Experimental and Theoretical Investigation of Laser Pretreatment on

Subscriber access provided by UNIV OF SOUTHERN INDIANA

Surfaces, Interfaces, and Applications

Experimental and Theoretical Investigation of Laser Pretreatment on Strengthening the Heterojunction Between Carbon Fiber Reinforced Plastic and Aluminum Alloy Yuyao Li, Shun Meng, Qianming Gong, Yilun Huang, Jianning Gan, Ming Zhao, Bin Liu, Lei Liu, Guisheng Zou, and Daming Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Experimental and Theoretical Investigation of Laser Pretreatment on Strengthening the Heterojunction Between Carbon Fiber Reinforced Plastic and Aluminum Alloy Yuyao Li †,‡,§, Shun Meng ∥, Qianming Gong †,‡,§*, Yilun Huang †,‡,§, Jianning Gan †,‡,§, Ming Zhao †,‡,§, Bin Liu ∥, Lei Liu ⊥, Guisheng Zou ⊥*, Daming Zhuang †,‡,§ †

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China ‡

State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China §

Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, PR China ∥

⊥

School of Aerospace Engineering, Tsinghua University, Beijing 100084, PR China

Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

KEYWORDS Heterojunction, Laser pretreatment, Carbon fiber reinforced plastic, Shear strength, Finite element analysis

ABSTRACT:

Besides aluminum alloy, lightweight carbon fiber reinforced plastics (CFRP) have been adopted progressively in automobiles in order to save energy and reduce emission, so constructing a reliable heterojunction between aluminum alloys and CFRP comes to be the key issue. In this study, ultra-fast picosecond infrared (IR) laser and excimer ultraviolet (UV) laser were introduced to pretreat the joint surface to enhance the adhesive strength. Scanning electron microscope, white light interferometry and X-ray photoelectron spectroscopy examinations indicated that, due to the energy absorptivity for the two lasers was different, the variation of the roughness, wettability and chemical composition were a little different for the patterned surface. Correspondingly, the shear strengths of the adhesive joints were increased from 5.6 MPa to 24.8 MPa and 21.9 MPa for IR and UV laser pretreated samples, respectively. Furthermore, finite element analysis was adopted to evaluate the effects of strengthened mechanical interlocking and fortified chemical bonding force on the enhancement of joint strength. It was shown that chemical bonding, instead of mechanical interlocking, played the dominant role in reinforcing the heterogeneous joints. As a whole, picosecond IR laser was more preferable for surface pretreatment in adhesive heterojunctions due to its higher processing and enhancing efficiency.


2

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure TOC (8cm*4cm) INTRODUCTION With the pressing demand of car lightweight, composites like carbon fiber reinforced plastics (CFRP) have been adopted increasingly to replace traditional steel in order to improve fuel efficiency of automobiles by reducing their dead-weights. Accordingly, heterogeneous joints between different materials are increased gradually and thus how to enhance the heterogeneous joints or heterojunctions comes to be a key issue. Currently, both mechanical joint and adhesive bonding are used in heterojunctions. But with regard to the joints between composite and light metal parts, mechanical joints such as riveting and bolting would always cause damage to the strength of composites. Comparably, on the one hand, adhesive bonding has the advantage of lower weight and cost1, 2, higher resistance to contact corrosion3 and galvanic corrosion4, which could prolong the service lifespan under cycling loads. On the other hand, adhesive bonding gives fairly uniform stress distribution without destroying carbon fibers in reinforced plastics5. However, adhesive bonding is also limited for its low shear strength6. In terms of this issue, surface pretreatment is an effective strategy to strengthen the interfacial bonding of heterojunctions. Appropriate surface pretreatment can not only remove all contaminants on the surface (e.g., dusts, micro-organisms, lubricants, etc.), but also modify the surface status such as improving the wettability, surface energy and introducing functional groups in the surface. All these may conduce


3


Page 4 of 32

to achieve strong combination between the molecules of adhesive and substrates6, 7. Big progress has been made in promoting heterojunctions with the help of some surface pretreatments such as shot blasting, silane coupling agent8, acidification9, atmospheric pressure plasma10, peel ply treatment11 and laser scanning12. Among them, laser scanning attracts much attention recently because it can construct abundant micro-nano structures in the surface efficiently, extensively and accurately with no pollution. Actually, some researchers have proved that laser pretreatment on the surface is beneficial to enhance the adhesive joints between homogenous metal materials such as tungsten-copper13, aluminum-steel14, dual-phase steel15 and Ti-6Al-4V alloy16. Meanwhile, some researchers attempted to apply this strategy to improve the heterogeneous junction between composites and metals. Tan et al17 took fiber laser to connect the thermoplastics with steel directly, and they attributed the limited improvement to the porosity produced in the jointing zone due to the reaction and solidification shrinkage of the polymer under laser treatments. Besides, Genna et al18 promoted the adhesive bonding of carbon fiber reinforced thermoplastic PPS plastics, which is difficult to bond as known to all, by a fiber laser with different powers and various angles between the beam traveling and fiber braiding direction. Finally, the apparent shear strength was increased remarkably from 2 MPa for the untreated sample to 6.2 MPa for the laser-pretreated one. Although some adhesive is not indispensable for the connection between the same materials, for example, the joint strength of CFRP laminates could be promoted to 22.73 MPa just by direct co-bonding of the two counterparts after laser treatment 19, for heterojunctions, such as the connection between plastics and metallic materials, some specific adhesive is generally required. And in this case, surface pretreatment is necessary in most instances to improve the adhesive bonding.


