This study investigates the effect of different surface treatments on the fracture behavior of epoxy-aluminum joints under mode-I loading. Six surface treatments including degreasing, abrasion with varying grit sizes, alkaline etching, acid etching, a combination of alkaline and acid etching, and a combination of abrasion and acid etching were applied to aluminum surfaces before bonding. Surface morphology, roughness parameters, total surface free energy, contact angle, and elemental composition were analyzed. Results showed that the ultimate fracture load and fracture energy initially increased and then decreased with increasing surface roughness. The most effective treatments were found to be acidic etching in combination with alkaline etching and abrasion due to their synergetic effects. These treatments removed the natural oxide layer and created a porous oxide layer, enhancing surface roughness, increasing adhesive-substrate contact areas, and providing more sites for mechanical interlocking. Compared to control joints, significant improvements were observed, including a 40% and 31% increase in maximum fracture load, a 74% and 53% increase in initiation fracture energy, and a 65% and 47% increase in propagation fracture energy, respectively. Etching treatments demonstrated superior effects on the fracture behavior of aluminum adhesive joints compared to abrasion methods, leading to cohesive failure after etching treatments.
Research Article
A comparative study of the effect of various surface treatments on the fracture behavior of aluminum adhesive joints under mode I loading
https://doi.org/10.21203/rs.3.rs-3149150/v1
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Surface treatment
Aluminum adhesive joints
Mode-I fracture energy
The capacity to connect and combine components is a crucial aspect in the manufacturing procedure. Mechanical fastening through mechanical joining is a commonly utilized method. Nevertheless, there are several drawbacks associated with using these fasteners, such as potential damage to drilling components, increased weight of the structure, and the introduction of substantial localized stresses [1]. Furthermore, considering the utilization of human labor, this procedure is a time-consuming manual approach with a lack of resilience. On the other hand, adhesive joints offer advantages such as uniform stress distribution, enhanced fatigue resistance, reduced structural weight, and the ability to join dissimilar materials. [2, 3]. The factors mentioned above make adhesive joints highly appealing as a valuable and significant method in industries such as aviation, marine, and automotive. However, it is important to note that adhesive joints are typically the weakest point in a structure due to their relatively low fracture tolerance and sensitivity to harsh environments.
Numerous factors, including the mechanical properties of the adhesive material, adhesive thickness, bonding surface condition, and joint design, significantly impact the performance of adhesive joints. To achieve the desired bonding strength, fracture resistance, and long-term durability, surface treatment methods are considered among the most critical factors to be taken into account.
Various processes are utilized for surface preparation and modification which the most popular methods of surface modification and preparation are chemical etching, vapor degreasing, and mechanical abrasion. Mechanical treatment involves roughening the surface through techniques such as grit-blasting or sanding. This roughness often improves adhesion. Nevertheless, the presence of residual debris and mechanical damage on the substrates can have negative effects on bonding, and the durability of these joints is often compromised. Vapor degreasing primarily focuses on eliminating oils and other organic contaminants from the roughened surface. Alkaline degreasing involves immersing the substrate in an appropriate alkaline solution. The purpose of this pretreatment is to eliminate the unstable aluminum oxide/hydroxide film and effectively clean stubborn oils and greases from the bonding surfaces.
Many researchers have considered effects of various surface treatment methods on adhesively bonded joints. Boutar et al. [4] conducted a study on the resistance of an aluminum one-component polyurethane adhesive joint specifically for the automotive industry. They focused on evaluating the influence of surface roughness and adhesive thickness on the joint's performance. According to their findings, there is a surface roughness that offers the best static strength and fatigue life. Zhan et al. [5] examined the impact of surface roughness and pretreatment on the tensile-shear strength of adhesive joints made of 2060 Al-Li alloy. Different amounts of mechanical abrasion and phosphoric acid anodizing were used to treat the substrates, which produced surfaces with varying degrees of surface roughness. The findings demonstrated that with the increase of surface roughness of Al–Li alloy, the tensile-shear strength of the adhesive joints increased, and the failure modes changed from interfacial failure to cohesive failure. Sun et al. [6] conducted a study investigating the impact of different pattern geometries created on the surface using lasers on the fracture behavior of adhesive joints. They found that the presence of longitudinal and transverse grooves on the bonding surface resulted in an increase in the mode-I fracture energy value. In other words, these pattern geometries enhanced the fracture resistance of the adhesive joints. Shokrian et al. [7] examined the impact of aluminum surface treatment on the tensile strength of bonded joints. According to their research, using HCl acid-based etching enhanced the aluminum surface's micro-roughness and nano-texture, which eventually produced the maximum shear strength seen in bonded joints. Gude et al.[8] investigated the correlation between surface characteristics (roughness and surface free energy) and mechanical properties of the adhesive joints. Experimental results indicated that mechanical interlocking was the main adhesion mechanism for the mode I testing, whereas the shear strength did not increase with a rougher surface and the surface free energy exhibited a more significant effect on shear strength. Prolongo et al. [9] examined the effect of various pretreatment methods applied on the surface characteristics of aluminum substrates and on the adhesive strength of epoxy–aluminum joints. Results showed the sulphuric acid-ferric sulphate etch provided an improved joint strength compared to other methods. Musiari et al. [10] assessed the durability of laser-treated treated aluminum bonded joints. They conducted an investigation to evaluate the mechanical properties of aluminum joints treated with various parameter configurations.
