- Email: [email protected]
CFRP adhesive joints
Journal Pre-proof Investigation of the differences between photochemical and photothermal laser ablation on the shear strength of CFRP/CFRP adhesive joints Erhan Akman, Yalçın Erdoğan, Mustafa Özgür Bora, Onur Çoban, Belgin Genc Oztoprak, Arif Demir PII:
S0143-7496(20)30008-7
DOI:
https://doi.org/10.1016/j.ijadhadh.2020.102548
Reference:
JAAD 102548
To appear in:
International Journal of Adhesion and Adhesives
Revised Date:
4 December 2019
Accepted Date: 9 January 2020
Please cite this article as: Akman E, Erdoğan Y, Bora MÖ, Çoban O, Oztoprak BG, Demir A, Investigation of the differences between photochemical and photothermal laser ablation on the shear strength of CFRP/CFRP adhesive joints, International Journal of Adhesion and Adhesives, https:// doi.org/10.1016/j.ijadhadh.2020.102548. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Investigation of the differences between photochemical and photothermal laser ablation on the shear strength of CFRP/CFRP adhesive joints Erhan Akmana,*, Yalçın Erdoğanb, Mustafa Özgür Boraa, Onur Çobana, Belgin Genc Oztopraka, Arif Demirc,d a
Aviation Materials Research and Development Laboratory, Faculty of Aeronautics and Astronautics, Kocaeli University, Kocaeli 41285, Turkey b
Electro Optic System Engineering, Kocaeli University, Kocaeli 41380, Turkey c
d
Department of Physics, Kocaeli University, Kocaeli 41380, Turkey
BEAM Ar-Ge Optics, Laser and Spectroscopy, KOU Technopark, Kocaeli 41275, Turkey
Abstract Ensure high bonding strength and durability is very important for composite materials that are widely used by many different industries. In order to achieve this, the surfaces of the composite materials should be prepared for the adhesion process. Surface preparation with laser is a promising technique for the adhesive joint. However, the diversity offered by the advances in laser technologies and the multiplicity of parameters that must be controlled in laser material processing makes the process somewhat complex. In the present study, two different lasers have been used with different wavelengths (355 nm (UV) and 10600 nm (IR)) and pulse durations (4,4 ns and 5 µs), to increase the lap shear strength of the adhesive joint of carbon fiber reinforced polymer (CFRP) materials. While the ultraviolet (UV) laser with high photon energy creates photo-chemical ablation, infrared laser (IR) causes photo-thermal ablation due to their low photon energy. CFRP samples treated with different lasers and the accumulated laser fluences have been adhesively joined and the results evaluated experimentally. According to the results, differences in laser ablation mechanism cause structural differences on CFRP surfaces. While resolidified epoxy was observed on CO2 laser treated surfaces, no residues were detected on UV laser treated surfaces. However, to avoid delamination or fiber damage risks an optimization of laser parameters are recommended to be performed for both laser systems. Keywords: CFRP; Adhesive bonding, Laser surface pre-treatment
*Corresponding Author: E. Akman, Tel:+902623513310-303, Email: [email protected]
1. Introduction Superior features of the carbon fiber reinforced polymers (CFRP) such as high strength, specific stiffness, density and small thermal expansion coefficient, low price and availability etc. ensure that they are preferred in the aviation and automotive industries [1, 2]. If the structure to be used has a complex geometry, the composite materials should be joined using screws or rivets as conventional methods. But to use screws or rivets the composite structure should be drilled which represents is one of the most important machining processes on composites [3]. Drilling of the composite materials causes stress concentrations and reduces structural integrity [2, 4]. Adhesive bonding is an alternative technique that offers advantages such as lower structural weight, lower manufacturing cost and improved damage tolerance [4]. However, adhesive bonding has also included challenges such as surface preparation of the adherend to obtain robust and predictable bonds [5]. A suitable adherend surface for adhesion is defined as: free from contaminants, lack of weak border layers, with surface roughness that can provide mechanical interlocking and a surface with enhanced wetting [6]. Generally, contaminations such as fluorine and silicon on CFRP surface originate from mold release agents that are applied during the manufacturing process [7]. As an alternative to traditional methods such as sandblasting, abrading etc., due to better controllability, non-contact, ease of automation and environmental friendly, laser surface treatment has become an important technique [7, 8]. CFRP composites are made from materials with different physical properties, causing differentiation also in absorption properties and thermal conduction [9]. Therefore, lots of experiments have been conducted with different wavelengths (Ultraviolet to Infrared) and operation modes of laser as pulsed or continuous irradiation [8-10]. The differences in wavelength and the operation mode especially pulse duration cause a difference in the mechanism of ablation. While the UV laser with high photon energy creates photo-chemical ablation determined as cold ablation, IR laser
causes photo-thermal ablation due to their low photon energy [11]. During the photo-thermal ablation, the heat is conducted to the bulk of the matrix material by the fibers, leading to a large heat affected zone (HAZ) where the fiber/matrix interface is weakened by thermal degradation of the polymer [12]. This effect can be minimized using pulsed mode laser with optimized parameters [13]. When the pulse duration is as short as that of femtosecond lasers, high precision modification of CFRP materials with minimal thermal damage is possible [12]. When the studies described in the literature are examined, it seems that many studies have been done to determine the most suitable laser features. In their study Nattapat et al. have used continuous wave (CW) carbon dioxide (CO2) laser in spite of thermally acting tools [14], due to the comparatively lower price and operating costs of low power CW CO2 laser system. According to their results it is possible to remove the resin layer without exceeding the damage threshold of the underlying fibers. And, the adhesive joint strength of laser-treated surfaces has been increased compared to sanded surfaces. Fischer et al. have used pulsed CO2 and UV laser to investigate heat deposit and absorption behavior at the two wavelengths [9]. According to their results the shear strengths of the pre-treated specimens are in the same range as the abraded reference specimens, so both lasers are a suitable method for the surface pre-treatment of CFRP material to improve the adhesive bonding strength. But they emphasize the risk of delamination because the CO2 laser deposits more heat to the material. Another study described by Fischer et al. [2] an excimer laser with 308 nm wavelength has been used to prepare the CFRP surface for adhesive joint. They used different laser energies and pulse overlap to increase the shear strength. The results obtained indicate that the tensile strength of the reference sample is reached or even exceeded. They draw also attention to the risk of delamination when inappropriate parameters are used in their study [2].
When the studies in the literature are examined, it has been observed that the adhesive strength of the samples treated with different lasers is sometimes in the same range as the abraded ones and sometimes exceeding. Although, it is stated that the UV laser is more suitable for surface treatment of composite materials due to the photo-chemical ablation mechanism, there is no notable difference in the test results. These results reveal the need for optimization of laser parameters. In this study two different pulsed lasers, III. Harmonic of Nd:YAG (λ=355 nm, as UV) and CO2 (λ=10600 nm, as IR)) were used to prepare the CFRP surfaces for adhesive joint. The focus of the study is determining the suitable accumulated laser fluences which should create optimum surface structure without damaging the carbon fibers for both wavelengths and also reasons for achieving higher shear strength of photo-chemical ablation mechanism.
2. Materials and methods In this study, unidirectional carbon fiber-reinforced epoxy composite [0]5 laminates (CFRP) were used. CFRP laminates were supplied from Kompozitsan, Izmir/Turkey. Fabricated CFRP laminates were manufactured by using resin transfer molding (RTM) method. The nominal thickness of the CFRP laminates is 1,8 mm. The CFRP characteristics were the following: the diameter of fibers = 7 µm, number of prepeg layers = 5, fiber areal weight = 245 g/m2, prepreg fabric type = unidirectional, carbon fiber mesh, type of resin = F-1564 epoxy resin, glass transition temperature of the resin = 80 ºC. Two different lasers were used for the surface treatment of the laminates. The CO2 laser (IR) which emits pulses of 5-400 µs duration range, repetition rate range between 5 kHz and 100 kHz and 10600 nm wavelength, with low photon energy (0,12 eV). To ablate the desired surface area on the samples with CO2 laser, a galvanometric system was used which provides the computer controlled movement of the laser beam, together with a 160 mm focal length F-Theta lens to focus the laser beam.
