the Alodine® 1201 solution for 2–5 min. After the desired coating period, the sample was removed, rinsed with water, and dried. The resultant color of the surface of the aluminum sample was gold or tan. This process is generally referred to as a chromate conversion coating, which creates a protective organic layer. During the chromate conversion process, a thin layer of aluminum oxide is formed along with the other compounds, as shown in the following reaction:
2Al + 2Na2CrO4 → Al2O3 + 2Na2O + Cr2O3

2.2.7. UV Treatment

Once sanding was performed, the aluminum specimens were further treated with UV lights. The samples were placed carefully on a slab and then, in batches, exposed to UV lamps (UVA-340) at controlled and elevated temperatures (70–105 °C) for 2, 4, and 8 days. Heat mainly came from the UV light source. The bonding area of the samples was exposed to UV light, while the unexposed area of the sample was covered with thin aluminum foil.
The high-energy photons from the UV lamps resulted in breaking the molecular organic bonds at and on the treated surface. Then, the UV treatment process cleaned contaminants from the exposed surface. For the joint evaluation between dissimilar materials, only the aluminum specimens were UV-treated, while the composite surface was lightly sanded to eliminate free-edge effects [25]. A selective UV wavelength resulted in superior adhesion strength of the carbon–epoxy composites; however, the laminated composite specimens under a longer period of UV treatment showed interfacial delamination.

2.2.8. Water Contact Angle Measurements

Water contact angle measurements were conducted using a goniometer (KSV CAM 100) with a precision syringe dispenser, high-resolution camera, monochromatic light, and CAM 100 software. A precisely measured droplet of water (5 µL) was dispensed on the flat and clean sample surface following an image recording using the camera. Monochromatic light was used to generate a sharp image with a clean edge. The CAM 100 software recorded the image data and measured the water contact angle values. For each water contact angle data, at least five tests were performed, and the test results were averaged.

2.2.9. Adhesive Bonding Processes

Epoxy adhesive bonding was prepared using Magnolia 6380 A/B epoxy at 100:27 ratios. Two parts of Magnolia adhesive (6380 A and 6380 B) were mixed in the appropriate ratio for 5–7 min and then applied on the aluminum and composite surfaces before reaching the gelation time. For the evaluation of composite-to-composite adhesive bonding, both surfaces were prepared using variations in the types of treatment and duration. Figure 2 shows the composite-to-composite and composite-to-aluminum single-lap joints prepared using the Magnolia 6380 epoxy and hardener. In all cases of joining, the composite samples were cleaned with detergent, and the aluminum surfaces were cleaned with various techniques and then coated with various combinations of materials, as shown previously in Table 1. Detergent cleaning on the composite was performed only to the degree that contaminants were removed from the surface.

2.2.10. Mechanical Testing

The average joint strength was measured using a universal MTS 318 tensile testing machine at a crosshead speed of 0.5 mm/min. The average joint strength of the adhesive was measured using the average shear stress method. For a single-lap joint, the average shear stress (τ) was calculated using the following equation:
τ = P b l
where P is the applied load, b is the joint width, and l is the bond length. The strength measurement of this technique assumes that the applied loads are concentric, and the adhesive only deforms in the shear direction. For each data point, at least five tests were conducted, and the test results were averaged to increase their reliability.