The failure of the bonding surface can be prevented by roughening the substrate surface. Surface roughening provides the beneficial mechanical interlocking between the aluminum substrate and adhesive, but over-roughening would reduce the wettability of the substrates; thus, the strength of the joints was weakened. This condition could lead to voids at the bonding interface, accelerating crack propagation and diminishing interfacial force strength [11, 12]. For instance, Boutar conducted a study to examine how surface characteristics affect the strength of adhesively bonded aluminum joints. The results indicated that as the surface roughness of the aluminum increased, the quality of the bonding decreased. In other words, higher surface roughness was associated with a decrease in the strength of the adhesively bonded joints [11]. Da Silva et al. [13] conducted a study to investigate the influence of grooves and scratches on joint strength, comparing the effects on brittle adhesive and ductile adhesives. The results revealed that scratches or grooves created by grinding the aluminum substrate were found to be harmless, and in some cases, they even decreased the maximum joint strength. Liu et al. [14] investigated of surface roughening on the lap shearing strength and failure behavior of adhesively bonded aluminum sheet joints. The results showed that the lap shearing strength of adhesively bonded joints increased and then decreased with the surface roughness of the aluminum substrate. They reported When the surface roughness was too large, the failure mode of the joint turned from the mixed failure mode to the interfacial failure mode, which decreased the strength of the joints.
According to literature, although many efforts have been made to consider effects various types of surface pretreatment on the adhesive bonding properties, a comparative and comprehensive study of effects of various surface treatment on the fracture energy for crack initiation and propagation under mode I loading remains largely unreported. Additionally, it appears that the effectiveness of specific pre-treatments may vary depending on the joint geometry. For instance, some researches show that using sulphuric acid-ferric sulphate-based treatment provides results comparable to those of chromic-sulphuric acid treatment in both, lap shear and peel tests [15], but performs worse in wedge test [16]. Hence, it is important to note that the results obtained from lap shear tests or peel tests, which are commonly used to evaluate joint strength, may not be directly applicable to Double Cantilever Beam (DCB) tests used to assess fracture behavior under mode I loading.
In this study, the effect of various surface treatments on the fracture behavior of aluminum adhesive joints were examined. To run a comparative and comprehensive examination, six various surface treatments including degreasing, abrasion with various grit sizes, alkaline etching, acid etching, the combination of alkaline and acid etching and the combination of abrasion and acidic etching were applied on aluminum surfaces before bonding. Several surface parameters including the surface morphology, surface roughness, surface free energy, and contact angle of aluminum adhesive joints were investigated. Moreover, the elemental composition of specimens was measured by XRD. The fracture energy of aluminum adhesive joints was determined and the effect of surface treatment on the fracture behavior adhesive joint was studied.
2.1. Materials
In this study aluminum alloy 2024 was chosen as a substrate material. The epoxy resin named KER 828 (Kumho P&B Chemicals Inc., Korea) was used as adhesive for bonding the substrates. Furthermore, the curing agent used was triethylenetetramine (TETA) with a weight mixing ratio of 10%.
2.2. Surface treatments
To conduct a comparative study of effects of various surface treatment on the fracture behavior of aluminum adhesive joints, six surface treatments methods were employed, as follow:
1. Solvent degreasing: The degreasing cycle includes 8 min wash soak, 8 min ultrasonic wash, 8 min vapor soak and 8 min dry. The specimens which were treated by this method were considered as the control joints.