During the surface treatment, plates were fixed at the focal point of the F-Theta lens, at this condition spot diameter of the beam was approximately 200 µm. The III. harmonic of the Nd:YAG laser (UV) with relatively high photon energy (3,50 eV) produces pulses at a pulse length of 4,4 ns and 20 Hz repetition rate. Two axis computer control linear motion system was used during the Nd:YAG laser treatment of the laminates. III. Harmonic of the Nd:YAG laser beam was irradiated on the surface of the material without focus. Under these conditions, the laser beam spot area on the material surface is approximately 5 mm2. In both systems, an area of 25,4 mm x 25,4 mm was irradiated in air by scanning the surface as parallel lines with a spot size and travel speed as described in Fig. 1. To compare the laser treatment effect, reference CFRP laminates were prepared by roughening their surface with sandpaper 220 grit size. For the characterization of all prepared CFRP surfaces, surface topographies and contact angles were obtained by non-contact laser profilometer and homemade contact angle goniometer, works static contact angle measurement method, with Drop Shape Analysis plugin [15], respectively. The measurement of the contact angle was repeated five times with using pure water for each sample to obtain the statistical result. Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) measurements were also performed to determine the chemical changes occuring on the CFRP surfaces due to the laser treatments. Two component structural epoxy paste adhesive (Loctite Hysol ® EA 9396TM) was used to bond the CFRP laminates. Curing temperature was 66 ºC with curing duration as 60 min. in an oven at atmospheric pressure. A fixture consisting of three axis linear translation stages was prepared to provide precision control of the bondline as seen in Fig. 1. The bondline tolerance of the samples using this fixture was ±10 µm. Optical microscope images of bondline thickness are available in our previous study [11]. The effect of the accumulated laser fluence and wavelength on the bonding strength of CFRP/CFRP adhesive bonding was
investigated by using a lap shear test according to ASTM D5868-01 standard. By adding to the grips (Fig. 1-c) on both sides of the test sample, the load transferred along the bondline region which prevented the bending load effect to the sample. Dartec universal test machine was used which had a capacity of 60 kN for determining the adhesive shear strength of different accumulated laser fluences and wavelengths. Test speed was selected as 13 mm/min. After mechanical tests, camera was used to observe the failure modes which occurred on the CFRP composite surfaces. All the tests in the study were repeated five times using for each sample group to obtain the statistical results.
Figure 1. Experimental setup of the CO2 laser treatment (a) and UV laser treatment (b) of the CFRP surfaces, the bonding fixture (c) used for the adhesive joint and set prepared for lap shear test.
3. Results and discussion From the literature, it can be seen that there are many parameters that affect the strength of the adhesive
bonding
such
material, bondline thickness,
as
nature
contamination,
of stress
the and
adhesive
and
environmental
the adherend factors
and
mostly surface treatment of the adherend [6]. The bonding thickness was determined as the
first step. As mentioned in our previous study, the bondline thickness of the adhesive was changed between 30-500 µm, and the highest strength was obtained at 50 µm [11] for CFRP/CFRP adhesive bonding. Therefore, all samples were jointed at a bonding thickness of 50 µm. During the surface preparation of the CFRP for the adhesive joint, the main parameter is the accumulated laser fluence which determines the ablation rate of the samples. While the laser fluences that are not sufficient does not clean and treated the surface as much as necessary, the accumulated laser fluences overload may cause delamination [9]. In our experiment, the accumulated laser fluences were changed between 6-33 J/cm2 for the CO2 laser and 160 mJ/cm2 - 480 mJ/cm2 for the UV laser. The SEM microscope images of the surfaces treated with different fluences and the wavelengths are shown in Fig. 2.
Figure 2. SEM images of the samples Untreated (a), Abraded (b) treated using UV (c), (d), (e) and CO2 (f), (g), (h) lasers with different accumulated fluences. Sa values in the pictures indicate the roughness of the surfaces.
As seen in Fig. 2-b, it appears that there are damages which disturb the continuity of the fibers at the surface of the abraded samples. The effect of the different accumulated fluence on the surfaces was tried to determine by calculating the area of the exposed fibers. As seen in SEM images for both of the wavelengths, at the low laser fluences (Fig. 2-c-f) the area (5,5% for UV laser, 7% for IR laser) of the exposed fiber is small (white arrows). An increase in the accumulated laser fluence caused the increase in the area (68 % for UV laser, 79 % for IR laser) of exposed fibers (white arrows) as expected (Fig. 2-d-g). When the fluence increment was continued, while the area of the exposed fibers reached to % 94,1 (white arrows in Fig. 2e) on the surface of the sample treated using UV laser, all of the fibers were exposed on the sample treated using IR laser (Fig. 2-h). As shown in Fig. 2-e-h, in the samples treated at a high accumulated fluence of both lasers, it is seen that the epoxy around the fibers which holds the fibers together is partially (black arrows) removed. In this case, it is believed that the fibers cannot adhere to the bulk structure and the risk of delamination can be observed. On the other, it was determined in our previous article that the increase in laser fluence reduces the surface contamination rate [11]. Also, the roughness of the treated surfaces at both wavelengths showed an increasing trend according to untreated sample (see Fig 2-a) with the increment of laser fluences excluding 6 J/cm2 in which the roughness reduced less than 0.5 µm due to the polymer polishing effect (see Fig. 2-f).
When the single lap shear strength test results have been investigated, the shear strength of the samples treated with UV laser showed an increasing tendency until the 400 mJ/cm2 with the laser fluence. After this point, a small decrease was observed as shown in Figure 3-a. The shear strength of the samples treated with IR laser is significantly reduced after 9 J/cm2 laser fluence. This situation is interpreted as degrading the epoxy matrix (see Fig 2-e-h), which binds the fiber layers together and showed more effect on samples prepared using CO2 laser
wavelength due to the photothermal ablation behavior as seen in Fig 3. Ejection of the epoxy matrix between the fibers can be considered as the upper limit of laser ablation since, after this point, fiber damages have begun to be observed [11]. 30 100
Shear strength Contact angle
25
60 15 40
10
Contact angle (º)
Shear strength (MPa)
80 20
20
5
0
0 Untreated Abraded
160
240
330
360
400
440
480
Laser fluences (mJ/cm2)
a) 30 Shear Strength Contact Angle
100
25
60 15 40
10
Contact angle (º)
Shear strength (MPa)
80 20
20
5
0
0 Untreated
b)
Abraded
6 9 Laser fluences (J/cm2)
18
33
Figure 3. Shear strength and contact angle variation of the samples prepared with different laser fluence for a) UV and b) CO2 laser.
