epoxy composite surfaces by atmospheric plasma

Applied Surface Science 283 (2013) 843–850

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Adhesion enhancement of Al coatings on carbon/epoxy composite surfaces by atmospheric plasma J.F. Coulon ∗ , N. Tournerie, H. Maillard ECAM RENNES - Louis de Broglie, Campus de Ker Lann, CS 29 128, 35 091 Rennes Cedex 09, France

a r t i c l e

i n f o

Article history: Received 5 April 2013 Received in revised form 4 July 2013 Accepted 5 July 2013 Available online 13 July 2013 Keywords: Adhesion strength Pull-off test Atmospheric plasma Surface free energy Metallic coating Carbon/epoxy composite

a b s t r a c t Adhesion strengths between aluminium thin film coatings and manufactured carbon/epoxy composite surfaces were measured by assessing fracture tensile strengths using pull-off tests. The effect of the substrate roughness (nm to ␮m) of these composite surfaces on adhesion was studied by examining the surface free energies and adhesion strengths. The adhesion strengths of the coatings varied significantly. To improve the coating adhesion, each composite surface was treated with atmospheric plasma prior to deposition, which resulted in an increase in the surface free energy from approximately 40 mJ/m2 to 70 mJ/m2 because the plasma pretreatment led to the formation of hydrophilic C O and C O bonds on the composite surfaces, as demonstrated by X-ray photoelectron spectroscopy analyses. The adhesion strengths of the coatings were enhanced for all surface roughnesses studied. In our study, the effect of mechanical adhesion due to roughness was separated from the effect of modifying the chemical bonds with plasma activation. The adhesion ability of the pure resin was relatively weak. Increasing the surface roughness largely improved the adhesion of the resin surface. Plasma treatment of the pure resin also increased the surface adhesion. Our study shows that plasma activation effectively enhances the adhesion of manufactured composites, even when the surface roughness is on the order of microns. The ageing of the surface activation was also investigated, and the results demonstrate that atmospheric plasma has potential for use in the pretreatment of composite materials. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Carbon fibre/epoxy composites have long been used as structural materials in the aerospace industry and have also found application in the upmarket automotive and boat industries, wind energy and sporting goods. This industrial growth is due to the improved mechanical properties and lower densities of composite materials compared to those of traditional metal alloys. For example, currently, composites account for 50% of the mass of the empty structure of new civil aircrafts [1]. Industrial composite surface treatments must prepare this new substitute material for processes as different as joining, coating, painting and even demoulding. Depending on the processes to which the surfaces are subjected, adhesion or non-adhesion properties are one of the main concerns. Polymer resins and composites are known to exhibit poor adhesion properties because of their low surface free energy, which is usually less than 45 mJ/m2 [2,3]. Therefore, there is a growing demand for better adhesion of surface coatings to these polymer materials. Numerous studies have reported improvement in polymer adhesion using either wet or dry pretreatments, especially low

∗ Corresponding author. Tel.: +33 299058466. E-mail address: [email protected] (J.F. Coulon). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.028

vacuum plasma treatments [4–7]. Moreover, atmospheric plasma treatments have also been successfully applied to enhance polymer adhesion [8,9]. To obtain good adhesion, especially for organic coating/organic substrate systems mostly used in printing or adhesive joints, it is well known that the wettability of the polymer surfaces must be improved [10,11]. However, few studies address improving the adhesion of metal to polymer surfaces. In this case, a higher surface free energy (and higher wettability) might not be sufficient to enhance the adhesion of the coating [12,13]. Better adhesion requires the formation of metal–O–polymer bonds as reported for several metal/polymer systems, which are employed in different applications such as aluminium metallisation, electronic devices or aluminium barrier layers in food packaging [12–19]. Our laboratory studies surface modifications of manufactured composites with the goal of improving the adhesion or nonadhesion of the surface. We previously reported that manufactured surfaces could be altered to achieve a non-wetting behaviour with surface energies as low as 15–20 mJ/m2 after treatment with cold fluorinated plasmas [20] or after the addition of specific fluorinated species while curing the epoxy resin [21]. In contrast, this study focuses on increasing the free surface energy of manufactured carbon/epoxy composites in an effort to increase the adhesion properties of a thin layer made of conductive material such as aluminium. For this purpose, we

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Polar surface energy Dispersive surface energy

80

1.5 cm 12-14 mm

Surface energy (mJ.m-2)