3. Results and Discussion

3.1. Water Contact Angle Analysis

The water contact angle is a measure of surface energy/tension and the nature of bonding on the substrates. A lower water contact angle suggests a higher surface energy/tension, which will affect adhesion strength and shear stress at the interface of the finished surface and adhesive. Figure 3 shows the results of plasma treatment on the wettability of the composite surfaces before and after sandpapering. As shown, 12 min of plasma treatment reduced the water contact angle from 75.3° to 39.9° for the base surfaces and from 61.2° to 23.4° for the sandpapered samples. The reduction in contact angle is an indication of improved wettability and adhesion of the surface as well as increased bond strength of the joints between composite-to-composite and composite-to-metal surfaces [26].
Figure 4 shows the effect of plasma and surface treatments on the wettability of the aluminum alloy surfaces. It can be observed that plasma treatment reduced the contact angle values for all types of surface preparations, including sanding, UV treatment, etching, and Alodine® coating. Plasma treatment in conjunction with Alodine® coating works differently. The water contact angles of the base, sandpapered, UV-treated, plasma-treated, and Alodine®-coated Al alloy samples are 78.5°, 60.4°, 69.0°, 70.3°, and 72.2°, respectively, before the plasma treatments; however, after 12 min of plasma treatment, these values are reduced to 33.3°, 23.4°, 40.8°, 40.2°, and 48.5°, correspondingly. The Alodine® coating creates a corrosion-inhibiting chromate (Cr6+) coating with oxygen plasma, which triggers other functional groups (hydroxyl and oxygen) on the surface to create a better bonding with the adhesives. Therefore, this is a favorable treatment to apply immediately after surface preparation. Compared to the other surface preparation techniques, plasma treatment was less effective with the Alodine® coating. During the adhesion process, the liquid–solid attraction depends on the surface energies of the substrate, adhesive, and interface. Adhesive surface energy is generally constant; therefore, adhesive bonding strength is a strong function of the condition of the substrate surface. It is noted that surface energies between the interface of the substrate and the adhesive play a critical role in adhesive dispersion and strength. Madeira et al. showed that, as the surface energy of the latex-coated packaging paperboard increases, the adhesion strength of the paperboard, polyethylene, also increases [27]. Increased polar groups on the surface due to oxidative treatment, a similar approach to this plasma treatment, resulted in enhanced surface energy and bonding. The reduced contact angle through plasma treatment is related to increased surface energy (functional groups) and, therefore, enhanced adhesion strength.

3.2. Effects of Surface Treatment on Adhesive Joint Strength

3.2.1. Composite-to-Composite Adhesive Joint Strength Analysis

The representative stress–strain diagram obtained from the composite-to-composite joint strength test is shown in Figure 5. A total of five samples for each type of surface preparation were tested, and the curve shown in Figure 4 represents the average of these measurements. As can be observed, the sandpapered base samples and sandpapered- with-plasma-treated samples gave ultimate tensile strengths of 18.4 MPa and 26.1 MPa, respectively. The curve profiles show that any surface treatment in addition to simply sanding resulted in more than 50% reduction in the failure strain. However, the plasma treatment had little effect on the failure strain. The plasma treatment duration only resulted in increased bond strengths without sacrificing much in the failure strain. Reduced strain suggests a reduced shape deformation and increased durability and stability of the samples under loads. Generally, the bonded structural members are axially loaded, particularly in aerospace applications, so the tensile and shear stresses can be well tolerated with surface treatments on similar and dissimilar materials.
In addition to tensile strength, shear strength and bending strength are also critical in understanding the performance of adhesive joints. While this study focuses on tensile strength, which is relevant to the axial loading typically encountered in aerospace applications, joints in real-world environments are subjected to a combination of tensile, shear, and bending forces. Shear strength measures the joint’s ability to resist sliding forces between the adherents, while bending strength evaluates the joint’s ability to withstand bending moments. These strengths are critical for joints subjected to complex loading conditions, such as those encountered in aerospace applications where tensile, shear, and bending forces act simultaneously. Therefore, while our study primarily focuses on tensile strength, further research exploring shear and bending strengths will provide a more comprehensive assessment of adhesive joint performance in real-world conditions.
The effects of the plasma treatment on the joint strength of the composite-to-composite surfaces with different surface preparations are illustrated in Figure 6. The single-lap shear strength of the composite-to-composite surface with 12 min of plasma treatment reached 24.7 MPa compared to the baseline adhesive joint with no plasma treatment, which was 18.4 MPa. It was observed that plasma and UV treatments enhanced bonding strength by increasing surface energy, promoting better adhesion between the composite-to-composite joints. Our results indicate that plasma treatment, when combined with sanding, improves the joint strength by approximately 35% due to increased surface functionalization. A study by Sun et al. showed the effects of plasma treatment parameters (height/distance, power, plasma speed, and temperature) on adhesive joint strengths [28]. That study showed that, as the height of the plasma nozzle was increased, the joint strengths also increased gradually and then reduced with a further increase in height. The study also suggested that a surface temperature between 138 °C and 175 °C was ideal for the plasma treatment process. Plasma treatment has been shown to improve bonding strength without altering the physical properties, including surface roughness and brittleness. In this case, the subsequent improvement in joint strength is an indication of the increased surface functional groups at atmospheric conditions [29].