4

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Due to the absorptivity of different laser energy for the composites is different, especially for the thermoplastic or thermosetting matrix, researchers explored laser treatment with different wavelengths. The influence of infrared (IR) and ultraviolet (UV) laser treatment on the roughness of the surface and bonding strength for CFRP is a little different20-22. Generally, the heat effect of IR laser is more violent and it would cause damage to the bonding between the resin matrix and the fibers. But for IR laser with different wavelengths, 3 μm is suggested to be more preferable to 10.6 μm because the former is in favor of energy adsorption for epoxy resin23. In addition, some researchers focused on the durability of the joints24. While in principle, the surface modification is determined by the energy exchange between the laser and the surface. Takahashi et al25 noticed the difference of heat conduction during IR and UV laser treatment on CFRP and they obtained respective heat conduction model. Since the pulse widths of adopted lasers were in the range of several to dozens of nanoseconds, much longer than photoelectric relaxation, it would cause severe heat effects because there was enough time for energy exchange between photons and crystal lattice, especially for IR laser. Three-dimensional numerical simulation was adopted to analyze the different removal speed of carbon fiber and resins in CFPR by IR laser26. By this token, femtosecond laser would be the best choice for surface pretreatment. Actually, Oliveria et al.27 have investigated the surface treatment of CFRP by femtosecond laser radiation. They acquired periodic surface structures by selectively removing the epoxy resin on the carbon fibers by the laser with a super-short pulse width of 550 fs while the mechanical properties were not reported in the article. Comparably, picosecond laser, an alternative ultra-fast laser, is well developed as a mature tool in industry presently and can run more cost-efficiently than femtosecond laser. Similar to femtosecond laser, negative heat effect could be ignored during surface pretreatment for CFRP by picosecond laser.


5


Page 6 of 32

So in this paper, we investigated the effect of picosecond IR laser pretreatment on the surface morphology of CFRP and consequent adhesive heterojunctions between CFRP and aluminum alloys with UV laser as the contrast. Besides the regular surface morphology observation and mechanical property examination, surface chemistry and wettability of the laser scanned surface were also analyzed to probe the intrinsic reasons that resulted in improved heterogeneous bonding. Particularly, based on aforementioned analyses, we employed finite element analysis (FEA) to determine whether the micro mechanical interlocking or chemical bonding played the dominant role in bringing about the increased shear strength. It was a little different from previous work about FEA in adhesive joints. Most of them were focused on static loading, environmental behaviors, fatigue loading, joint deformation, geometrical parameters, dynamic characteristics and interface morphology of the joints28-36, thus in this study, the comparison about the influence of mechanical interlocking and chemical bonding on the adhesive joints may indicate an appropriate strategy for surface pretreatment.

MATERIAL AND METHODS Sample preparation 3.00 mm thick 6061 aluminum alloy (JINFA Copper Aluminum Co. LTD, Shenzhen) and CFRP (Weisheng composite materials co. LTD in Wuxi) were chosen as the counterparts of the specially designed joint (Figure 1). The CFRP laminates were orthogonal plies of unidirectional tape composed of carbon fibers with epoxy resin as the matrix by mold pressing technology, and the fiber volume ratio was about 70%. An acrylic adhesive named AO420 was adopted as the binder between CFRP and aluminum alloys.


6

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Before surface pretreatment, the samples were cleaned with ethanol. Two kinds of laser devices were chosen and the one was Nd:YVO4 picosecond laser (PX100-2-GH, made by Edge Wave in Germany), an IR laser with the wave length of 1064 nm. The laser beam spot was circular and the focal point was 50 μm in diameter with the pulse duration of 12 picoseconds. The other one was a proven, industrial excimer laser (LPXpro 305, made by Coherent in America), a UV laser with the wavelength of 248 nm. The laser beam was an equilateral triangle with a side length of 120 nm and the pulse duration was 25 ns which was in the range of photoelectric relaxation time37. The laser parameters are shown in Table 1. The absorptivity of the energy from IR and UV laser by aluminum were 6 % and 0.8 % respectively, which indicated that IR laser was more suitable for pretreating the surface of aluminum alloys38. The results of probe trials indicated that a patterned micro-pits in the surface of aluminum alloys was effective enough to ensure that the tensile failure occurred at the interface between the adhesive and CFRP instead of at the interface between the alloys and adhesive. Hence in this study, we kept the patterned micro-pits in the surface of aluminum alloys unchanged all the way and instead, we only concentrated on exploring the laser scanning parameters for CFRP. Since carbon fibers were very sensitive to IR laser for their extraordinarily high absorptivity of IR photons, the minimum energy fluence of 0.04 J∙cm-2 and high frequency of 2000 kHz were the only applicable choice for CFRP during IR laser scanning. In contrast, based on the limitation of the much lower frequency of UV laser, we adopted a lower scanning speed in order to make sure the laser release enough energy on the CFRP surface. In the preliminary experiments (Table S1), different parameters for line space and repeating times that affected the treatment speed (being calculated according to Equation S1) were attempted to explore the appropriate laser treatment parameters for CFRP. The set of test parameters in Table 1, being selected from Table S1, might


7


Page 8 of 32

not be the optimal processing parameters which were not the gist of this work, it conduced to differentiate the influence of the pretreatments brought about by ultrafast picosecond IR laser and traditional UV laser. Particularly, IR laser treatment demonstrated a distinct advantage over UV laser treatment on treatment speed (Table S2). Figure 1 illustrated the laser scanning process and the schematic diagram of related shear strength testing. Table 1. Laser treatment parameters for aluminum alloys and CFRP. D for line space, V for scanning speed, f for laser frequency, ρ for laser fluency and R for repeating times.

Al CFRP

Wavelength(nm)

D(μm)

V(m∙s-1)

f(kHz)

ρ(J∙cm-2)

R(times)

Treatment speed (mm2∙s-1)

1064

80

2

1000

0.71

20

8

1064

80

2

2000

0.04

1

160

248

200

1.5x10-4

0.01

3

1

0.03

Figure 1. Schematic diagram for (a) laser processing and (b) shear strength testing. Shear strength testing Figure 1b provides the single lap-shear specimen configuration. CFRP pieces were cut into the same size by a manual saw (SYJ-200H Shenyang Kejing Auto-Industrial Co., LTD) and aluminum alloy pieces were cut by a precision saw (WSQ 50, Shenyang Mike Material Processing Equipment Co., LTD). The adhesive bonded specimens were prepared as follows: (1) Cleaning the surface of the specimens using ultrasonic bath with alcohol and pure water for 5 minutes respectively to remove grease and impurities; (2) Laser treatment in air according to preset program; (3) Mixing two components of the adhesive homogeneously in a disposable paper cup with a glass rod, and