70 60 50 40 30 20 10 -30 -25 -20 -15 -12 -10 -7 -5 -2 0 2 5 7 10 12 15 20 25 30

0 Posion (mm)

Fig. 1. Drawing of the gap between nozzle and surface and its effect area detailed on the chart of surface free energy measurements versus lateral distance to the torch scan axis (position).

applied air atmospheric plasma on samples with different roughnesses and assessed the resulting adhesion of metallic aluminium thin coatings with pull-off tests. Thus, the effects of the surface roughness, chemistry and thermodynamics on the adhesion have been examined. This study provides new insights into the three main adhesion mechanisms involved (mechanical interlocking, chemical bonding and thermodynamic adhesion [3,22,23]) for thin films on polymer surfaces. 2. Materials and methods 2.1. Materials Carbon/epoxy composites, referred to as composites, were manufactured by an aeronautics company using a resin transfer moulding process. The fibre reinforcement was approximately 60 wt%. The resin, which was supplied by Hexcel, was an epoxy thermoset resin, RTM6, with a cured density of 1.14 g/cm3 . The resin was a monocomponent system in which the chemicals were premixed and already degassed. The chemical composition is based on tetrafunctional molecules such as tetraglycidylmethylenedianiline and a mixture of aromatic amines as a hardener. Two types of composite materials produced using different manufacturing processes were studied. The first type, composite A, was made according to a standard process involving the use of a technical fabric whose weft left a negative imprint on the surface resin after removal, inducing high surface roughness. The second type of composite, composite B, was manufactured according to a new process designed to ensure a flatter surface. Two different levels of surface roughness were thus examined. Additionally, model epoxy resin samples, referred to as resin, were prepared in the laboratory using the same RTM6 resin as that used for the composites. The chosen curing cycle was 120 min at 180 ◦ C as recommended by the resin supplier. Samples were placed in a secondary vacuum at a pressure of 10−4 mbar and room temperature for 24 h for main desorption. The resin and composite samples were all rinsed with demineralised water and cleaned for 10 min in a pure alcohol ultrasound bath to remove any released agents and dust. 2.2. Atmospheric plasma surface treatment An atmospheric plasma treatment was applied using the Ultra Light System arc plasma torch from AcXysTM . The discharge was obtained by introducing filtered air into two cylindrical concentric electrodes and applying a high voltage (1.5 kV) and low frequency power (100 kHz). The air inlet pressure was 4 bar. An outlet in

the outer electrode creates an afterglow region, which is used for treatments, at a nozzle exit. This discharge is under non-local thermodynamic equilibrium (non-LTE) and hence produces a cold plasma [24], which can be applied on heat-sensitive substrates such as polymers. The afterglow contains activated neutral and metastable species but is devoid of ions, which ensures that the effects on the surface are mainly chemical in nature. The temperature at the nozzle exit does not exceed 200 ◦ C. The atmospheric plasma surface activation process was optimised to maximise the effect of the treatment on the sample surface and prevent thermal degradation. The experiments were performed on model resin surfaces prepared in the laboratory. To optimise the process, a single strip of the surface was exposed to plasma (see Fig. 1) to compare the untreated and activated surfaces using the same sample. The influence of the operating parameters, such as the gap between the nozzle and the surface, scan speed, number of scans and shift between the scan lines, was investigated. The effect of the treatment was quantified by measuring the surface free energy. Additionally, the roughness was measured to confirm the lack of thermally induced roughening of the surface. These experiments led to the optimised profile shown in Fig. 1. The surface free energy increased from 40 mJ/m2 to 65 mJ/m2 after a single plasma scan with the following optimised parameters: a gap between the nozzle and the surface of 1.5 cm and a scan speed of 0.2 cm/s. This increase is entirely due to the increase in the polar surface energy component from 8 mJ/m2 to 40 mJ/m2 as shown in Fig. 1. Under these conditions, the treatment was effective over a width of 12–14 mm with a constant high surface energy obtained over the whole width of the strip. An abrupt change in the surface energy was observed at the limit of the afterglow plume, indicating that the treatment was directed straight at the surface. An additional scan at the surface induced a subsequent increase in the surface free energy up to 70 mJ/m2 . Beyond two scans, the treatment no longer had an effect on the surface energy, i.e., no further change in the surface energy was observed. Based on these results, further sample preparations were performed to achieve the maximum effect of the treatment. The first scan was completed by translating the torch by 5 mm between scan strips to ensure that each point on the surface was exposed at least twice to the plasma, thus achieving saturation with the treatment. A second scan was performed perpendicularly to the first in order to improve the homogeneity of the treatment. 2.3. Analytical tools The surfaces roughness was evaluated with an Alpha-Step D120 profilometre of the KLA-Tencor series and with a Veeco atomic