3.2.2. Composite-To-Aluminum Joint Strength Analysis

The adhesive strengths of the composite-to-aluminum joints with various surface treatments are presented in Figure 7. The test results suggest that surface sanding (with P 600 grit size) plays a key role in enhancing the quality of the adhesive joint, which may be because of the mechanical interlocking mechanisms and improved surface area for better interactions. When plasma treatment was included on the aluminum surface, the joint strength improved by 11% (18.63 MPa vs. 20.39 MPa); this improvement was 28% with UV treatment for 8 days of exposure. The joint strength with sandblasting, acetone cleaning, and UV treatment together for 8 days resulted in a 57% improvement in adhesion strength compared to the baseline strength. This treatment suggests that UV light treatment is superior to plasma treatment on the aluminum surface. UV treatment on the aluminum surface resulted in a significant improvement in adhesion strength, with an approximately 30% increase (23.89 MPa vs. 18.63 MPa) compared to untreated surfaces. These findings highlight the impact of surface treatments on bonding performance, aligning with the literature that emphasizes the role of increased surface energy in adhesion. Previously, the surface preparation using UV light was found to be an effective tool for increasing specific surface area by increasing the adhesive bond strength of polycarbonate and poly (ether-ether ketone) (PEEK) samples by an average of 34% [30]. UV radiation exposure on the substrate surface generates high energy, which breaks the covalent bonds on the surface and introduces functional groups such as hydroxyl, carbonyl, or carboxyl. These covalent groups increase the surface energy and improve its wettability and adhesion. Among plasma treatments, oxygen plasma results in greater surface treatment, which suggests that O2/H2O plasma treatment can react with surface hydroxyl groups and create the layer for up to three covalent bonds, which facilitate bonding with epoxy adhesives [31]. The best treatment (plasma 12 min, sanded, cleaned) showed statistically significant improvements in both failure load and joint strength compared to the base (no plasma) group. The mean failure load for the best treatment was 15.43 kN, and the mean joint strength was 24.46 MPa. With p-values of 0.00195 for failure load and 0.00112 for joint strength, both values are below 0.05, confirming that the differences are statistically significant.

3.3. Failure Modes Analysis and Comparisons

The analysis of joint-area failure modes is an effective tool for understanding the effect of surface treatment. Figure 8a,b present the composite-to-composite joints, and Figure 8c,d present the lap shear fracture surfaces of the composite-to-aluminum joints. Joint-area failure modes can be adhesive, cohesive, and adhesive–cohesive (mixed failure modes) [32]. In these studies, our expectation was to minimize the adhesive failure modes by improving the surface treatment processes (detergent cleaning, UV exposure, oxygen plasma cleaner, sandblasting, and chromium conversion coating) for aluminum alloy substrates.
The fracture surface of the composite-to-composite joints (combination of sanding, detergent cleaning, and plasma treatment) suggests that their failure is dominated by adhesive failure. Figure 9 demonstrates the composite-to-aluminum and composite-to-composite lap joint failure modes under an axial tensile load [33,34,35,36]. In the composite-to-aluminum joint, failure is initiated in the adhesive on the aluminum side, and then, the crack propagates to the composite side. This type of behavior suggests that, because the adhesive modulus is sufficiently high, the adhesive failure seems brittle, at which point the interlaminar failure begins on the aluminum–adhesive interface. The composite-to-aluminum joint also shows a substrate failure. In the composite-to-aluminum joint failure, more adhesives were attached to the aluminum surface than to the composite surface, and cohesive failure was more prominent on the composite surface. Stronger aluminum–adhesive–cohesive bonding suggests the effectiveness of treatment between composite and metal surfaces. This improvement can be useful for aerospace, automotive, and energy applications [37,38,39,40,41,42].
Table 2 summarizes the joint strength of the composite-to-composite and composite-to-aluminum substrates and compares the current test results with other published studies. The joint strengths of the composite-to-composite and composite-to-aluminum surfaces were 25.3 and 23.9 MPa, correspondingly. This is significantly higher than many of the joint strengths. For the CFRP surface treatments and joint strengths, various methods have been developed. During the autoclave manufacturing of CFRP, peel piles are commonly applied to the composite surface for better bonding. These peel plies, when removed before adhesive bonding, play a crucial role in controlling surface morphology [45]. Some peel piles, impregnated with resin and referred to as wet peel piles, can influence surface chemistry by regulating the volatilization of superficial resin formation during CFRP manufacturing. If further adjustments of surface roughness are made using some techniques such as abrasion and grit blasting along with the peel piles, adhesion strengths can be significantly higher. However, in many cases, industry tends to avoid these steps due to their time-consuming nature and the subsequent need for additional surface cleaning and labor. Also under investigation are advanced pre-treatment technologies utilizing high-energy processes, such as laser ablation [46,47] and high-intensity plasma treatment [48,49], which may substantially improve the bonding properties of similar and dissimilar materials.