8

Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coating the adhesive onto the surface of counterparts; (4) Two shims were assembled to make sure the tensile strength was loaded in a line; (5) The assembled samples were pressed under 10MPa for 45 minutes to make sure the adhesive cured and the thickness of the adhesive was controlled to be 1mm with the help of spacer blocks. According to the testing standard GBT7124-2008, the shear strength testing was accomplished by a universal mechanical testing machine with a loading rate of 5 mm/min at room temperature. At least five samples were prepared to acquired valid data. Characterizations Scanning electron microscope (SEM, Gemini SEM 500 from Zeiss) was used to observe the surface and fracture morphology. To examine the surface topography, white light interferometer (WLI, NexviewTM from Zygo) was employed. X-ray photoelectron spectroscopy (XPS, 250Xl from Thermo Fisher Co.) was used to analyze the variations of the elemental position and chemical groups in the surface before and after laser treatment. The X-ray source employed was an Al Kα X-ray source (hν=1487 eV) generated from aluminum anode operating at the emission voltage of 15 kV with the power of 200 W, and the spot size was 650 μm. Meanwhile, surface contact angles with water were detected to evaluate the modification of the wettability of the surface (OCA 25, Dataphysics) for good wettability was beneficial to the spreadability of the adhesive. Finite element analyses Generally, roughening the surface may create mechanical interlocking between the adhesive and the connecting counterparts. And at the same time, laser treatment would introduce active chemical groups in the surface. Then, whether mechanical or chemical force dominated the reinforcement came to be the key point and so herein FEA was adopted to address this issue. The simulation model was a simplification of pure mechanical lap joint for comparing the contribution from the mechanical lap joint and chemical bonding, which was different from


9


previous models adopted to simulate the numerical value of the joints28,

Page 10 of 32

32.

The model was

composed of 3 parts, i.e., CFRP, aluminum alloys and the adhesive, and the calculations were accomplished with the help of Abaqus. Each part was meshed, and the mesh was refined for the zones with high stress concentration such as the interface between the adhesive and adherends. Ignoring marginal effects, the stress condition was supposed to be equivalent on the flat surface due to the absolutely plain stress. Considering the failure always happened in the interface between the adhesive and CFRP, the contact was only set on the CFRP surface. The maximum value of the tensile force and stress could be obtained by applying unidirectional stretching stress on the model with fixing boundary condition for the CFRP foil. Besides, the fracture energies of CFRP and aluminum alloy were not set up in the FEA for the maximal load obtained during mechanical tests was far below the breaking strength of the substrate (aluminum alloys or CFRP). Moreover, since it might be impossible to obtain the exact friction coefficient between the adhesive and the adherend, five different frictional coefficients of 0.01, 0.04, 0.1, 0.2 and 0.4 were introduced in the FEA based on the information from related references35, 39-41 and the self-lubricant character of carbon materials. All the results (Table S3) showed that the mechanical strength contributed no more than 10 % to measured values, which demonstrated the variations of the friction coefficients would not alter the qualitative conclusion that chemical bonding force played the dominant role in the heterojunctions. Thus, the frictional coefficient of 0.1 was selected ultimately for further discussion. The Young moduli are 70 GPa for aluminum alloys, 40 GPa for CFRP, and 17 GPa for the adhesive. The Poisson ratio was 0.35 both for CFRP and aluminum, 0.414 for the adhesive. The physical parameters of aluminum alloys were provided by the manufactures and those of CFRP and adhesives were obtained by practical measurements.


10

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSIONS Surface morphology and topography The surface morphology and topography of the pristine and laser treated sample characterized by SEM and WLI were demonstrated in Figure 2. Therein Figure 2a and 2b showed the surface of pristine CFRP and aluminum alloys. Some irregular pores in CFRP surface might be ascribed to residual blisters in epoxy resin or shrinkage during preparation process. The roughness of pristine CFRP and aluminum alloys was about 0.9 μm. Surface morphology and three-dimensional topography of laser scanned samples were examined by SEM and WLI, respectively. For IR laser pre-treated CFRP (Figure 2c, 2d), the epoxy decomposed or vaporized thoroughly leaving nothing on the bare carbon fibers. At the same time, the surface and the ends of the fibers were ablated a little by IR laser. The roughness increased from 0.9 μm to about 2.0 μm. In contrast, in the surface of UV laser pre-treated CFRP sample (Figure 2e, 2f), only part of the epoxy was removed and carbon fibers were left undamaged. The residual epoxy on the carbon fibers was uniformly distributed and the roughness increased a little to approximately 1.5 μm. It might affect the bonding between the adhesive and the CFRP which is discussed later. Essentially, the different surface morphology and topography created by IR and UV laser could be attributed to the selective absorptivity of different components in CFRP and different photon energy of the lasers. It was reported that the photon energy was 1.17 eV in the laser of 1064 nm and 5.00 eV in that of 248 nm, the CFRP consisted of covalent bonds mostly which had a binding energy more or less of 5.78×10-19 J or 3.61 eV42. As a result, IR laser photon energy was not high enough to break the covalent bonds in CFRP so that the energy was adsorbed by the epoxy and the carbon fibers instead to some extent. Consequentially, the accumulated heat would lead to the thermal decomposition of epoxy43 and partially etching or ablation of the carbon


11


Page 12 of 32

fibers44. By comparison, photon energy of UV laser was high enough to break the bonds in CFRP directly without heat accumulation. Hence the epoxy in superficial area was vaporized by breaking covalent bonds directly without causing any damage to carbon fibers underneath the epoxy. Due to the slight heat effect, the epoxy not undergoing laser scanning would stick to the carbon fibers surface as usual (as the arrow pointed in Figure 2e)45. But the thing in common was that, the contact surface area would be increased which was corresponding to the roughening of the surface of CFRP after both kinds of laser pretreatments46. Different from CFRP, aluminum alloy was laser patterned with well-aligned pits. The interval of the pits was 80 μm and the radius of the hole was about 24 μm. The average depth of the pits is 20 μm and the average roughness was about 5.3 μm (Figure 2g,2h)


12

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM images for pristine CFRP (a), pristine aluminum alloy (b). SEM and WLI images for CFRP after IR laser treatment (c)(d), UV laser treatment (e)(f), and for aluminum alloy after IR laser treatment (g)(h).