One of the important tests that provide information about the suitability of the surfaces to the adhesive joint is the surface contact angle measurement. According to the literature, it is stated that the increase in wettability of the surfaces affects the adhesion strength positively [6]. Surface chemistry, roughness and pollution affect the contact angle of the surfaces. Oxygen content on the surface increases wettability and therefore adhesion at fiber interfaces. On the other hand, contact angle is affected by the change in surface roughness in accordance with the nature of the surface itself [16]. Surface contamination, which is generally caused by fluorine and silicon, adversely affects surface energy and reduces adhesion [11]. The contact angle of the abraded sample increased due to roughness compared to the untreated sample. When the contact angle results of the surfaces prepared using UV laser were examined (Fig. 3a), the contact angle increased with the increase in laser fluence. This increment was continued until 330 mJ/cm2 laser fluence and then started to decrease and it has reached the lowest value at the 400 mJ/cm2 laser fluence. At this point, the contact angle increased slightly after the increase in the laser fluence and remained constant thereafter. This situation can be explained as; laser fluence increment up to 330 mJ/cm2 causes the obtained further rough surfaces that enhance the surface hydrophobicity. However, at the 400 mJ/cm2 and above laser fluences the combined effects from the surface roughness, removal of surface contaminants and the change of the surface chemistry led to the behavior of the contact angle in the graph (Fig. 3-b) [7]. On the other hand, while the contact angles were approximately same with the untreated sample at the low accumulated laser fluence of CO2 treated surfaces, 33 J/cm2 laser fluence caused a reduction on the contact angle of the CFRP surfaces. This change in contact angle can be explained by the removal of the contamination [11] and epoxy around the fibers, the filling of the water droplet to these volumes. On the other hand, this behaviour may be an indicative of delamination which is one of the important risks in photo-thermal ablation. The
contact angle results indicate that CO2 laser treated surfaces are more suitable for bonding. However, only the contact angle information is not sufficient to identify an appropriate surface for bonding, as is well known. The improvement in adhesion strength is also correlated with chemical modification in addition to removal of contaminants and increased roughness of surfaces [17]. The chemical composition of the surface should also be identified. Polar groups (hydroxyl- (OH) and carbonyl-(-C=O)) that can be formed on the surface can increase the adhesion strength via the beneficial effect of physisorption [18]. The FTIR-ATR spectrum of the surfaces is shown on Fig. 4. It is known that graphite-like materials such as carbon fiber absorb the electromagnetic radiation in the entire infrared region [19]. So, peak intensities in the FTIR-ATR spectra appear very low compared to other samples spectra because all fibers were exposed on the sample surface treated with sand paper (abraded) and 33 J/cm2 CO2 laser fluence. To determine the amount of functional group bands (-OH, CHx, C=O etc.) on the surface integral band areas can be calculated from the FTIR-ATR spectra by using baseline corrected peak intensity, the area under the peaks has been calculated and given in Table 1. As shown in Table 1, an increase of the CO2 laser fluences causes the decreases of the ratio of hydroxyl group [20] which tends to increase with the increment of UV laser fluences. However, when the laser reached the highest energy density for our experiment, the –OH ratio is decreased which can be explained by the fragmentation of polymeric chains at this ablation levels. Park et al. [18] determined the threshold value as ≈500 mJ/cm2 for the sheet-molding compound which was composed of thermosetting polyester resin and chopped glass fibers. One of the reasons for the relatively low adhesion strength of CO2 laser treated samples may be due to the low rate of functional groups.
240
480 mJ/cm2
230
440 mJ/cm2
220
400 mJ/cm2
T (a.u)
160 mJ/cm2
x 2,4
x 2,3
x 2,22 x 2,2
210
Abraded
x 2,1
200
Untreated
x2
190
a) 180 3700
3200
33 J/cm2
2700 2200 Wavenumber (cm-1)
1700
1200
700
x 2,5
240
18 J/cm2
x 2,4
230
9
J/cm2
x 2,3
220
T (a.u)
6 J/cm2
x 2,15
210
Abraded
x 2,1
200
Untreated
x2
190
b) 180
3700
3200
2700 2200 Wavenumber (cm-1)
1700
1200
700
Figure 4. FTIR-ATR spectrum of the sample Untreated, abraded and treated different laser fluence for a) UV laser and b) CO2 laser. For better visualization of the graph, the data is multiplied by the specified coefficients. The bands at 1022 cm-1 and 1241 cm-1 corresponds to the C–O stretching, which represents the molecular backbone of the modified epoxy resin containing the anisole function group [21]. Increase in accumulated laser fluences for both wavelengths caused decreased the C-O
band, however sample treated with 9 J/cm2 IR laser fluence has maximum C-O peak intensity. 1608 cm-1 corresponds to deformation vibration of absorbed H2O molecules. While the entire UV laser treated samples spectra have H2O content band, which was not observed at the samples treated with accumulated fluence higher than 9 J/cm2 using IR laser. The band at labelled 1730 cm-1 was assigned C=O stretching vibrations of carboxylic groups [22] and as the increased accumulated laser fluence for both IR and UV caused to decreasing peak intensity. While the bands at 2860 cm-1 and 2963 cm-1 are due to methyl group C–H (CH3) stretching vibration, the band at 2925 cm-1 is methylene C–H (CH2) stretching [23]. Although there was no significant change in the CHx band intensities in the other spectra, a decrease was observed in the CHx band with the increase of the accumulated fluence of IR laser as seen Table 1. The intensity of the 3200-3500 cm-1 band corresponding to the hydroxyl stretching ν(OH) which was minimum at the abraded sample and reduced in the IR laser-treated samples. On the other hand, it is observed that the area of the O-H band tends to increase with the accumulated laser fluence in UV laser-treated samples. It is thought that the higher adhesion strength of the samples treated with UV laser is due to the more O-H content on the surface versus to IR laser treated samples. The peak at 3415 cm-1 corresponds to the N–H stretching vibration [24], and was observed only the samples treated at 9 J/cm2 and 18 J/cm2 accumulated fluences of IR laser. Polar groups (such as -OH, C=O etc) indicate the hydrophilic functional groups of the surface while nonpolar groups (such as CHx) indicate the hydrophobic functional groups as shown Table 1. and their ratio define of the surface wettability character. Although, laser treated surfaces both CO2 and UV has shown same trend in the spectral data in the table, polar/nonpolar group ratio was increased only UV laser data and this results support the shear strength results.
Table 1. The calculated area under the peaks of the FTIR-ATR spectrum for different fluences and the wavelengths. Pretreatment process
-OH
C=O
C-H
Polarity Ratio
-
152,57
2,191
107,11
1,44
Abraded
220 grid
18,88
0,534
17,7
1,09
6 J/cm2
186,244
1,247
188,608
0,99
2
9 J/cm
111,940
0,826
130,970
0,86
18 J/cm2
69,612
0,297
114,801
0,61
33 J/cm2
28,602
0,047
20,867
1,37
160 mJ/cm2
105,201
2,035
131,561
0,82
2
400 mJ/cm
110,731
0,281
89,381
1,24
440 mJ/cm2
170,976
0,057
115,289
1,48
480 mJ/cm2
131,976
0,037
93,69
1,40
UV Laser
CO2 Laser
Untreated
Another point to note is the difference in the laser ablation mechanism which depends on laser wavelength and pulse duration. While the UV Nd:YAG laser beam interact with the material as photo-chemical, CO2 laser interaction with the material is photo-thermal. High magnification optical microscope images were given in Fig. 5-a, due to the nature of UV laser with nanosecond pulse duration, no residues are observed on the fibers. On the other hand, in the samples treated with CO2 laser, it is seen that the surfaces of the fibers are coated with the re-solidifying epoxy as a thin film form (see Fig 5-b). The heating of the fiber matrix above the glass transition temperature results in degradation of the interphase between the fiber and the matrix [25]. This may be the most important reason for the low adhesive shear strength results of the samples treated with CO2 laser.
10 µm
10 µm
a)
50 µm
b)
50 µm
Figure 5. High magnification optical microscope images of surfaces treated with a) UV laser at 440 mJ/cm2 and b) CO2 laser at 9 J/cm2. Fig. 6 shows optical microscope images taken after single lap shear tests of CFRP / CFRP bonded surfaces of different UV and CO2 laser wavelengths. In the study conducted with UV wavelength, two different failure mechanisms were observed; the first of these, adhesive failure on the surface of the adhesive / CFRP material observed in the low UV laser fluence (Fig. 6-a). The second one is the fiber tear failure which appeared at high laser fluence is (Fig.6-b-c). The difference between the shear strengths obtained in low and high laser fluences showed itself as adhesive failure damage. It is foreseen that a good adhesion cannot be achieved as the separation from the adherend interface from the adhesive is observed in this type of damage. As stated in Ref [10], compared to conventional adhesive bonds were mostly either adhesion failure or cohesive failure occurs, depending if the surface pretreatment is adequate, on the UV laser treated CFRP also two different kinds of delamination occurred. On specimens treated with higher UV laser fluences where the resin layer was not thoroughly removed the top resin layer is ripped off. The samples treated with higher intensities tend to show delamination beneath the uppermost ply (Fig 6-b-c). Fiber tear failure occurs exclusively within the fiber reinforced plastic matrix at higher UV laser fluences. This damage mode characterized by the appearance of reinforcing fibers on both ruptured surfaces [26]. Based on optic microscope investigations, the improvement on the adhesive shear
strength of CFRP/CFRP joints with higher UV laser fluences is due to fiber tear failure mechanism. This type of failure mechanism is driven by mechanical obstruction by the no epoxy residues on the fibers and good adhesion between fibers and adhesive. The changes in failure mode from adhesive failure to fiber tear failure at the CFRP/CFRP joints require higher energy, as adhesive bond strength exists at the level of mechanical interlocking. For CO2 laser treated CFRP/CFRP joints under 6 J/cm2 and 9 J/cm2 fluences (Fig. 6-d-e) the dominant damage mechanism is occurred as cohesive failure. The fracture patterns revealed that the CO2 laser treated CFRP/CFRP joints under 6 J/cm2 and 9 J/cm2 fluences produced high performance adhesion which are observed as completely cohesive failure indicating an increased at adhesive bonding strength compared to the higher CO2 laser fluences such as 18 J/cm2 (Fig. 6-f). Adhesive failure
a)
160 mj/cm2
5 mm
b)
400 mj/cm2
5 mm
e)
500 μm
Fiber tear damage
c)
440 mj/cm2,
500 μm
Fiber tear damage
5 mm
d)
f)
500 μm
Figure 6. Microscopic images of fractured surfaces of adhesive bonded joints with (a, b, c) UV and (d, e, f) CO2 laser treated CFRP samples: effect of accumulated laser fluence.