2–3) Mechanical Abrasion: The bonding surfaces were sanded with sandpapers with grit sizes of 80 and 400 and then wiped with methyl ethyl ketone (MEK).
4) Alkaline Cleaning: The bonding surfaces were pretreated as in 1 method followed by immersion in 100 g/l NaOH solution, at 60 C for 1 min, and rinsed in tap water.
5) The combination of abrasion and acidic etching treatment: The bonding surfaces were pretreated as in method 2 (abraded with a 400 grit size) followed by immersion in a water solution with 185 ml/l sulphuric acid (97%) and 127 g/l ferric sulphate, at 65 C for 8 min, and rinsed in tap water (percentages by weight: 48% H2O, 37% H2SO4 and 15% FeSO4).
6) The combination of acidic and alkaline etching treatment: The bonding surfaces were pretreated as in method 4 followed by immersion in a water solution with 185 ml/l sulphuric acid (97%) and 127 g/l ferric sulphate, at 65 C for 8 min, and rinsed in tap water (percentages by weight: 48% H2O, 37% H2SO4 and 15% FeSO4).
The details of the surface treatments investigated are listed in Table 1.
|
Number |
Denotation |
Surface treatments type |
|---|---|---|
|
1 |
Control |
Solvent degreasing |
|
2 |
MA-60 |
Mechanical abrasion |
|
3 |
MA-400 |
Mechanical abrasion |
|
4 |
AC |
Solvent degreasing + Alkaline cleaning |
|
5 |
ET- AC |
Alkaline cleaning + Etching treatment |
|
6 |
ET-MA |
Mechanical abrasion + Etching treatment |
2.3. Measurements of surface characteristics
The surface morphology of the prepared substrates was studied by scanning electron microscopy method (SEM, VEGA/TESCAN) equipped with energy dispersive x-ray spectroscopic analysis (EDS). Although the SEM images can provide a useful results of the surface topography, they are incapable of providing an accurate evaluation of the surface roughness levels. To assess the surface roughness of the samples throughout this study, roughness parameters Ra and Rz and Rq for the substrate were measured. Ra represents the arithmetic mean of the absolute values of the longitudinal coordinates of the roughness profiles, Rz represents the arithmetic mean value of the individual roughness depth of successive sampling lengths and Rq represents the root-mean square roughness. Two surface roughness parameters were measured parallel to load direction using a profilometer (TR-200Plus) to further evaluate the surface of the specimens. The measuring principle is based on probing the surface with an inductive stylus with a tip of a radius of 2 µm and a measuring force is 0.75 mN. Before the test, the samples were placed on the testing platform and fixed with a clamping device. For each sample, five points were selected for testing, each point was tested for three times and average value was regarded as the ultimate result.
Ordinarily, the considerable level of wettability of the aluminum substrate surface is a precondition for good adhesion. It is generally determined by the surface free energy of the substrates to be bonded. The contact angle measurement method was adopted to compare the change of surface free energy of the aluminum substrates after various surface treatments. The static contact angles of different specimens were measured by a goniometer VCA Optima by AST Products, Inc, using distilled water as the polar liquid and diiodomethane as a nonpolar liquid at the temperature and humidity of 25◦C and 30%, respectively. At least three valid measurements for each specimen and liquid were recorded and the average angle measurements for both liquids were introduced in the SE2500 software which returned the values for the surface energies of each specimen according to the Owens-Wendt (OW) method [17].
2.4. Fabrication and testing of double-cantilever beam specimens
To fabricate DCB adhesive joints, aluminum substrates with the dimensions of 200 × 25 mm2 were bonded using the KER 828 epoxy resin. In these joints, the bondline thickness was controlled by placing 0.6 mm steel wires at the two ends the lower substrate. A 15 µm thick aluminum film was placed on the adhesive surface to act as an initial crack in the joint, resulting in an initial crack length 80 mm from the load line. An appropriate fixture was employed in order to apply a controlled pressure on the joints during the curing process by two adjustable springs. The curing cycle for the adhesive was 60 ◦C for 90 min. After curing, the excess adhesive on the sides of the joints was removed. The dimensions of DCB specimens are shown in Fig. 1. To prepare specimens for quasi-static tensile tests, 2 piano hinges were bonded to the ends of the substrates to apply the opening load on the specimens. One lateral side of each joint was painted with a fine white spray to form a uniform and thin layer on the substrate surface, resulting in an easier detection of crack propagation during mode I testing.