13


Page 14 of 32

Wettability Although the intrinsic shear strength of the adhesive is crucial to the joint strength of heterojunctions, the wettability of the adhesive to the adherends is a nonnegligible factor that determines the bonding quality. So in this study, the contact angles of water were measured to evaluate the variation of the wettability of the counterparts before and after laser pretreatment (Table 2, Figure S1). Although the surface tension of acrylic adhesive was different from water, their chemical polarities were comparable and thus the contact angles of water was referentially valuable as well. The surface of the pristine CFRP was hydrophobic, after laser pretreatment it turned to be hydrophilic. The contact angle decreased considerably from 104.0o to 65.6o and 57.8o after IR and UV laser scanning, respectively. Even for hydrophilic aluminum alloy, its contact angle decreased from 86.8o to 59.3o after IR laser treatment. Based on the well-known Young equation, which describes the relationships among the free energies of the liquid and solid against their saturated vapor and of the interface between liquid and solid47, 𝛾𝑠𝑣 ― 𝛾𝑠𝑙 = 𝛾𝑙𝑣 ∙ 𝑐𝑜𝑠𝜃

(1)

We could find out that for CFRP, 𝛾𝑠𝑣 increased or 𝛾𝑠𝑙 decreased after laser pretreatment. In fact, laser scanning would roughen the surface so that surface energy would be increased accordingly. Moreover, not only the outermost epoxy on the surface of CFRP was removed by laser pretreatment, but also some hydrophilic functional groups were introduced in the surface of CFRP48 and as a result, 𝛾𝑠𝑙 decreased correspondingly.


14

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Contact angles for pristine CFRP, IR laser treated CFRP, UV laser treated CFRP, pristine aluminum alloy and IR laser treated aluminum alloy.

Samples Contact angles (°)

CFRP

Aluminum

Pristine

IR laser treated

UV laser treated

Pristine

IR laser treated

104.0

65.6

57.8

86.8

59.3

Similarly, for aluminum alloy, its wettability conformed to Wenzel regime49 (Equation S2) which demonstrated the influence of the surface roughness on the contact angles. Specifically, if the materials are primarily hydrophilic, the increasing of roughness would promote their hydrophilicity further. Vice versa, for hydrophobic materials, the increased roughness would result in increased hydrophobicity. Marco A50 and Jiangyou L51 also reported that the contact angles for laser structured surfaces of Al and Cu decreased to some extent. Since the wettability improvement is positively related to the spreadability of the adhesive, modified surface of CFRP and aluminum alloys would facilitate the sufficient contact between the adhesive and the counterparts of the heterojunction which conduced to enhance the bonding strength. XPS analyses It was reasonable that roughening the surface and improving its wettability could facilitate the full contact and strengthen the mechanical interlocking between the adhesive and the adherends. While in addition, the change of surface chemical state would affect the chemical bonding in the heterogeneous joint as well. But the variations might be different for IR laser and UV laser pretreatment. Specifically, after being pretreated by UV laser, the ratio of elemental carbon to oxygen in the surface of CFRP was just the same, While the corresponding ratio for IR laser pretreated sample was much higher and the percentage of oxygen declined dramatically (Figure 3). This discrepancy was consistent with the amount of epoxy remained on the surface of CFRP,


15


Page 16 of 32

the more the epoxy removed after laser treatment, the higher the ratio of C/O. These results confirmed the SEM and WLI observations.

Figure 3. XPS spectra of Pristine, IR laser treated and UV laser treated CFRP


16

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Deconvolution of C 1s and O 1s XPS spectra of Pristine CFRP (a) and (b), IR laser pretreated CFRP (c) and (d), UV laser pretreated CFRP (e) and (f)


17


Page 18 of 32

Although the ratios of O were different for IR and UV laser pretreated samples (Figure 3), further detailed deconvolution of O peaks indicated that laser introduced functional groups were similar (Figure 4b, 4d and 4f). Compared with the single C-O characteristic peak at the binding energy of 532.86 eV for the pristine CFRP, new peaks at about 531.90 eV and 532.18 eV appeared for IR and UV laser pretreated samples, respectively. They should all be attributed to C=O bond52 and this demonstrated that laser pretreatment could increase the active chemical groups which conduced to strengthen chemical bonding in adhesive joint8. Different from the peak fitting results of O1s peak, the deconvolution results of C1s were slightly different for IR and UV laser treated samples. For pristine CFRP, the two characteristic peaks at 284.79 eV and 286.52 eV should be attributed to C-C and C-O, respectively36. After being pretreated by IR laser, additional C=O bond appeared at about 285.56 eV53 in the surface of CFRP, while for UV pretreated samples, besides the original C-C bond, the initial C-O bond was replaced by O-C=O with a binding energy of 286.00 eV54 (Figure 4a,4c and 4e), which could also improve the reactivity of CFRP surface. As for the peak at 288.04 eV, some researchers suggested it to be radical carbon residue on the carbon fiber derived from epoxy carbonization at an instantaneous high temperature45. In contrast, because of the intense heat effect during IR laser scanning, almost no carbon remained (Figure 2c), so there was no peak at 288.04 eV. Besides, the content of silicone (Si) in the surface was also one of the important factors that affected the mechanical properties of the heterojunctions between CFRP and aluminum alloys for Si was a potential surface contaminant from mold release agent. It might build up into a coating layer on the CFRP surface21 and cause negative effects on the fracture toughness of the joints55. The peak positions of Si 2s and Si 2p were at 153 eV and 103 eV, respectively (Figure S2). These results were consistent with those in the previous published work56. Elemental variations showed


18

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that Si content decreased from 6.60 % in the pristine CFRP to 1.98 % and 4.82 % after IR laser and UV laser treatment, respectively (Figure 3). As a result, besides the increasing of contact area after laser treatment, the decreasing of Si content also contributed to improve the heterojunction strength, especially after IR laser treatment. As a whole, both IR and UV laser scanning could break the chemical bonds in epoxy and introduce some reactive groups such as C=O and O-C=O, which could interact with –COOH in acrylate structural adhesive57. At the same time, laser treatment also decreased the silicone release agent on the surface. Thus, laser treatment intensified the chemical bonding between the adhesive and the adherends. Different from CFRP, because of its evident oxidizability with considerable oxide layer, the chemical composition of laser scanned aluminum alloy surface changed little (Figure 5) and the slight increase of Al/O might be ascribed to thinning or partial eradication of aluminum oxide layer46.

Figure 5. XPS spectra of pristine aluminum and IR laser treated aluminum.