Conclusion The present study investigates the optimum surface pretreatment of CFRP adhesive-bonded by using CO2 laser and UV laser. From the test results, two different lasers show an improvement of the adhesive bonding strength compared to both untreated and abraded samples. Release agent could not be completely removed from the surface with a low fluence (6 J/cm2) of the CO2 laser. The increase in the CO2 laser fluence (9 J/cm2) caused the reduction of surface pollution, roughening and chemical change and thus reached the maximum value of shear strength (23,72 MPa). On the other hand, by using UV laser, the maximum value (27,5 MPa) was reached at 400 mJ/cm2 laser fluence. This means an increase of 23,87% compared to untreated samples and a 21,1% increase compared to abraded samples. Although the surface roughness obtained by both lasers is approximately the same, we think that the difference in the shear strength results is mainly due to two reasons; -
The first is that the UV laser wavelength increases the -OH groups, which increases the shear strength, although the CO2 laser has a reducing effect.
-
The second effect is the difference in the ablation mechanisms of the lasers used which depends on the wavelength and pulse duration. The state of the CO2 laser is a photothermal mechanism which has led to the formation of residues on the fiber surfaces that have solidified again after melting and do not have strong adhesion to the fiber. In the process of photochemical ablation by UV laser, no residues were observed on fiber surfaces.
According to the results, optimal parameters are; for CO2 laser; 2 W power at 5 kHz repetition rate, 5 µs pulse duration, 200 mm/s scanning speed and for UV laser; 20 mJ pulse energy, 20 Hz repetition rate, 4,4 ns pulse duration and 15 mm/s scanning speed. To summarize, to avoid
delamination risk or fiber damage, optimization of laser parameters is necessary to be performed for both laser systems. In addition, the durability behavior of joints is another issue that needs to be studied and has not been planned in this study will be dealt with in future research. Acknowledgements This work was supported by The Scientific and Technological Research Council of Turkey (TUBİTAK) under grant number 215M775. Authors wish to thank Dr. Guralp Ozkoc for his kindly support of FTIR analysis in Plastic and Rubber Technology Lab at Chem Eng Dept in Kocaeli University.
References [1] Zhan X, Gao C, Lin W, Gu C, Sun W, Wei Y, Laser Cleaning Treatment and its Influence on the Surface Microstructure of CFRP Composite Material, J Powder Metall Min 2017;6:16. [2] Fischer F, Kreling S, Dilger K, Surface Structuring of CFRP by using Modern Excimer Laser Sources, Physics Proc 2012;39:154-160. [3] Wolynski A, Herrman T, Mucha P, Haloui H, L’huillier J, Laser ablation of CFRP using picosecond laser pulses at different wavelengths from UV to IR, Physics Proc 2011;12:292301. [4] Banea MD, da Silva LFM, Adhesively bonded joints in composite materials: an overview, J Mater Des Appl 2008;223:1-18.
[5] Palmieri FL, Belcher MA, Wohl CJ, Blohowiak KY, Connell JW, Laser ablation surface preparation for adhesive bonding of carbonfiber reinforced epoxy composites, Int J Adhes Adhes 2016;68:95-101. [6] Buchman A, Rotel M, Dodiuk-Kenig H, Nd:YAG Laser Surface Treatment of Various Materials to Enhance Adhesion. In: Mittal KL, Bahners T, editors. Laser Surface Modification and Adhesion. New York: Wiley;2015, p. 3-54. [7] See TL, Liu Z, Cheetham S, Dilworth S, Li L, Laser abrading of carbon fibre reinforced composite for improving paint adhesion, Appl. Phys. A 2014;117:1045-1054. [8] Kreling S, Fischer F, Delmdahl R, Gäbler F, Dilger K, Analytical characterization of CFRP laser treated by excimer laser radiation, Physics Proc 2013;41:282-290. [9] Fischer F, Kreling S, Jaschke P, Frauenhofer M, Kracht D, Dilger K, Laser Surface PreTreatment of CFRP for Adhesive Bonding in Consideration of the Absorption Behaviour, J Adhes 2012:88;350–363. [10] Fischer F, Kreling S, Dilger K, Pre-Treatment of CFRP for Adhesive Bonding Using Laser Radiation, ECCM15;2012;1-8. [11] Akman E, Erdoğan Y, Bora MÖ, Çoban O, Genc Oztoprak B, Demir A, Investigation of accumulated laser fluence and bondline thickness effects on adhesive joint performance of CFRP composites, Int J Adhes Adhes 2019;89:109-116. [12] Oliveira V, Sharma SP, de Moura MFSF, Moreira RDF, Vilar R, Surface treatment of CFRP composites using femtosecond laser radiation, Opt Lasers Eng 2017;94:37-43. [13] Riveiro A, Quintero F, Lusquiños F, del Val J, Comesaña R, Boutinguiza M, Pou J, Experimental study on the CO2 laser cutting of carbon fiber reinforced plastic composite, Composites Part A 2012;43:1400-1409.
[14] Nattapat M, Marimuthu S, Kamara AM, Nekouie Esfahani MR, Laser Surface Modification of Carbon Fiber Reinforced Composites, Mater Manuf Processes 2015:30;1450– 1456. [15] Stalder AF, Melchior T, Müller M, Sage D, Blu T, Unser M, Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops, Colloid Surface A 2010;364:72-81. [16] Busscher HJ, Van Pelt AWJ, de Boer P, de Jong HP, Arends J, The effect of surface roughening of polymers on measured contact angles of liquids, Colloids Surf 1984;9:319–31. [17] Sezer HK, Short Review on Laser Texturing and Cleaning Carbon Fibre Composites for Aerospace Applications, Journal of Polytechnic 2016;19:623-631. [18] Park JK, Mukherjee K, Excimer Laser Surface Treatment of Sheet Molding Compound for Adhesive Bonding, Mater Manuf Process 1998;13:359-368. [19] Ohwaki T, Ishida H, Comparison Between FT-IR and XPS Characterization of Carbon Fiber Surfaces, J Adhesion 1995;52:167-186. [20] Hartwig A, Vitr G, Dieckhoff S, Hennemann OD, Surface treatment of an epoxy resin by CO2 laser irradiation, Angew Makromol Chem 1996;238:177-189. [21] Wu Z, Li S, Liu M, Wang Z, Liu X, Liquid oxygen compatible epoxy resin: modification and characterization, RSC Adv 2015;15:1–9. [22] Silverstein RM, Webster FX, Kiemle DJ, Bryce DL, Spectrometric Identification of Organic Compounds, 8th ed.; Wiley: Hoboken, NJ, USA, 2015, 92. [23] Asmatulua R, Erukalaa KS, Shindea M, Alarifib IM, Gorjic MR, Investigating the effects of surface treatments on adhesion properties of protective coatings on carbon fiber-reinforced composite laminates, Surf Coat Tech 2019:380;125006 (1-11).
[24] Li W, Xiang D, Wang L, Jones EH, Zhao C, Wang B, Li Yuntao, Simultaneous enhancement of electrical conductivity and interlaminar fracture toughness of carbon fiber/epoxy composites using plasma treated conductive thermoplastic film interleaves, RSC Adv;2018:8, 26910–26921. [25] Wolfrum J, Eibl S, Lietch L, Rapid evaluation of long-term thermal degradation of carbon fibre epoxy composites, Compos Sci Technol 2009; 69: 523-530. [26] Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints. ASTM D5573-99, 2012.