The DCB specimens were tested according to standard ISO 25217 [18] to determine Mode-I fracture energy (G1C). Tests were carried out under displacement control at a displacement rate of 2.5 mm/min using the SANTAM STM-150 universal testing machine. Figure 2 shows the test setup for DCB specimens. The crack length (a) was monitored using a traveling optical microscope and the corresponding load and displacement were recorded simultaneously by the testing machine. The G1C values for the onset of crack propagation were measured using the simple beam theory. At least five specimens were tested in each test case to determine the average G1C.
3.1. Load-displacement behavior
The representative load-displacement behavior of the adhesive joints prepared with various surface treatment methods is shown in Fig. 3. Moreover, Fig. 4 shows the average maximum load for different specimens. As can be seen in Fig. 3, firstly, the load- displacement behavior associated with all specimens experienced a strong fluctuation as the load passed the peak value. This behavior usually displays an unstable crack propagation during failure called stick-slip behavior. From the data in Fig. 4, several observations can be made. Firstly, among all the joints, the ET-AC and ET-MA joints, which produced similar loading traces exhibited the greatest peak load values. These joints experience 31% and 40% improvement in the maximum load value compared to the control joint, respectively. Secondly, for the prepared specimens using MA- 400 and AC methods, 16% and 27% increases in the load level obtained in comparison with the control joints, respectively. However, the load carrying capacity for specimens treated with 60 grit size (MA- 60) decreased considerably and the peak load value reached 43.5 N. These specimens experienced a 30% decrease in the load level.
3.2. Fracture energy
3.2.1. Fracture energy in crack initiation
According to standard ISO 25217, simple beam theory (SBT) can be employed for the determining mode-I fracture energy (G1C) when the crack growth follows a stick-slip pattern. Therefore, as in this study stick-slip pattern was observed as the dominant crack growth scheme, the simple beam theory was utilized for obtaining the fracture energy. According to the simple beam theory, the fracture energy (GIC) is obtained using Eq. (1).
$${G}_{IC}=\frac{4 {P}^{2}}{E{B}^{2}} \left(\frac{3{a}^{2}}{{h}^{3}}+\frac{1}{h}\right) \left(1\right)$$
where P is the applied force, E is the elasticity modulus of substrates, b is the substrate width, h is the substrate thickness and a is the crack length. The initiation GIC values for the joints manufactured with different treatments are shown in Fig. 5. As can be seen in Fig. 5, the critical strain energy release rates, GIC, for the all specimens (except MA- 60) was higher than the values obtained for the control joints. The differences obtained between fracture energy at crack initiation of the specimens tested on the variously pretreated surfaces highlight the importance of the topography of the surfaces. The value of initiation GIC for the degreased joints was 0.75 ± 0.06 N/mm, which was considered the reference value in this study. For the specimens prepared with AC and MA- 400 methods, 22% and 42% increase in the initiation GIC obtained compared to the control joints, respectively. The results showed that the acidic etching method in the combination with the alkaline etching and abrasion (ET- AC and ET- MA) were the most effective treatment methods to improve the initiation GIC. These specimens experienced 53% and 74% improvement in the initiation GIC values compared to control joints, respectively. The lowest value of initiation GIC obtained for specimens abraded with a grit size of 60 (MA- 60). A decrease of 17% was observed in the value of decreased GIC of these samples compared to the degreased specimens.
3.2.2. Fracture energy in crack propagation
The mode-I fracture energy during crack propagation was attained for the specimens prepared with various surface treatment methods using Eq. 1. Furthermore, different specimens were compared using resistance curves, which indicated a relationship between crack length and GIC. The results of mode-I fracture energy are shown in Fig. 6. which shows the averages and standard deviations of GIC propagation, calculated based on resistance curves. According to Fig. 6, fracture energy values during crack propagation followed the same pattern as the fracture energy in crack initiation. The value of propagation fracture energy (Propagation GIC) for the control joints was 0.86 ± 0.11 mm/N, which was the lowest value among all the joints prepared with various treatments (except MA- 60 joints). The value of propagation GIC for the ET- MA joints was 1.42 ± 0.12 N/mm, an increase of about 65% compared to the control joints. ET- AC joints also remarkably elevated the propagation fracture energy with a relative increase of 47%. The AC and the MA- 400 treatment had an improvement of 14% and 37% over the control joints respectively. However, the lowest propagation GIC value of 0.74 ± 0.09 was obtained for the surfaces abraded with 60 grit size (MA- 60), which showed a decrease of 14%, compared to the control joints. Table 2 presents the values and percentages changes in the maximum loads and fracture energies of different specimens compared to the degreased specimen.