19


Page 20 of 32

Shear strength The shear strength of the heterojunction increased remarkably after laser pretreatment in comparison to the pristine one. Corresponding stress-distance curves were shown in Figure S3a. To be specific, the shear strengths were 21.89 MPa and 24.82 MPa for UV and IR pretreated samples, respectively. They were 293.8 % and 346.4 % higher than that of untreated heterojunction (Figure 6). Furthermore, compared with other state-of-art treatments such as plasma58, peel-ply59, sanding and grist blast60 (Table S4), the shear strength of laser treatment was not the highest although it is competitive among them. To be specific, peel-ply, a frequently used treatment in industry, is quite applicable and competitive although the rate of increase is not that distinct. Sanding treatment is another effective pretreating technology while its application might be limited by its manual operation mode. Thus, as a whole, if considering the difference about the adhesives, testing standards, and treatment efficiencies among the different processing means, it can be still deemed that laser treatment is a competitive and promising processing technology for its convenience, flexibility, high efficiency and remarkable improvement in heterojunctions with adhesive as the bonding medium. Besides, laser pretreatment has been reported in some previous work (Table S5). Reitz et al.22 reported similar results that the shear strength was increased from 7.2 MPa to 14.9 MPa and 15.6 MPa, respectively, for UV and nano-second IR laser pretreated samples. While some reports showed that the shear strength could be increased to 20.2 MPa by nano-second IR laser pretreatment20 or be promoted much from 7.5 MPa to 25.0 MPa by UV laser pretreatment21. Since the types of adhesives adopted and the testing methods in the published works were different, the detailed numerical results were not that comparable. The more noteworthy was the difference of the failure modes for the laser pretreated samples.


20

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Although all the sandwich-like heterojunctions were broken in the CFRP side, the failure modes were different (Table 3, Figure S4 and S5). Especially, the difference between the failure modes could be differentiated by the schematic diagrams in the column of “Patterns” in Table 3. For the pristine samples, they always failed at the interface between the CFRP and the adhesive, which was called adhesion failure (AF)6. Obviously, this type of failure occurred due to the relatively weak bonding between the adhesive and the untreated smooth surface layer of epoxy in CFRP. In contrast, after being pretreated by UV laser, the fracture mode transformed into a blend mode of AF and cohesion substrate failure (CSF) mode22, which happened at the interlamination of CFRP instead of at the interface between the adhesive and CFRP. In this case, AF mode was dominant. While after being scanned by IR laser, the fracture behavior was basically in accordance with CSF mode, in which the carbon fibers were peeled off from the CFRP surface and sticking to the adhesive. The primary cause of the different failure modes should be correlated with the variation of the bonding strength in the sandwich-like structure after laser pretreatment. For UV laser scanned sample, only part of the matrix epoxy was removed (Figure 2e), the bonding between carbon fibers and epoxy was stronger than that between the adhesive and epoxy-partially-removed surface, so that AF mode still played the leading role. Different from UV laser, heat effect of IR laser was more intense, only bare carbon fibers left after laser scanning (Figure 2c). Because of chemically activated carbon fibers, the bonding between carbon fibers and the adhesive was much stronger than that between orthogonal layers in CFRP, so it was easy to result in delamination in CFRP instead of at the interface. It was in common for both IR and UV treated samples that not only mechanical interlocking but also wettability and chemical bonding were enhanced, nevertheless, for UV treated samples, the


21


Page 22 of 32

loose granular carbon residue on the carbon fiber might cause negative effects in adhesive jointing (inset in Figure 2e). so in general, the picosecond IR laser pretreatment exhibited better performance than UV laser pretreated ones not only for the higher enhancement of joint strength, but also for the higher processing efficiency of picosecond IR laser.

Figure 6. Shear strengths of pristine counterparts of CFRP/aluminum alloys, and laser pretreated ones of CFRP/aluminum alloys.


22

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Fracture surface, patterns, failure modes and binding force for the heterojunctions before and after different pretreatments. Finite element analyses Based on the experimental analyses above, we could attribute the boosted heterogeneous jointing strength to the strengthened mechanical interlocking, the improved wettability and fortified chemical bonding between the adhesive and the laser scanned surface. However, quantitatively, which one was the decisive mechanism left to be a pending issue. Some researchers believed that the interface morphology played an important role in reinforcing the bonding between the adhesive and the adherends based on theoretical calculations35, 36, while the effect of chemical bonding could not be ignored either. Since the contribution of chemical bonding was hard to quantify, in this work, only shear stress contributed by pure mechanical interlocking was simulated by finite


23


Page 24 of 32

element analyses. The contribution from chemical bonding was acquired indirectly by subtracting the calculated mechanical contribution from the practical measured shear strength.

Figure 7. The model of the UV laser pre-treatment (a)(b) and the IR laser pre-treatment (c)(d) The model (Figure 7) was built based on the adhesive, laser pretreated CFRP and aluminum alloy. The modeling of the laser pretreated surface was based on the results of WLI detections. Since the roughness of the pristine CFRP surface was too low, the mechanical interlocking force by FEA was set to be zero approximately. Because the plain stress state of the specimen during stretching was the same, the model could be simplified in a contrast to the whole. The forcedistance curves of experimental and finite element analysis were shown in Figure S3. The maximum stress of the UV laser pretreated specimen is 0.80 MPa and that of the IR laser pretreated specimen is 0.82 MPa when the frictional coefficient was set to be 0.1. So accordingly, the contributions from chemical bonding could be obtained accordingly in Table 4. Obviously, chemical bonding always plays the leading role in promoting the strength of heterojunctions no matter which kind of laser adopted. This might be due to the height of the regular asperities in the laser pretreated surface was not high enough to produce strong mechanical


24

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interlocking at the interface61. The ratio of the contribution from mechanical interlocking by finite element analyses is only 3.63 % for UV laser pretreated specimen and 3.31 % for IR pretreated one. In accordance with this, chemical bonding force increased from 5.46 MPa for the pristine heterojunction to 21.10 MPa and 24.00 MPa for UV and IR laser pretreated specimens, respectively. In this sense, it should be the increased contact surface area, laser-induced functional groups and consequential improved wettability that led to the enhanced jointing strength for the heterojunction.