S0143-7496(20)30008-7
DOI:
https://doi.org/10.1016/j.ijadhadh.2020.102548
Reference:
JAAD 102548
To appear in:
International Journal of Adhesion and Adhesives
Revised Date:
4 December 2019
Accepted Date: 9 January 2020
Please cite this article as: Akman E, Erdoğan Y, Bora MÖ, Çoban O, Oztoprak BG, Demir A, Investigation of the differences between photochemical and photothermal laser ablation on the shear strength of CFRP/CFRP adhesive joints, International Journal of Adhesion and Adhesives, https:// doi.org/10.1016/j.ijadhadh.2020.102548. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Investigation of the differences between photochemical and photothermal laser ablation on the shear strength of CFRP/CFRP adhesive joints Erhan Akmana,*, Yalçın Erdoğanb, Mustafa Özgür Boraa, Onur Çobana, Belgin Genc Oztopraka, Arif Demirc,d a
Aviation Materials Research and Development Laboratory, Faculty of Aeronautics and Astronautics, Kocaeli University, Kocaeli 41285, Turkey b
Electro Optic System Engineering, Kocaeli University, Kocaeli 41380, Turkey c
d
Department of Physics, Kocaeli University, Kocaeli 41380, Turkey
BEAM Ar-Ge Optics, Laser and Spectroscopy, KOU Technopark, Kocaeli 41275, Turkey
Abstract Ensure high bonding strength and durability is very important for composite materials that are widely used by many different industries. In order to achieve this, the surfaces of the composite materials should be prepared for the adhesion process. Surface preparation with laser is a promising technique for the adhesive joint. However, the diversity offered by the advances in laser technologies and the multiplicity of parameters that must be controlled in laser material processing makes the process somewhat complex. In the present study, two different lasers have been used with different wavelengths (355 nm (UV) and 10600 nm (IR)) and pulse durations (4,4 ns and 5 µs), to increase the lap shear strength of the adhesive joint of carbon fiber reinforced polymer (CFRP) materials. While the ultraviolet (UV) laser with high photon energy creates photo-chemical ablation, infrared laser (IR) causes photo-thermal ablation due to their low photon energy. CFRP samples treated with different lasers and the accumulated laser fluences have been adhesively joined and the results evaluated experimentally. According to the results, differences in laser ablation mechanism cause structural differences on CFRP surfaces. While resolidified epoxy was observed on CO2 laser treated surfaces, no residues were detected on UV laser treated surfaces. However, to avoid delamination or fiber damage risks an optimization of laser parameters are recommended to be performed for both laser systems. Keywords: CFRP; Adhesive bonding, Laser surface pre-treatment
*Corresponding Author: E. Akman, Tel:+902623513310-303, Email: [email protected]
1. Introduction Superior features of the carbon fiber reinforced polymers (CFRP) such as high strength, specific stiffness, density and small thermal expansion coefficient, low price and availability etc. ensure that they are preferred in the aviation and automotive industries [1, 2]. If the structure to be used has a complex geometry, the composite materials should be joined using screws or rivets as conventional methods. But to use screws or rivets the composite structure should be drilled which represents is one of the most important machining processes on composites [3]. Drilling of the composite materials causes stress concentrations and reduces structural integrity [2, 4]. Adhesive bonding is an alternative technique that offers advantages such as lower structural weight, lower manufacturing cost and improved damage tolerance [4]. However, adhesive bonding has also included challenges such as surface preparation of the adherend to obtain robust and predictable bonds [5]. A suitable adherend surface for adhesion is defined as: free from contaminants, lack of weak border layers, with surface roughness that can provide mechanical interlocking and a surface with enhanced wetting [6]. Generally, contaminations such as fluorine and silicon on CFRP surface originate from mold release agents that are applied during the manufacturing process [7]. As an alternative to traditional methods such as sandblasting, abrading etc., due to better controllability, non-contact, ease of automation and environmental friendly, laser surface treatment has become an important technique [7, 8]. CFRP composites are made from materials with different physical properties, causing differentiation also in absorption properties and thermal conduction [9]. Therefore, lots of experiments have been conducted with different wavelengths (Ultraviolet to Infrared) and operation modes of laser as pulsed or continuous irradiation [8-10]. The differences in wavelength and the operation mode especially pulse duration cause a difference in the mechanism of ablation. While the UV laser with high photon energy creates photo-chemical ablation determined as cold ablation, IR laser
causes photo-thermal ablation due to their low photon energy [11]. During the photo-thermal ablation, the heat is conducted to the bulk of the matrix material by the fibers, leading to a large heat affected zone (HAZ) where the fiber/matrix interface is weakened by thermal degradation of the polymer [12]. This effect can be minimized using pulsed mode laser with optimized parameters [13]. When the pulse duration is as short as that of femtosecond lasers, high precision modification of CFRP materials with minimal thermal damage is possible [12]. When the studies described in the literature are examined, it seems that many studies have been done to determine the most suitable laser features. In their study Nattapat et al. have used continuous wave (CW) carbon dioxide (CO2) laser in spite of thermally acting tools [14], due to the comparatively lower price and operating costs of low power CW CO2 laser system. According to their results it is possible to remove the resin layer without exceeding the damage threshold of the underlying fibers. And, the adhesive joint strength of laser-treated surfaces has been increased compared to sanded surfaces. Fischer et al. have used pulsed CO2 and UV laser to investigate heat deposit and absorption behavior at the two wavelengths [9]. According to their results the shear strengths of the pre-treated specimens are in the same range as the abraded reference specimens, so both lasers are a suitable method for the surface pre-treatment of CFRP material to improve the adhesive bonding strength. But they emphasize the risk of delamination because the CO2 laser deposits more heat to the material. Another study described by Fischer et al. [2] an excimer laser with 308 nm wavelength has been used to prepare the CFRP surface for adhesive joint. They used different laser energies and pulse overlap to increase the shear strength. The results obtained indicate that the tensile strength of the reference sample is reached or even exceeded. They draw also attention to the risk of delamination when inappropriate parameters are used in their study [2].
When the studies in the literature are examined, it has been observed that the adhesive strength of the samples treated with different lasers is sometimes in the same range as the abraded ones and sometimes exceeding. Although, it is stated that the UV laser is more suitable for surface treatment of composite materials due to the photo-chemical ablation mechanism, there is no notable difference in the test results. These results reveal the need for optimization of laser parameters. In this study two different pulsed lasers, III. Harmonic of Nd:YAG (λ=355 nm, as UV) and CO2 (λ=10600 nm, as IR)) were used to prepare the CFRP surfaces for adhesive joint. The focus of the study is determining the suitable accumulated laser fluences which should create optimum surface structure without damaging the carbon fibers for both wavelengths and also reasons for achieving higher shear strength of photo-chemical ablation mechanism.
2. Materials and methods In this study, unidirectional carbon fiber-reinforced epoxy composite [0]5 laminates (CFRP) were used. CFRP laminates were supplied from Kompozitsan, Izmir/Turkey. Fabricated CFRP laminates were manufactured by using resin transfer molding (RTM) method. The nominal thickness of the CFRP laminates is 1,8 mm. The CFRP characteristics were the following: the diameter of fibers = 7 µm, number of prepeg layers = 5, fiber areal weight = 245 g/m2, prepreg fabric type = unidirectional, carbon fiber mesh, type of resin = F-1564 epoxy resin, glass transition temperature of the resin = 80 ºC. Two different lasers were used for the surface treatment of the laminates. The CO2 laser (IR) which emits pulses of 5-400 µs duration range, repetition rate range between 5 kHz and 100 kHz and 10600 nm wavelength, with low photon energy (0,12 eV). To ablate the desired surface area on the samples with CO2 laser, a galvanometric system was used which provides the computer controlled movement of the laser beam, together with a 160 mm focal length F-Theta lens to focus the laser beam.