4. Conclusions

This study, employing various surface treatment and preparation techniques, investigated the failure modes and joint strengths of single-lap joints bonded with adhesive. The primary goal here was to create a surface with effective wettability and adhesion to facilitate proper adhesive spreading, thereby enhancing the bond strength between similar and dissimilar materials. The findings indicate that the joint strength of a composite-to-composite single-lap joint surpasses that of a composite-to-aluminum single-lap joint. Sanding played a crucial role in elevating joint quality by preparing a suitable surface. Sanding with sandpaper resulted in a roughened surface, increasing the surface area for bonding. This process removed the oily layer from the bonding surface, improved surface reactivity through enhanced mechanical interlocking and surface area, and increased the bond strength by reinforcing intermolecular forces. This study demonstrated that combining different surface preparations yielded superior results in terms of joint strength. For the composite-to-composite single-lap joints, the combination of surface sanding and plasma cleaning produced very high joint strength compared to individual treatments. Optimal results were achieved with a 12 min plasma treatment following surface sanding. In the case of the composite-to-aluminum joints, the composite underwent detergent cleaning surface treatment, and a chromium coating was applied solely to the aluminum. For this type of joint, a combination of surface treatments was necessary to enhance the bond quality. Surface sanding combined with plasma treatment showed positive results. However, the highest bond strength quality was achieved when surface sanding on the aluminum was paired with 8 days of UV treatment. The composite-to-composite bonding with sanding and plasma treatment exhibited a joint strength of 25.3 MPa, whereas composite-to-aluminum bonding with sanded, cleaned, and UV treatment for 8 days showed a joint strength of 23.9 MPa. The failure of composite-to-composite joints is dominated by adhesive failure. In composite-to-aluminum joints, failure is initiated in the adhesive on the aluminum side, and then, the crack propagates to the composite side. The findings from this study suggest that plasma and UV surface treatments can significantly enhance joint strength and reliability, which is crucial in aerospace and automotive applications. Integrating these methods could replace traditional mechanical fastening, providing more durable, stress-resistant joints. This approach may support industry goals of reducing maintenance costs and achieving lighter, fuel-efficient designs. Future research could explore the durability of treated surfaces under environmental stresses, alternative adhesives, or alternative composite materials. These investigations could lead to practical applications, such as improved joint performance in various industrial settings.