Table 4. The contributions of chemical bonding and mechanical interlocking to the shear strength for the pristine, the UV and IR laser pretreated heterojunctions. Measured value

Mechanical bonding

Chemical bonding

(MPa)

force by FEA (MPa)

force (MPa)

Smooth surface

5.56

0

5.56

UV laser treated surface

21.90

0.80

21.10

IR laser treated surface

24.82

0.82

24.00

CONCLUSION In this study, ultrafast picosecond IR laser was introduced to strengthen the heterogeneous joint between aluminum alloy and CFRP with nanosecond UV laser as the contrast. Because of the slight heat effect of picosecond IR laser and selective absorption of laser energy by CFRP, the surface morphology, roughness and chemical composition of IR laser pretreated sample was a little different from those of UV laser pretreated one. The roughness of IR laser pretreated surface was a little higher than that of UV laser pretreated one, while the contact angle of water decreased a little more for UV laser pretreated CFPR sample, which implied better wettability and


25


Page 26 of 32

spreadability of the adhesive. On the one hand, the increased surface roughness, enlarged surface area and improved spreadability strengthened the mechanical interlocking in the heterojunction. On the other hand, some functional groups such as C=O and O-C=O in the surface introduced by laser pretreatment could react with –COOH in the adhesive and thus fortified the chemical bonding between the adhesive and CFRP. As a result, the shear strengths of the heterojunctions were greatly improved by 346.4% for picosecond IR laser pretreated samples and 293.8% for UV laser pretreated ones. Fracture surface analyses of the heterojunctions indicated that CSF mode was the only failure mode for IR laser pretreated samples which further demonstrated the relatively better reinforcing efficiency for picosecond IR laser. Moreover, the FEA results indicated that ratio of the contribution by mechanical interlocking to the joint strength was lower than 4% in both cases which suggested that fortified chemical bonding played the dominant role in strengthening the heterogeneous joint. As a whole, picosecond IR laser was more preferable to UV laser as the pretreating instrument for its higher processing efficiency and better enhancing effects in strengthening heterogeneous joints.

AUTHOR INFORMATION Corresponding Author * E-mail: (Qianming Gong) [email protected] * E-mail: (Guisheng Zou) [email protected] Notes The authors declare no competing financial interest.


26

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China [grant numbers 2017YFB1103300, 2017YFB1104900], the National Natural Science Foundation of China [grant numbers 51772165, 51520105007], and Tsinghua University Initiative Scientific Research Program [grant number 20141081146]. SUPPORTING INFORMATION The equations about treatment speeds and Wenzel regime, images about contact angles and fracture surfaces, deconvolution of Si XPS spectra, force-distance curves, preliminary and supplementary results about laser treatment, comparisons with previous work. REFERENCES 1. Baldan, A., Adhesively-Bonded Joints and Repairs in Metallic Alloys, Polymers and Composite Materials: Adhesives, Adhesion Theories and Surface Pretreatment. J Mater Sci 2004, 39 (1), 1-49. 2. Banea, M. D.; da Silva, L. F. M., Adhesively Bonded Joints in Composite Materials: An Overview. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications 2016, 223 (1), 1-18. 3. Molitor, P.; Barron, V.; Young, T., Surface Treatment of Titanium for Adhesive Bonding to Polymer Composites: A Review. International Journal of Adhesion and Adhesives 2001, 21 (2), 129-136. 4. Haga, O.; Koyama, H.; Kawada, K., Mechanical Properties of a New Type Super Hybrid Material. Advanced Composite Materials 1996, 5 (2), 139-149. 5. Kretsis, G.; Matthews, F. L., The Strength of Bolted Joints in Glass-Fiber Epoxy Laminates. Composites 1985, 16 (2), 92-102. 6. Budhe, S.; Banea, M. D.; de Barros, S.; da Silva, L. F. M., An Updated Review of Adhesively Bonded Joints in Composite Materials. International Journal of Adhesion and Adhesives 2017, 72, 30-42. 7. Kanerva, M.; Saarela, O., The Peel Ply Surface Treatment for Adhesive Bonding of Composites: A Review. International Journal of Adhesion and Adhesives 2013, 43, 60-69. 8. Sang, J.; Sato, R.; Aisawa, S.; Hirahara, H.; Mori, K., Hybrid Joining of Polyamide and Hydrogenated Acrylonitrile Butadiene Rubber through Heat-Resistant Functional Layer of Silane Coupling Agent. Applied Surface Science 2017, 412, 121-130. 9. Gonzalez-Canche, N. G.; Flores-Johnson, E. A.; Cortes, P.; Carrillo, J. G., Evaluation of Surface Treatments on 5052-H32 Aluminum Alloy for Enhancing the Interfacial Adhesion of