During the surface treatment, plates were fixed at the focal point of the F-Theta lens, at this condition spot diameter of the beam was approximately 200 µm. The III. harmonic of the Nd:YAG laser (UV) with relatively high photon energy (3,50 eV) produces pulses at a pulse length of 4,4 ns and 20 Hz repetition rate. Two axis computer control linear motion system was used during the Nd:YAG laser treatment of the laminates. III. Harmonic of the Nd:YAG laser beam was irradiated on the surface of the material without focus. Under these conditions, the laser beam spot area on the material surface is approximately 5 mm2. In both systems, an area of 25,4 mm x 25,4 mm was irradiated in air by scanning the surface as parallel lines with a spot size and travel speed as described in Fig. 1. To compare the laser treatment effect, reference CFRP laminates were prepared by roughening their surface with sandpaper 220 grit size. For the characterization of all prepared CFRP surfaces, surface topographies and contact angles were obtained by non-contact laser profilometer and homemade contact angle goniometer, works static contact angle measurement method, with Drop Shape Analysis plugin [15], respectively. The measurement of the contact angle was repeated five times with using pure water for each sample to obtain the statistical result. Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) measurements were also performed to determine the chemical changes occuring on the CFRP surfaces due to the laser treatments. Two component structural epoxy paste adhesive (Loctite Hysol ® EA 9396TM) was used to bond the CFRP laminates. Curing temperature was 66 ºC with curing duration as 60 min. in an oven at atmospheric pressure. A fixture consisting of three axis linear translation stages was prepared to provide precision control of the bondline as seen in Fig. 1. The bondline tolerance of the samples using this fixture was ±10 µm. Optical microscope images of bondline thickness are available in our previous study [11]. The effect of the accumulated laser fluence and wavelength on the bonding strength of CFRP/CFRP adhesive bonding was
investigated by using a lap shear test according to ASTM D5868-01 standard. By adding to the grips (Fig. 1-c) on both sides of the test sample, the load transferred along the bondline region which prevented the bending load effect to the sample. Dartec universal test machine was used which had a capacity of 60 kN for determining the adhesive shear strength of different accumulated laser fluences and wavelengths. Test speed was selected as 13 mm/min. After mechanical tests, camera was used to observe the failure modes which occurred on the CFRP composite surfaces. All the tests in the study were repeated five times using for each sample group to obtain the statistical results.
Figure 1. Experimental setup of the CO2 laser treatment (a) and UV laser treatment (b) of the CFRP surfaces, the bonding fixture (c) used for the adhesive joint and set prepared for lap shear test.
3. Results and discussion From the literature, it can be seen that there are many parameters that affect the strength of the adhesive
bonding
such
material, bondline thickness,
as
nature
contamination,
of stress
the and
adhesive
and
environmental
the adherend factors
and
mostly surface treatment of the adherend [6]. The bonding thickness was determined as the
first step. As mentioned in our previous study, the bondline thickness of the adhesive was changed between 30-500 µm, and the highest strength was obtained at 50 µm [11] for CFRP/CFRP adhesive bonding. Therefore, all samples were jointed at a bonding thickness of 50 µm. During the surface preparation of the CFRP for the adhesive joint, the main parameter is the accumulated laser fluence which determines the ablation rate of the samples. While the laser fluences that are not sufficient does not clean and treated the surface as much as necessary, the accumulated laser fluences overload may cause delamination [9]. In our experiment, the accumulated laser fluences were changed between 6-33 J/cm2 for the CO2 laser and 160 mJ/cm2 - 480 mJ/cm2 for the UV laser. The SEM microscope images of the surfaces treated with different fluences and the wavelengths are shown in Fig. 2.
Figure 2. SEM images of the samples Untreated (a), Abraded (b) treated using UV (c), (d), (e) and CO2 (f), (g), (h) lasers with different accumulated fluences. Sa values in the pictures indicate the roughness of the surfaces.
As seen in Fig. 2-b, it appears that there are damages which disturb the continuity of the fibers at the surface of the abraded samples. The effect of the different accumulated fluence on the surfaces was tried to determine by calculating the area of the exposed fibers. As seen in SEM images for both of the wavelengths, at the low laser fluences (Fig. 2-c-f) the area (5,5% for UV laser, 7% for IR laser) of the exposed fiber is small (white arrows). An increase in the accumulated laser fluence caused the increase in the area (68 % for UV laser, 79 % for IR laser) of exposed fibers (white arrows) as expected (Fig. 2-d-g). When the fluence increment was continued, while the area of the exposed fibers reached to % 94,1 (white arrows in Fig. 2e) on the surface of the sample treated using UV laser, all of the fibers were exposed on the sample treated using IR laser (Fig. 2-h). As shown in Fig. 2-e-h, in the samples treated at a high accumulated fluence of both lasers, it is seen that the epoxy around the fibers which holds the fibers together is partially (black arrows) removed. In this case, it is believed that the fibers cannot adhere to the bulk structure and the risk of delamination can be observed. On the other, it was determined in our previous article that the increase in laser fluence reduces the surface contamination rate [11]. Also, the roughness of the treated surfaces at both wavelengths showed an increasing trend according to untreated sample (see Fig 2-a) with the increment of laser fluences excluding 6 J/cm2 in which the roughness reduced less than 0.5 µm due to the polymer polishing effect (see Fig. 2-f).
When the single lap shear strength test results have been investigated, the shear strength of the samples treated with UV laser showed an increasing tendency until the 400 mJ/cm2 with the laser fluence. After this point, a small decrease was observed as shown in Figure 3-a. The shear strength of the samples treated with IR laser is significantly reduced after 9 J/cm2 laser fluence. This situation is interpreted as degrading the epoxy matrix (see Fig 2-e-h), which binds the fiber layers together and showed more effect on samples prepared using CO2 laser
wavelength due to the photothermal ablation behavior as seen in Fig 3. Ejection of the epoxy matrix between the fibers can be considered as the upper limit of laser ablation since, after this point, fiber damages have begun to be observed [11]. 30 100
Shear strength Contact angle
25
60 15 40
10
Contact angle (º)
Shear strength (MPa)
80 20
20
5
0
0 Untreated Abraded
160
240
330
360
400
440
480
Laser fluences (mJ/cm2)
a) 30 Shear Strength Contact Angle
100
25
60 15 40
10
Contact angle (º)
Shear strength (MPa)
80 20
20
5
0
0 Untreated
b)
Abraded
6 9 Laser fluences (J/cm2)
18
33
Figure 3. Shear strength and contact angle variation of the samples prepared with different laser fluence for a) UV and b) CO2 laser.
One of the important tests that provide information about the suitability of the surfaces to the adhesive joint is the surface contact angle measurement. According to the literature, it is stated that the increase in wettability of the surfaces affects the adhesion strength positively [6]. Surface chemistry, roughness and pollution affect the contact angle of the surfaces. Oxygen content on the surface increases wettability and therefore adhesion at fiber interfaces. On the other hand, contact angle is affected by the change in surface roughness in accordance with the nature of the surface itself [16]. Surface contamination, which is generally caused by fluorine and silicon, adversely affects surface energy and reduces adhesion [11]. The contact angle of the abraded sample increased due to roughness compared to the untreated sample. When the contact angle results of the surfaces prepared using UV laser were examined (Fig. 3a), the contact angle increased with the increase in laser fluence. This increment was continued until 330 mJ/cm2 laser fluence and then started to decrease and it has reached the lowest value at the 400 mJ/cm2 laser fluence. At this point, the contact angle increased slightly after the increase in the laser fluence and remained constant thereafter. This situation can be explained as; laser fluence increment up to 330 mJ/cm2 causes the obtained further rough surfaces that enhance the surface hydrophobicity. However, at the 400 mJ/cm2 and above laser fluences the combined effects from the surface roughness, removal of surface contaminants and the change of the surface chemistry led to the behavior of the contact angle in the graph (Fig. 3-b) [7]. On the other hand, while the contact angles were approximately same with the untreated sample at the low accumulated laser fluence of CO2 treated surfaces, 33 J/cm2 laser fluence caused a reduction on the contact angle of the CFRP surfaces. This change in contact angle can be explained by the removal of the contamination [11] and epoxy around the fibers, the filling of the water droplet to these volumes. On the other hand, this behaviour may be an indicative of delamination which is one of the important risks in photo-thermal ablation. The
contact angle results indicate that CO2 laser treated surfaces are more suitable for bonding. However, only the contact angle information is not sufficient to identify an appropriate surface for bonding, as is well known. The improvement in adhesion strength is also correlated with chemical modification in addition to removal of contaminants and increased roughness of surfaces [17]. The chemical composition of the surface should also be identified. Polar groups (hydroxyl- (OH) and carbonyl-(-C=O)) that can be formed on the surface can increase the adhesion strength via the beneficial effect of physisorption [18]. The FTIR-ATR spectrum of the surfaces is shown on Fig. 4. It is known that graphite-like materials such as carbon fiber absorb the electromagnetic radiation in the entire infrared region [19]. So, peak intensities in the FTIR-ATR spectra appear very low compared to other samples spectra because all fibers were exposed on the sample surface treated with sand paper (abraded) and 33 J/cm2 CO2 laser fluence. To determine the amount of functional group bands (-OH, CHx, C=O etc.) on the surface integral band areas can be calculated from the FTIR-ATR spectra by using baseline corrected peak intensity, the area under the peaks has been calculated and given in Table 1. As shown in Table 1, an increase of the CO2 laser fluences causes the decreases of the ratio of hydroxyl group [20] which tends to increase with the increment of UV laser fluences. However, when the laser reached the highest energy density for our experiment, the –OH ratio is decreased which can be explained by the fragmentation of polymeric chains at this ablation levels. Park et al. [18] determined the threshold value as ≈500 mJ/cm2 for the sheet-molding compound which was composed of thermosetting polyester resin and chopped glass fibers. One of the reasons for the relatively low adhesion strength of CO2 laser treated samples may be due to the low rate of functional groups.