3.3. Effect of pre-treatment on aluminum surface
The effectiveness of the substrate treatments can depend on the nature of aluminum alloy. In this work aluminum alloy 2024 was employed as the substrate for the fabrication of joints. Table 3 indicates the elemental compositions of aluminum alloy 2024 containing Mg and Cu as main alloying elements.
|
Elements |
Al |
Cu |
Mg |
Fe |
Si |
Others |
|---|---|---|---|---|---|---|
|
A2024 |
92.88 |
4.67 |
1.34 |
0.25 |
0.15 |
0.71 |
Most chemical pre-treatments applied to aluminum substrates are based on corrosion processes from acid or alkaline environments in order to dissolve the natural alumina layer, to clean and activate the aluminum surface, and to help in generation of a new porous alumina layer that will well adhere. In general, due to the presence of elements or compounds with different electrochemical potentials the corrosion susceptibility A2000 series alloys is high [19]. This implies that A2024 alloy should be affected by chemical etching. The surface morphology of the treated A2024 aluminum alloy samples was studied by SEM/EDS analysis and the results obtained are displayed in Fig. 7–8. Moreover, the surface elemental composition of the samples prepared with various surface treatment is shown in Table 4. According to Fig. 7- a, it can be seen that the surface of the degreased alloy (control) is approximately smooth and only shallow scratches and rolling lines arising from thermo-mechanical processing of the alloy are visible. According to Table 4, the low O content on the degreased aluminum samples indicated that a thin oxide layer (Al2O3) covered the aluminum surface [19]. The abrasion (MA- 60 and MA- 400) did not affect profoundly to the elemental composition of aluminum alloy according to Table 4. This is because the abrasion process is a mechanical treatment to increase the surface roughness without modifying the porosity or composition of the samples. As can be seen in Fig. 7-b and c more roughened surface obtained for MA- 60 and MA-400 specimens compared to the control joints. It can be seen that the intermetallic particles became more visible as a result of alkaline etching and these specimens revealed a scalloped appearance (Fig. 8-a). Moreover, according to Table 4, the O content increased after alkaline etching compared to degreased aluminum alloy that may be attributed to the corrosion products during alkaline etching [9]. Also, the concentration of intermetallic particles i.e. Fe and Si significantly increased after alkaline etching. The increase of intermetallic particles contents on the aluminum surface after alkaline etching indicates that the oxide layer was considerably destroyed. For specimens prepared with acid etching (Fig. 8- b and c) a homogenous surfaces obtained and less intermetallic particles with smaller size were observed. The low oxygen concentration measured on aluminum alloy and increase in Al content confirms the low thickness of the formed alumina layer [9]. In principle, the generation of a higher porous oxide layer could imply a high adhesive strength, increasing the contact area and providing some degree of mechanical interlocking [19]. Using acid etching after alkaline solution (ET- AC) removed surface intermetallic and the corrosion products formed on the aluminum surface after alkaline etching. As a result, the concentration of intermetallic particles i.e. Si and Fe decreased and the concentration of Al increased.
|
Element % |
Al |
O |
Si |
Fe |
Cu |
Mn |
Mg |
|---|---|---|---|---|---|---|---|
|
Control |
83.0 |
2.4 |
0.1 |
0.7 |
11.3 |
0.9 |
1.8 |
|
MA- 60 |
83.7 |
2.5 |
0.1 |
1.0 |
9.6 |
1.1 |
2.0 |
|
MA- 400 |
83.9 |
2.6 |
0.1 |
0.9 |
9.5 |
1.0 |
1.9 |
|
AC |
71.3 |
6.2 |
1.1 |
3.2 |
13.3 |
2.0 |
2.9 |
|
ET- AC |
87.7 |
2.9 |
0.5 |
- |
5.9 |
0.9 |
2.1 |
|
ET- MA |
88.7 |
1.7 |
0.4 |
1.6 |
4.2 |
1.1 |
2.2 |
Moreover, Table 5provides information about surface roughness parameters specimens prepared with various surface treatments. As can be seen, the lowest surface roughness obtained for control specimens (0.32 ± 0.09 µm). Mechanical abrasion increased the surface roughness and the maximum surface roughness parameters obtained for MA- 60 specimens (2.1 ± 0.28 µm). For AC specimens 0.97 ± 0.21 µm value of surface roughness obtained. Finally, for specimens prepared with the acidic etching in combination with alkaline solution and abrasion 1.18 ± 0.21 µm and 1.69 ± 0.17 µm value of surface roughness obtained, indicating the generation of alumina layer with more porosity on the aluminum surface. In addition, Fig. 8. shows the effect of surface roughness on the fracture energy of aluminum adhesive joints prepared with various surface treatments. According to Fig. 9, the fracture energy values increased with an improvement in surface roughness value and then increased. The lowest fracture energy value obtained for the MA- 60 specimens the had the highest surface roughness parameters. Clearly, it is possible to conclude that an increase in roughness may not necessarily increase the fracture energy and load-bearing capacity of the joint.