27


Page 28 of 32

Thermoplastic-Based Fiber Metal Laminates. International Journal of Adhesion and Adhesives 2018, 82, 90-99. 10. Mohan, J.; Ramamoorthy, A.; Ivanković, A.; Dowling, D.; Murphy, N., Effect of an Atmospheric Pressure Plasma Treatment on the Mode I Fracture Toughness of a Co-Cured Composite Joint. The Journal of Adhesion 2014, 90 (9), 733-754. 11. Arenas, J. M.; Alía, C.; Narbón, J. J.; Ocaña, R.; González, C., Considerations for the Industrial Application of Structural Adhesive Joints in the Aluminium–Composite Material Bonding. Composites Part B: Engineering 2013, 44 (1), 417-423. 12. Rotel, M.; Zahavi, J.; Tamir, S.; Buchman, A.; Dodiuk, H., Pre-Bonding Technology Based on Excimer Laser Surface Treatment. Applied Surface Science 2000, 154, 610-616. 13. Jiang, D.; Long, J.; Han, J.; Cai, M.; Lin, Y.; Fan, P.; Zhang, H.; Zhong, M., Comprehensive Enhancement of the Mechanical and Thermo-Mechanical Properties of W/Cu Joints Via Femtosecond Laser Fabricated Micro/Nano Interface Structures. Materials Science and Engineering: A 2017, 696, 429-436. 14. Alfano, M.; Pini, S.; Chiodo, G.; Barberio, M.; Pirondi, A.; Furgiuele, F.; Groppetti, R., Surface Patterning of Metal Substrates through Low Power Laser Ablation for Enhanced Adhesive Bonding. The Journal of Adhesion 2014, 90 (5-6), 384-400. 15. Rotella, G.; Alfano, M.; Schiefer, T.; Jansen, I., Evaluation of Mechanical and Laser Surface Pre-Treatments on the Strength of Adhesive Bonded Steel Joints for the Automotive Industry. Journal of Adhesion Science and Technology 2016, 30 (7), 747-758. 16. Palmieri, F. L.; Watson, K. A.; Morales, G.; Williams, T.; Hicks, R.; Wohl, C. J.; Hopkins, J. W.; Connell, J. W., Laser Ablative Surface Treatment for Enhanced Bonding of Ti6Al-4V Alloy. Acs Appl Mater Inter 2013, 5 (4), 1254-1261. 17. Tan, X.; Zhang, J.; Shan, J.; Yang, S.; Ren, J., Characteristics and Formation Mechanism of Porosities in CFRP During Laser Joining of CFRP and Steel. Composites Part B: Engineering 2015, 70, 35-43. 18. Genna, S.; Leone, C.; Ucciardello, N.; Giuliani, M., Increasing Adhesive Bonding of Carbon Fiber Reinforced Thermoplastic Matrix by Laser Surface Treatment. Polymer Engineering & Science 2017, 57 (7), 685-692. 19. Leone, C.; Genna, S., Effects of Surface Laser Treatment on Direct Co-Bonding Strength of CFRP Laminates. Composite Structures 2018, 194, 240-251. 20. Zhan, X.; Li, Y.; Gao, C.; Wang, H.; Yang, Y., Effect of Infrared Laser Surface Treatment on the Microstructure and Properties of Adhesively CFRP Bonded Joints. Optics & Laser Technology 2018, 106, 398-409. 21. Rauh, B.; Kreling, S.; Kolb, M.; Geistbeck, M.; Boujenfa, S.; Suess, M.; Dilger, K., UV-Laser Cleaning and Surface Characterization of an Aerospace Carbon Fibre Reinforced Polymer. International Journal of Adhesion and Adhesives 2018, 82, 50-59. 22. Reitz, V.; Meinhard, D.; Ruck, S.; Riegel, H.; Knoblauch, V., A Comparison of IR- and UV-Laser Pretreatment to Increase the Bonding Strength of Adhesively Joined Aluminum/CFRP Components. Composites Part A: Applied Science and Manufacturing 2017, 96, 18-27. 23. Blass, D.; Nyga, S.; Jungbluth, B.; Hoffmann, H. D.; Dilger, K., Composite Bonding PreTreatment with Laser Radiation of 3 Microm Wavelength: Comparison with Conventional Laser Sources. Materials (Basel) 2018, 11 (7). 24. Palmieri, F. L.; Belcher, M. A.; Wohl, C. J.; Blohowiak, K. Y.; Connell, J. W., Laser Ablation Surface Preparation for Adhesive Bonding of Carbon Fiber Reinforced Epoxy Composites. International Journal of Adhesion and Adhesives 2016, 68, 95-101.


28

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Takahashi, K.; Tsukamoto, M.; Masuno, S.; Sato, Y., Heat Conduction Analysis of Laser CFRP Processing with IR and UV Laser Light. Composites Part A: Applied Science and Manufacturing 2016, 84, 114-122. 26. Ohkubo, T.; Sato, Y.; Matsunaga, E.-i.; Tsukamoto, M., Three-Dimensional Numerical Simulation During Laser Processing of Cfrp. Applied Surface Science 2017, 417, 104-107. 27. Oliveira, V.; Sharma, S. P.; de Moura, M. F. S. F.; Moreira, R. D. F.; Vilar, R., Surface Treatment of CFRP Composites Using Femtosecond Laser Radiation. Optics and Lasers in Engineering 2017, 94, 37-43. 28. He, X., A Review of Finite Element Analysis of Adhesively Bonded Joints. International Journal of Adhesion and Adhesives 2011, 31 (4), 248-264. 29. Zhao, X.; Adams, R. D.; da Silva, L. F. M., A New Method for the Determination of Bending Moments in Single Lap Joints. International Journal of Adhesion and Adhesives 2010, 30 (2), 63-71. 30. Mubashar, A.; Ashcroft, I. A.; Critchlow, G. W.; Crocombe, A. D., Modelling Cyclic Moisture Uptake in an Epoxy Adhesive. The Journal of Adhesion 2009, 85 (10), 711-735. 31. Jen, Y.-M.; Ko, C.-W., Evaluation of Fatigue Life of Adhesively Bonded Aluminum Single-Lap Joints Using Interfacial Parameters. International Journal of Fatigue 2010, 32 (2), 330-340. 32. Rudawska, A., Adhesive Joint Strength of Hybrid Assemblies: Titanium Sheet-Composites and Aluminium Sheet-Composites—Experimental and Numerical Verification. International Journal of Adhesion and Adhesives 2010, 30 (7), 574-582. 33. Pereira, A. M.; Ferreira, J. M.; Antunes, F. V.; Bártolo, P. J., Analysis of Manufacturing Parameters on the Shear Strength of Aluminium Adhesive Single-Lap Joints. Journal of Materials Processing Technology 2010, 210 (4), 610-617. 34. Alfano, M.; Furgiuele, F.; Pagnotta, L.; Paulino, G. H., Analysis of Fracture in Aluminum Joints Bonded with a Bi-Component Epoxy Adhesive. J Test Eval 2011, 39 (2), 296-303. 35. Haghpanah, B.; Chiu, S.; Vaziri, A., Adhesively Bonded Lap Joints with Extreme Interface Geometry. International Journal of Adhesion and Adhesives 2014, 48, 130-138. 36. Zhang, Z.; Shan, J.; Tan, X., Evaluation of the CFRP Grafting and Its Influence on the Laser Joining CFRP to Aluminum Alloy. Journal of Adhesion Science and Technology 2017, 32 (4), 390-406. 37. Lawrence, E. O.; Beams, J. W., The Element of Time in the Photoelectric Effect. Phys Rev 1928, 32 (3), 0478-0485. 38. Chen, J.; Zhang, Q.-l.; Yao, J.-h.; Fu, J.-b., Study on Laser Absorptivity of Metal Material. Journal of Applied Optics 2008, 29 (5), 793-8. 39. Gray, P. J.; McCarthy, C. T., A Global Bolted Joint Model for Finite Element Analysis of Load Distributions in Multi-Bolt Composite Joints. Compos Part B-Eng 2010, 41 (4), 317-325. 40. McCarthy, C. T.; McCarthy, M. A.; Stanley, W. F.; Lawlor, V. P., Experiences with Modeling Friction in Composite Bolted Joints. Journal of Composite Materials 2005, 39 (21), 1881-1908. 41. Atre, A.; Johnson, W. S., Analysis of the Effects of Interference and Sealant on Riveted Lap Joints. J Aircraft 2007, 44 (2), 353-364. 42. Fischer, F.; Kreling, S.; Dilger, K., Surface Structuring of CFRP by Using Modern Excimer Laser Sources. Physics Procedia 2012, 39, 154-160. 43. Levchik, S. V.; Weil, E. D., Thermal Decomposition, Combustion and Flame-Retardancy of Epoxy Resins? A Review of the Recent Literature. Polymer International 2004, 53 (12), 1901-