240
480 mJ/cm2
230
440 mJ/cm2
220
400 mJ/cm2
T (a.u)
160 mJ/cm2
x 2,4
x 2,3
x 2,22 x 2,2
210
Abraded
x 2,1
200
Untreated
x2
190
a) 180 3700
3200
33 J/cm2
2700 2200 Wavenumber (cm-1)
1700
1200
700
x 2,5
240
18 J/cm2
x 2,4
230
9
J/cm2
x 2,3
220
T (a.u)
6 J/cm2
x 2,15
210
Abraded
x 2,1
200
Untreated
x2
190
b) 180
3700
3200
2700 2200 Wavenumber (cm-1)
1700
1200
700
Figure 4. FTIR-ATR spectrum of the sample Untreated, abraded and treated different laser fluence for a) UV laser and b) CO2 laser. For better visualization of the graph, the data is multiplied by the specified coefficients. The bands at 1022 cm-1 and 1241 cm-1 corresponds to the C–O stretching, which represents the molecular backbone of the modified epoxy resin containing the anisole function group [21]. Increase in accumulated laser fluences for both wavelengths caused decreased the C-O
band, however sample treated with 9 J/cm2 IR laser fluence has maximum C-O peak intensity. 1608 cm-1 corresponds to deformation vibration of absorbed H2O molecules. While the entire UV laser treated samples spectra have H2O content band, which was not observed at the samples treated with accumulated fluence higher than 9 J/cm2 using IR laser. The band at labelled 1730 cm-1 was assigned C=O stretching vibrations of carboxylic groups [22] and as the increased accumulated laser fluence for both IR and UV caused to decreasing peak intensity. While the bands at 2860 cm-1 and 2963 cm-1 are due to methyl group C–H (CH3) stretching vibration, the band at 2925 cm-1 is methylene C–H (CH2) stretching [23]. Although there was no significant change in the CHx band intensities in the other spectra, a decrease was observed in the CHx band with the increase of the accumulated fluence of IR laser as seen Table 1. The intensity of the 3200-3500 cm-1 band corresponding to the hydroxyl stretching ν(OH) which was minimum at the abraded sample and reduced in the IR laser-treated samples. On the other hand, it is observed that the area of the O-H band tends to increase with the accumulated laser fluence in UV laser-treated samples. It is thought that the higher adhesion strength of the samples treated with UV laser is due to the more O-H content on the surface versus to IR laser treated samples. The peak at 3415 cm-1 corresponds to the N–H stretching vibration [24], and was observed only the samples treated at 9 J/cm2 and 18 J/cm2 accumulated fluences of IR laser. Polar groups (such as -OH, C=O etc) indicate the hydrophilic functional groups of the surface while nonpolar groups (such as CHx) indicate the hydrophobic functional groups as shown Table 1. and their ratio define of the surface wettability character. Although, laser treated surfaces both CO2 and UV has shown same trend in the spectral data in the table, polar/nonpolar group ratio was increased only UV laser data and this results support the shear strength results.
Table 1. The calculated area under the peaks of the FTIR-ATR spectrum for different fluences and the wavelengths. Pretreatment process
-OH
C=O
C-H
Polarity Ratio
-
152,57
2,191
107,11
1,44
Abraded
220 grid
18,88
0,534
17,7
1,09
6 J/cm2
186,244
1,247
188,608
0,99
2
9 J/cm
111,940
0,826
130,970
0,86
18 J/cm2
69,612
0,297
114,801
0,61
33 J/cm2
28,602
0,047
20,867
1,37
160 mJ/cm2
105,201
2,035
131,561
0,82
2
400 mJ/cm
110,731
0,281
89,381
1,24
440 mJ/cm2
170,976
0,057
115,289
1,48
480 mJ/cm2
131,976
0,037
93,69
1,40
UV Laser
CO2 Laser
Untreated
Another point to note is the difference in the laser ablation mechanism which depends on laser wavelength and pulse duration. While the UV Nd:YAG laser beam interact with the material as photo-chemical, CO2 laser interaction with the material is photo-thermal. High magnification optical microscope images were given in Fig. 5-a, due to the nature of UV laser with nanosecond pulse duration, no residues are observed on the fibers. On the other hand, in the samples treated with CO2 laser, it is seen that the surfaces of the fibers are coated with the re-solidifying epoxy as a thin film form (see Fig 5-b). The heating of the fiber matrix above the glass transition temperature results in degradation of the interphase between the fiber and the matrix [25]. This may be the most important reason for the low adhesive shear strength results of the samples treated with CO2 laser.
10 µm
10 µm
a)
50 µm
b)
50 µm
Figure 5. High magnification optical microscope images of surfaces treated with a) UV laser at 440 mJ/cm2 and b) CO2 laser at 9 J/cm2. Fig. 6 shows optical microscope images taken after single lap shear tests of CFRP / CFRP bonded surfaces of different UV and CO2 laser wavelengths. In the study conducted with UV wavelength, two different failure mechanisms were observed; the first of these, adhesive failure on the surface of the adhesive / CFRP material observed in the low UV laser fluence (Fig. 6-a). The second one is the fiber tear failure which appeared at high laser fluence is (Fig.6-b-c). The difference between the shear strengths obtained in low and high laser fluences showed itself as adhesive failure damage. It is foreseen that a good adhesion cannot be achieved as the separation from the adherend interface from the adhesive is observed in this type of damage. As stated in Ref [10], compared to conventional adhesive bonds were mostly either adhesion failure or cohesive failure occurs, depending if the surface pretreatment is adequate, on the UV laser treated CFRP also two different kinds of delamination occurred. On specimens treated with higher UV laser fluences where the resin layer was not thoroughly removed the top resin layer is ripped off. The samples treated with higher intensities tend to show delamination beneath the uppermost ply (Fig 6-b-c). Fiber tear failure occurs exclusively within the fiber reinforced plastic matrix at higher UV laser fluences. This damage mode characterized by the appearance of reinforcing fibers on both ruptured surfaces [26]. Based on optic microscope investigations, the improvement on the adhesive shear
strength of CFRP/CFRP joints with higher UV laser fluences is due to fiber tear failure mechanism. This type of failure mechanism is driven by mechanical obstruction by the no epoxy residues on the fibers and good adhesion between fibers and adhesive. The changes in failure mode from adhesive failure to fiber tear failure at the CFRP/CFRP joints require higher energy, as adhesive bond strength exists at the level of mechanical interlocking. For CO2 laser treated CFRP/CFRP joints under 6 J/cm2 and 9 J/cm2 fluences (Fig. 6-d-e) the dominant damage mechanism is occurred as cohesive failure. The fracture patterns revealed that the CO2 laser treated CFRP/CFRP joints under 6 J/cm2 and 9 J/cm2 fluences produced high performance adhesion which are observed as completely cohesive failure indicating an increased at adhesive bonding strength compared to the higher CO2 laser fluences such as 18 J/cm2 (Fig. 6-f). Adhesive failure
a)
160 mj/cm2
5 mm
b)
400 mj/cm2
5 mm
e)
500 μm
Fiber tear damage
c)
440 mj/cm2,
500 μm
Fiber tear damage
5 mm
d)
f)
500 μm
Figure 6. Microscopic images of fractured surfaces of adhesive bonded joints with (a, b, c) UV and (d, e, f) CO2 laser treated CFRP samples: effect of accumulated laser fluence.