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force microscope (AFM) in the tapping mode. The arithmetic average roughness, Ra, was chosen as the representative roughness parameter. The Ra is the mean of 4 measurements made in 2 perpendicular directions. The analysed length was 30 mm. Microscopic observations were performed with a HITACHI S-3200 N variable pressure scanning electron microscope (VPSEM). Because the composites are non-conductive materials and the variable pressure mode does not require further metallisation of the surface, all observations were obtained with backscattered electrons at a working pressure of 15–20 Pa. X-ray photoelectron spectroscopic (XPS) analyses were performed using a VSW Scientific Instruments HA100 spectrometer with Mg K␣ radiation (1253.6 eV) under a vacuum of 10−9 mbar. The sample was introduced into the analysis chamber in 3 steps to ensure that surface species were completely desorbed from the surface of the composite by pumping before the sample entered the next chamber. To correct for charging effects, the spectra were shifted based on a mixed C C/C C bond peak position at 284.8 eV [25–27]. 2.4. Wettability and surface free energy Contact angle measurements were performed using the sessile drop technique to determine the wettability of the surface. The surface energies were calculated using the Owens–Wendt method developed in 1969 [2], which is based on the geometric mean equation and static contact angles of reference liquids. Two reference liquids, typically a polar liquid (distilled water) and an apolar liquid (diiodomethane), are necessary. Single drops (3 ␮l) of water and diiodomethane were placed on the surfaces of the samples. The contact angles, , of the droplets were calculated based on the diameter, D, and the height, h, of the droplet on the surface according to the following equation [28]:  = 2 arctan

 2h  D

.

The measurements were taken 10 s after the drop has made contact with the surface. The average values of 8 drops were used; the standard deviation was approximately 1◦ for the drop angle measurements and 2 mJ/m2 for the surface energy calculations. The samples were rinsed with pure alcohol prior to the measurement, except if they were treated with atmospheric plasma. 2.5. Thermally evaporated thin films Aluminium thin films were grown on the resin and composite surfaces from aluminium wires using an Alcatel thermal evaporator. The setup initial vacuum was 5 × 10−6 mbar, and the pressure did not exceed 10−5 mbar during the deposition. For each sample, aluminium films with a thickness of 1 ␮m were deposited simultaneously on both the untreated and plasma-modified surfaces to ensure identical growth conditions. The aluminium films were optically flat with a metallic aspect. The film thickness was controlled after deposition using the step height measurement feature of the Alpha-Step D-120 profilometre. The error in the thickness was less than 10%, while the difference in the thicknesses of the centre and the edges of a sample was less than 5%. 2.6. Pull-off test The pull-off test (also named the tensile adhesive test) has been developed in our laboratory and was chosen from the numerous adhesion measurement methods available [29]. Guidelines for adapting the pull-off test to a substrate/coating system with thick or thin metallic coatings deposited on different rigid or even flexible substrates have appeared in the literature through standards

Fig. 2. Pull-off test assembly as inserted into the tensile tester.

[30–32]. The reproducible strength values obtained by performing the pull-off test can be used as references to study the relative changes in adhesion in coating/substrate systems or even to evaluate direct adhesive bonding on uncoated substrates as reported by Moghadamzadeh et al. [33]. In any case, quantitative results must be used with caution because the perpendicular load generally does not correspond to that in the real application. The preparation of the pull-off test requires special attention to obtain a reliable tensile strength value as previously discussed by several authors [34–36]. We successfully applied the pull-off test to several thin films deposited on different substrates. For strongly adhesive coated films, the pull-off test was applied up to approximately 70 MPa. In any case, we considered the following: - Using a glue with a higher mechanical resistance than the one studied for the coating/substrate system. - Strict alignment of the stress applied to the adhesion interface under study, which was ensured by using a ball joint. - Stiffness of the substrate. - Thickness and cohesion of the coating, which may imply that the area of the coating and the dolly be the same. - Limitation of this test in the case of very thin coatings due to glue diffusion through the pores of the material. In this study, pull-off tests were performed as illustrated in Fig. 2. At least 5 specimens were tested, and the resulting pull strengths were averaged. Aluminium dollies that were 2 cm in diameter were glued to the metallic coating with an epoxy adhesive (Araldite 2011, bicomponent epoxy resin supplied by Huntsman). Araldite 2011 has been tested alone and found to support up to 20 MPa, making it adequate for this study. A special hanger allows for the aluminium dolly to be freely linked to the top of this assembly where a 14 mm-diameter ball joint supplied by Mycelium Roulement (France) allows the system to freely rotate on each axis. The