|
Specimens |
Ra (µm) |
Rq (µm) |
Rz (µm) |
|---|---|---|---|
|
Control |
0.32 ± 0.09 |
0.68 ± 0.12 |
2.88 ± 0.26 |
|
MA- 60 |
2.1 ± 0.28 |
2.65 ± 0.38 |
12.96 ± 0.41 |
|
MA- 400 |
0.88 ± 0.11 |
1.10 ± 0.15 |
6.22 ± 0.30 |
|
AC |
0.97 ± 0.21 |
1.28 ± 0.30 |
6.95 ± 0.35 |
|
ET-AC |
1.18 ± 0.21 |
1.54 ± 0.30 |
7.45 ± 0.35 |
|
ET- MA |
1.69 ± 0.17 |
1.98 ± 0.25 |
10.45 ± 0.50 |
3.4. Wettability
The surface free energy was determined using the Owens–Wendt method, which is based on direct measurements of the contact angle. The surface free energy was calculated using the Owens-Wendt method Eq. 2. To determine the contact angle, two measuring liquids (non-polar and polar) with known polar and dispersion component values were used. With the consideration of the surface tension balance equation of the solid–liquid–gas triple point (Eq. 3), total surface free energy (σs) and its component including polar component σsP dispersive component σsD were calculated.
$${\gamma }_{sl}={\sigma }_{s}+{\sigma }_{l}-2(\sqrt{{\sigma }_{s}^{D}+{\sigma }_{l}^{D}}+\sqrt{{\sigma }_{s}^{P}+{\sigma }_{l}^{P}} (2)$$
$${\sigma }_{s}={\gamma }_{sl}+{\sigma }_{l}*Cos\left(\theta \right) \left(3\right)$$
where θ is the contact angle, and γsl is surface tension of solid–liquid phase interface, and σs and σ1 correspond to the surface free energy of the substrate and the surface tension of the liquid, respectively. The superscripts D and P represent the dispersive and polar fractions, respectively, and the total surface energy is the sum of both fractions. The contact angles measured from the reference distilled water drop placed on different substrate surfaces after application of surface preparation are shown in Fig. 10. Moreover, Figs. 11 provides information the average contact angle and total surface energy of the bonding surfaces for various specimens. Clearly, the surface contact angles of the samples tend to increase first and then decrease gradually when the etching treatments were employed. According to Fig. 11, a contact angle of 90.2₀ was obtained for the control specimens. The MA- 60 experienced the largest contact angle value, however, ET-MA specimens had the lowest contact angle value. In other words, the surface wettability of the specimens was affected by the surface roughness. In contrast, the total surface free energy appears to represent the trend opposite to that of the surface contact angle. The theory of adhesive adsorption holds that the greater the surface free energy of the specimens, the stronger the bonding force with the external matter, a higher energy barrier at the interface for crack initiation and propagation and consequently, the better the bonding effect. Therefore, in terms of adsorption theory, the load bearing capacity and fracture energy specimens should decrease first and increase gradually with changing treatments from abrasion to etching. According to Table 2, MA- 60 specimens (with the lowest the lowest value of total surface free energy) experienced about 30% and 17.3% decrease in the load bearing capacity and fracture energy compare to the control joint. However, for ET- MA specimens that had the largest total surface free energy (45.7mJ/m2) a 40.3% and 74.6% improvement in in the load bearing capacity and fracture energy obtained in comparison to the control joint.