Conclusion The present study investigates the optimum surface pretreatment of CFRP adhesive-bonded by using CO2 laser and UV laser. From the test results, two different lasers show an improvement of the adhesive bonding strength compared to both untreated and abraded samples. Release agent could not be completely removed from the surface with a low fluence (6 J/cm2) of the CO2 laser. The increase in the CO2 laser fluence (9 J/cm2) caused the reduction of surface pollution, roughening and chemical change and thus reached the maximum value of shear strength (23,72 MPa). On the other hand, by using UV laser, the maximum value (27,5 MPa) was reached at 400 mJ/cm2 laser fluence. This means an increase of 23,87% compared to untreated samples and a 21,1% increase compared to abraded samples. Although the surface roughness obtained by both lasers is approximately the same, we think that the difference in the shear strength results is mainly due to two reasons; -
The first is that the UV laser wavelength increases the -OH groups, which increases the shear strength, although the CO2 laser has a reducing effect.
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The second effect is the difference in the ablation mechanisms of the lasers used which depends on the wavelength and pulse duration. The state of the CO2 laser is a photothermal mechanism which has led to the formation of residues on the fiber surfaces that have solidified again after melting and do not have strong adhesion to the fiber. In the process of photochemical ablation by UV laser, no residues were observed on fiber surfaces.
According to the results, optimal parameters are; for CO2 laser; 2 W power at 5 kHz repetition rate, 5 µs pulse duration, 200 mm/s scanning speed and for UV laser; 20 mJ pulse energy, 20 Hz repetition rate, 4,4 ns pulse duration and 15 mm/s scanning speed. To summarize, to avoid
delamination risk or fiber damage, optimization of laser parameters is necessary to be performed for both laser systems. In addition, the durability behavior of joints is another issue that needs to be studied and has not been planned in this study will be dealt with in future research. Acknowledgements This work was supported by The Scientific and Technological Research Council of Turkey (TUBİTAK) under grant number 215M775. Authors wish to thank Dr. Guralp Ozkoc for his kindly support of FTIR analysis in Plastic and Rubber Technology Lab at Chem Eng Dept in Kocaeli University.
References [1] Zhan X, Gao C, Lin W, Gu C, Sun W, Wei Y, Laser Cleaning Treatment and its Influence on the Surface Microstructure of CFRP Composite Material, J Powder Metall Min 2017;6:16. [2] Fischer F, Kreling S, Dilger K, Surface Structuring of CFRP by using Modern Excimer Laser Sources, Physics Proc 2012;39:154-160. [3] Wolynski A, Herrman T, Mucha P, Haloui H, L’huillier J, Laser ablation of CFRP using picosecond laser pulses at different wavelengths from UV to IR, Physics Proc 2011;12:292301. [4] Banea MD, da Silva LFM, Adhesively bonded joints in composite materials: an overview, J Mater Des Appl 2008;223:1-18.
[5] Palmieri FL, Belcher MA, Wohl CJ, Blohowiak KY, Connell JW, Laser ablation surface preparation for adhesive bonding of carbonfiber reinforced epoxy composites, Int J Adhes Adhes 2016;68:95-101. [6] Buchman A, Rotel M, Dodiuk-Kenig H, Nd:YAG Laser Surface Treatment of Various Materials to Enhance Adhesion. In: Mittal KL, Bahners T, editors. Laser Surface Modification and Adhesion. New York: Wiley;2015, p. 3-54. [7] See TL, Liu Z, Cheetham S, Dilworth S, Li L, Laser abrading of carbon fibre reinforced composite for improving paint adhesion, Appl. Phys. A 2014;117:1045-1054. [8] Kreling S, Fischer F, Delmdahl R, Gäbler F, Dilger K, Analytical characterization of CFRP laser treated by excimer laser radiation, Physics Proc 2013;41:282-290. [9] Fischer F, Kreling S, Jaschke P, Frauenhofer M, Kracht D, Dilger K, Laser Surface PreTreatment of CFRP for Adhesive Bonding in Consideration of the Absorption Behaviour, J Adhes 2012:88;350–363. [10] Fischer F, Kreling S, Dilger K, Pre-Treatment of CFRP for Adhesive Bonding Using Laser Radiation, ECCM15;2012;1-8. [11] Akman E, Erdoğan Y, Bora MÖ, Çoban O, Genc Oztoprak B, Demir A, Investigation of accumulated laser fluence and bondline thickness effects on adhesive joint performance of CFRP composites, Int J Adhes Adhes 2019;89:109-116. [12] Oliveira V, Sharma SP, de Moura MFSF, Moreira RDF, Vilar R, Surface treatment of CFRP composites using femtosecond laser radiation, Opt Lasers Eng 2017;94:37-43. [13] Riveiro A, Quintero F, Lusquiños F, del Val J, Comesaña R, Boutinguiza M, Pou J, Experimental study on the CO2 laser cutting of carbon fiber reinforced plastic composite, Composites Part A 2012;43:1400-1409.
[14] Nattapat M, Marimuthu S, Kamara AM, Nekouie Esfahani MR, Laser Surface Modification of Carbon Fiber Reinforced Composites, Mater Manuf Processes 2015:30;1450– 1456. [15] Stalder AF, Melchior T, Müller M, Sage D, Blu T, Unser M, Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops, Colloid Surface A 2010;364:72-81. [16] Busscher HJ, Van Pelt AWJ, de Boer P, de Jong HP, Arends J, The effect of surface roughening of polymers on measured contact angles of liquids, Colloids Surf 1984;9:319–31. [17] Sezer HK, Short Review on Laser Texturing and Cleaning Carbon Fibre Composites for Aerospace Applications, Journal of Polytechnic 2016;19:623-631. [18] Park JK, Mukherjee K, Excimer Laser Surface Treatment of Sheet Molding Compound for Adhesive Bonding, Mater Manuf Process 1998;13:359-368. [19] Ohwaki T, Ishida H, Comparison Between FT-IR and XPS Characterization of Carbon Fiber Surfaces, J Adhesion 1995;52:167-186. [20] Hartwig A, Vitr G, Dieckhoff S, Hennemann OD, Surface treatment of an epoxy resin by CO2 laser irradiation, Angew Makromol Chem 1996;238:177-189. [21] Wu Z, Li S, Liu M, Wang Z, Liu X, Liquid oxygen compatible epoxy resin: modification and characterization, RSC Adv 2015;15:1–9. [22] Silverstein RM, Webster FX, Kiemle DJ, Bryce DL, Spectrometric Identification of Organic Compounds, 8th ed.; Wiley: Hoboken, NJ, USA, 2015, 92. [23] Asmatulua R, Erukalaa KS, Shindea M, Alarifib IM, Gorjic MR, Investigating the effects of surface treatments on adhesion properties of protective coatings on carbon fiber-reinforced composite laminates, Surf Coat Tech 2019:380;125006 (1-11).
[24] Li W, Xiang D, Wang L, Jones EH, Zhao C, Wang B, Li Yuntao, Simultaneous enhancement of electrical conductivity and interlaminar fracture toughness of carbon fiber/epoxy composites using plasma treated conductive thermoplastic film interleaves, RSC Adv;2018:8, 26910–26921. [25] Wolfrum J, Eibl S, Lietch L, Rapid evaluation of long-term thermal degradation of carbon fibre epoxy composites, Compos Sci Technol 2009; 69: 523-530. [26] Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints. ASTM D5573-99, 2012.