29


Page 30 of 32

1929. 44. Sato, Y.; Tsukamoto, M.; Matsuoka, F.; Ohkubo, T.; Abe, N., Thermal Effect on CFRP Ablation with a 100-W Class Pulse Fiber Laser Using a PCF Amplifier. Applied Surface Science 2017, 417, 250-255. 45. Veltrup, M.; Lukasczyk, T.; Ihde, J.; Mayer, B., Distribution and Avoidance of Debris on Epoxy Resin During UV ns-Laser Scanning Processes. Applied Surface Science 2018, 440, 11071115. 46. Huang, Y.; Li, Y.; Gong, Q.; Zhao, G.; Zheng, P.; Bai, J.; Gan, J.; Zhao, M.; Shao, Y.; Wang, D.; Liu, L.; Zou, G.; Zhuang, D.; Liang, J.; Zhu, H.; Nan, C., Hierarchically Mesostructured Aluminum Current Collector for Enhancing the Performance of Supercapacitors. ACS Appl Mater Interfaces 2018, 10 (19), 16572-16580. 47. Owens, D. K.; Wendt, R. C., Estimation of Surface Free Energy of Polymers. Journal of Applied Polymer Science 1969, 13 (8), 1741-&. 48. Wu, G.; Ma, L.; Liu, L.; Wang, Y.; Xie, F.; Zhong, Z.; Zhao, M.; Jiang, B.; Huang, Y., Interfacially Reinforced Methylphenylsilicone Resin Composites by Chemically Grafting Multiwall Carbon Nanotubes onto Carbon Fibers. Composites Part B: Engineering 2015, 82, 5058. 49. Wenzel, R. N., Surface Roughness and Contact Angle. J Phys Colloid Chem 1949, 53 (9), 1466-1467. 50. Alfano, M.; Lubineau, G.; Furgiuele, F.; Paulino, G. H., Study on the Role of Laser Surface Irradiation on Damage and Decohesion of Al/Epoxy Joints. International Journal of Adhesion and Adhesives 2012, 39, 33-41. 51. Long, J.; Zhong, M.; Fan, P.; Gong, D.; Zhang, H., Wettability Conversion of Ultrafast Laser Structured Copper Surface. Journal of Laser Applications 2015, 27 (S2), S29107. 52. Ahmed, M. H.; Byrne, J. A.; McLaughlin, J. A. D.; Elhissi, A.; Ahmed, W., Comparison between FTIR and XPS Characterization of Amino Acid Glycine Adsorption onto Diamond-Like Carbon (DLC) and Silicon Doped DLC. Applied Surface Science 2013, 273, 507-514. 53. Singh, B.; Fang, Y.; Cowie, B. C. C.; Thomsen, L., Nexafs and Xps Characterisation of Carbon Functional Groups of Fresh and Aged Biochars. Organic Geochemistry 2014, 77, 1-10. 54. Zaldivar, R. J.; Kim, H. I.; Steckel, G. L.; Nokes, J. P.; Morgan, B. A., Effect of Processing Parameter Changes on the Adhesion of Plasma-Treated Carbon Fiber Reinforced Epoxy Composites. Journal of Composite Materials 2009, 44 (12), 1435-1453. 55. Markatos, D. N.; Tserpes, K. I.; Rau, E.; Brune, K.; Pantelakis, S., Degradation of Mode-I Fracture Toughness of CFRP Bonded Joints Due to Release Agent and Moisture Pre-Bond Contamination. J Adhesion 2014, 90 (2), 156-173. 56. Kreling, S.; Fischer, F.; Delmdahl, R.; Gäbler, F.; Dilger, K., Analytical Characterization of CFRP Laser Treated by Excimer Laser Radiation. Physics Procedia 2013, 41, 282-290. 57. Tao, R.; Alfano, M.; Lubineau, G., Laser-Based Surface Patterning of Composite Plates for Improved Secondary Adhesive Bonding. Composites Part A: Applied Science and Manufacturing 2018, 109, 84-94. 58. Rhee, K. Y.; Yang, J. H., A Study on the Peel and Shear Strength of Aluminum/CFRP Composites Surface-Treated by Plasma and Ion Assisted Reaction Method. Composites Science and Technology 2003, 63 (1), 33-40. 59. Holtmannspotter, J.; Czarnecki, J. V.; Wetzel, M.; Dolderer, D.; Eisenschink, C., The Use of Peel Ply as a Method to Create Reproduceable but Contaminated Surfaces for Structural Adhesive Bonding of Carbon Fiber Reinforced Plastics. J Adhesion 2013, 89 (2), 96-110.


30

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60. Prolongo, S. G.; Gude, M. R.; Del Rosario, G.; Urena, A., Surface Pretreatments for Composite Joints: Study of Surface Profile by SEM Image Analysis. Journal of Adhesion Science and Technology 2010, 24 (11-12), 1855-1867. 61. Byskov-Nielsen, J.; Boll, J. V.; Holm, A. H.; Højsholt, R.; Balling, P., Ultra-HighStrength Micro-Mechanical Interlocking by Injection Molding into Laser-Structured Surfaces. International Journal of Adhesion and Adhesives 2010, 30 (6), 485-488.


31


Figure TOC


Page 32 of 32

Experimental and Theoretical Investigation of Laser Pretreatment on

Recommend Documents