4.5. Fracture surface
The macroscopic features of the fracture surfaces of the specimens prepared with various surface treatments is shown in Fig. 12., where it is clearly visible that various failure modes obtained for the different surface treatments. For the control joints which the bonding surface was degreased a combination of adhesive and cohesive failure was observed (Fig. 12-a). As was shown in Fig. 7-a, for these specimens the bonding surface was smooth and the lowest surface roughness (0.32 ± 0.09 µm) obtained. The predominant failure mode for MA-60 specimens was adhesive failure indicating weak adhesion between the adhesive and aluminum substrate. As was shown above, the surface bonding of these specimens was over- roughened and the highest surface roughness values was obtained for them. Although, a moderate increase in the surface roughness can improve the mechanical interlocking and bonding area (As was seen for MA-400 specimens (Fig-12-c) which experienced a cohesive/adhesive failure mode and 15.9% and 22.6% increase in the maximum load and fracture energy compare to the control joint.), over- roughening can weaken the interlocking effect of the surface features. This is due to that the over-roughened surface will store more gas and when the adhesive gradually infiltrates the metal surface, it is not easy for the gas to escape from the interface. As a result, air bubbles forms at the interface between the adhesive and the metal surface, which prevented the adhesive integrating the metal surface further and causes the wettability decreases. In this way, the interlocking effect is also weakened [20]. After using alkaline etching (AC specimens), the failure mode shifted from adhesive/cohesive failure for the control joints to cohesive failure mode which is desirable and indicating good bonding at the interface (Fig. 12-d). These specimens experienced a 27.3% and 42.6% improvements in the failure load and fracture energy compare to the control joints. As was shown in Table 3, the changing in the elemental composition of aluminum surface obtained for AC specimens indicating using alkaline etching destroyed the natural aluminum layer and created a more rugged and bumpy bonding surface compare to mechanical abrasion. The predominant failure mode for the specimens prepared with acid etching was cohesive failure mode (Fig. 12-e and f). For these specimens, according to Table 3, after destroying the natural alumina film a more porous oxide layer (surface roughness parameter increased for acid etched specimens compare to control joints with a natural oxide layer) was formed on aluminum surface. Therefore, the presence of this porous layer on surface and the creation of a moderate surface bonding improved adhesive bonding at the interface. As was seen, the largest improvement in the maximum failure load and fracture energy values obtained for ET- MA specimens. This is because the moderate abrasion process would significantly increase the surface roughness, while the etching process will allow a uniform oxide layer which has open porous structures to form on the surfaces [6]. Both process have synergetic effect and consequently, would increase adhesive - substrate contact areas and provide more sites for mechanical interlocking to occur.
In this work, effects of various treatments including degreasing, abrasion, alkaline etching, acidic etching and the combination of them on the mode I fracture energy of aluminum adhesive joints were investigated experimentally. Double cantilever beam specimens were fabricated using aluminum-epoxy substrates and they were tested according to ISO 25217. It was shown that various treatments applied on the bonding surfaces of aluminum affected surface characteristics. It mainly influenced on the surface roughness, elemental compositions and aspect and porosity of oxide layer formed on the aluminum surface. Examination of surface characteristics showed that surface morphology and wettability play a crucial role in the fracture behavior of adhesive aluminum joints. The mechanical abrasion methods showed more roughened surface without changing in the elemental compositions compared to etching treatments and the maximum surface roughness (2.1 ± 0.28) obtained for MA- 60 specimens. However, the lowest ultimate fracture load and fracture energy obtained for them, a 30% and 17.3% decrease compare to the control joints, respectively. The results showed that applying etching methods improved the wettability and quality of bonding and consequently higher load bearing capacity and fracture energy. This was associate to destroying natural oxide layer and the creation of a porous oxide layer due to the presence of allowing elements, such as Cu or Mg that enhances the corrosion processes. After applying etching methods, the contact angles decreased from 90.6 for the control joint to 64.4 for the ET-MA specimens and total surface free energy increased from 38.3 to 45.7. The DCB test results indicated that the combination of mechanical abrasion and acidic etching had a synergetic effect on the surface characteristics of aluminum bonding surface and the maximum fracture load and fracture energy obtained for this specimens, a 40.3% and 74.6% improvement compare to the control joint. This is due to improvement of adhesive - substrate contact areas and the formation more sites for mechanical interlocking to occur.
Funding
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
Competing Interests
The authors have no relevant financial or non-financial interests to disclose.
Author Contributions
R.N. and M.M.H contributed equally to this work.
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