Author Contributions

Methodology, N.P.; Formal analysis, M.N.U. and A.S.R.; Investigation, A.S.R. and R.A.; Data curation, N.P.; Writing—original draft, M.N.U.; Project administration, R.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors greatly acknowledge Wichita State University and the National Institute of Aviation Research for the financial and technical support of the present study.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Processes 12 02623 g001
Figure 1. Schematic for a plasma treatment process [24].
Figure 1. Schematic for a plasma treatment process [24].
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Figure 2. Composite-to-composite and composite-to-aluminum single-lap joint process.
Figure 2. Composite-to-composite and composite-to-aluminum single-lap joint process.
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Figure 3. Effect of plasma treatment on wettability of composite surfaces before and after sandpapering.
Figure 3. Effect of plasma treatment on wettability of composite surfaces before and after sandpapering.
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Figure 4. Effects of various surface and plasma treatments on wettability of aluminum alloy surface.
Figure 4. Effects of various surface and plasma treatments on wettability of aluminum alloy surface.
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Figure 5. Stress–strain diagram of composite-to-composite adhesive bonding with sandpaper at various plasma treatment times.
Figure 5. Stress–strain diagram of composite-to-composite adhesive bonding with sandpaper at various plasma treatment times.
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Figure 6. Joint strength of plasma-treated composite-to-composite surface with different surface preparations (CT—detergent cleaned, ST—sand treatment).
Figure 6. Joint strength of plasma-treated composite-to-composite surface with different surface preparations (CT—detergent cleaned, ST—sand treatment).
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Figure 7. Joint strength of composite-to-aluminum surface with different surface preparations. (CT—detergent cleaned, ST—sand treatment, PT—plasma treatment, Coat—Alodine®-coated, UV—UV treatment).
Figure 7. Joint strength of composite-to-aluminum surface with different surface preparations. (CT—detergent cleaned, ST—sand treatment, PT—plasma treatment, Coat—Alodine®-coated, UV—UV treatment).
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Figure 8. Lap shear fracture surfaces: (a,b) composite-to-composite joints and (c,d) composite-to-aluminum joints.
Figure 8. Lap shear fracture surfaces: (a,b) composite-to-composite joints and (c,d) composite-to-aluminum joints.
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Figure 9. Demonstration of lap joint failure modes under axial tensile load: (a–c) composite-to-aluminum and (d,e) composite-to-composite [43,44].
Figure 9. Demonstration of lap joint failure modes under axial tensile load: (a–c) composite-to-aluminum and (d,e) composite-to-composite [43,44].
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Table 1. Surface preparation for joining composite-to-composite and composite-to-aluminum.
Table 1. Surface preparation for joining composite-to-composite and composite-to-aluminum.
Adherent 1Adherent 2Surface Preparation
on Adherent 1		Surface Preparation
on Adherent 2
CompositeCompositeCTCT
ST+CTST+CT
CT+PT (4 min)CT+PT (4 min)
CT+PT (8 min)CT+PT (8 min)
CT+PT (12 min)CT+PT (12 min)
ST+CT+PT (4 min)ST+CT+PT (4 min)
CompositeAluminumCTCT
CTST+CT
CTCoat
CTST+CT+PT (4 min)
CTST+CT+PT (8 min)
CTST+CT+UV (4 days)
CTST+CT+UV (8 days)
CT—detergent cleaned, ST—sand treatment, PT—plasma treatment, Coat—Alodine®-coated, UV—UV treatment.
Table 2. Comparison of current study with contemporary adhesive joint studies.
Table 2. Comparison of current study with contemporary adhesive joint studies.
Composite Surface PreparationMetal SurfaceAdhesiveJoint Strength (MPa)Reference
Sanded with P320 GritNot ApplicableEpoxy23.6[42]
CFRP Selective Laser IrradiationNot ApplicableEpoxy32[43]
Composite-to-
Composite Atmospheric Pressure Plasma Treatment		Not ApplicableEpoxy31.6[13]
Not ApplicableCold Rolled Steel Cleaned with AcetonePoly (ether sulphone), Poly (ether ketone), Poly (ether-ether ketone)8.1[44]
CFRP-CFRP
Sanding and Plasma Treatment		Not ApplicableEpoxy25.3Present Study
CFRP Sanded with P400 GritAluminum Sanded, Cleaned, UV Treatment for 8 DaysEpoxy23.9Present Study
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Paranjpe, N.; Uddin, M.N.; Rahman, A.S.; Asmatulu, R. Effects of Surface Treatment on Adhesive Performance of Composite-to-Composite and Composite-to-Metal Joints. Processes 2024, 12, 2623. https://doi.org/10.3390/pr12122623

AMA Style

Paranjpe N, Uddin MN, Rahman AS, Asmatulu R. Effects of Surface Treatment on Adhesive Performance of Composite-to-Composite and Composite-to-Metal Joints. Processes. 2024; 12(12):2623. https://doi.org/10.3390/pr12122623

Chicago/Turabian Style

Paranjpe, Nikhil, Md. Nizam Uddin, Akm Samsur Rahman, and Ramazan Asmatulu. 2024. "Effects of Surface Treatment on Adhesive Performance of Composite-to-Composite and Composite-to-Metal Joints" Processes 12, no. 12: 2623. https://doi.org/10.3390/pr12122623

APA Style

Paranjpe, N., Uddin, M. N., Rahman, A. S., & Asmatulu, R. (2024). Effects of Surface Treatment on Adhesive Performance of Composite-to-Composite and Composite-to-Metal Joints. Processes, 12(12), 2623. https://doi.org/10.3390/pr12122623

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