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Table 1 Roughness, surface free energy parameters and adhesion strength for the 3 studied materials. Sample

Average roughness Ra

Dispersive component

Polar component

Composite A Composite B Pure resin

5.0 ± 0.6 ␮m 0.8 ± 0.1 ␮m 15 ± 3 nm

Sd = 34.6 mJ/m2 Sd = 33.2 mJ/m2 Sd = 30.2 mJ/m2

S = 3.2 mJ/m2 p S = 5.2 mJ/m2 p S = 10.7 mJ/m2

p

Total surface energy

Adhesion strength

 S = 37.4 mJ/m2  S = 38.4 mJ/m2  S = 40.9 mJ/m2

4.9 ± 0.2 MPa 1.9 ± 0.2 MPa 0.06 ± 0.05 MPa

Fig. 3. SEM photographs of composite A (left, ×100) and composite B (right, ×500).

3.1. Untreated surfaces Table 1 reports the roughness of the different surfaces. Composite A has an Ra value of nearly 5 ␮m, and the roughness of composite B is approximately 1 ␮m. SEM photographs shown in Fig. 3 illustrate the topographical differences in the composite surfaces. The texture is mostly due to the use of a technical fabric in the case of the manufactured composite A as previously discussed in other work [37]. For the composite B, the flatter texture results from the new demoulding process, which aims to produce smoother surfaces. These surface topographies are independent of the inner carbon fibre frames, which are the same for both composites. In comparison to the composites, the resin is significantly flatter with a roughness of 15 nm. Table 1 also reports the surface free energies for our 3 substrates. The energy values are all less than or approximately equal to 40 mJ/m2 and consist of a mainly dispersive component, which is in agreement with our previous and other similar studies on carbon/epoxy composites [20,38]. The wettabilities corresponding to these moderate surface free energies show that the surfaces are not particularly hydrophilic or hydrophobic. As differences in the values of the polar and apolar components for the three samples are very slight, we consider that the difference of roughness exhibits no significant effect on the surface energy. However, the sample adhesion strengths are very different when coated with aluminium films as observed in Fig. 4. Pure resin with a roughness of a few nanometres provides very poor adhesion,

3.2. Activated surfaces 3.2.1. XPS analysis In the XPS analysis, carbon, oxygen and nitrogen were detected. Calculations based on the XPS data of the untreated surface gave atomic percentages of 84% carbon, 11% oxygen and 5% nitrogen,

Roughness and adhesion strength of the 3 materials 6

6,000

5

5,000 Ra (nm) Adhesion strength (MPa)

4,000

4

3,000

3

2,000

2

1,000

1

0

Adhésion strength (MPa)

3. Results and discussion

while the composite surfaces with roughnesses of 1 and 5 ␮m have adhesion strengths of 1.9 MPa and 5 MPa, respectively, when coated with identical aluminium films. Because the wettability and chemical composition of the resin are similar for the 3 samples, we conclude that the roughness controls the level of adhesion of the metallic coating and is directly proportional to the adhesion strength within the scale of our study. We can assume that the effect of the roughness on the practical adhesion reported here is due to pure mechanical interlocking as described by the mechanical theory of adhesion [22]. The anchoring effect which promotes the adhesion increases when the surface area or bulk energy dissipation phenomena increase [23]. The poor adhesion observed on the relatively smooth surface indicates that no other adhesion mechanisms (thermodynamic, diffusion, chemical. . .) occur in this system. As a result, the fractures between the aluminium and resin were entirely adhesive for all untreated surfaces irrespective of the roughness, indicating no chemical interactions between substrates and coatings.

Ra (nm)

whole assembly is inserted into the upper strength cell of an Instron 4204 tensile tester system (Fig. 2). Two aluminium rods hold the substrate in place at the bottom. Using a ball joint was necessary to eliminate any tangential component of the stress. Tensile experiments were performed with a 50 kN strength cell at a constant speed of 3 mm/min. The tensile experiments led to the easy rupture of the coating/substrate system due to the relatively low strengths required for these tests based on the mechanical strengths of the tools and glue. No distortion of the substrate or cohesive rupture was observed.

0 Composite A

Composite B

Resin

Fig. 4. Average roughness Ra of composites A, B and resin compared to adhesion strength of an aluminium coating on their surfaces. Standard deviation for adhesion strength was less than 0.2 MPa.

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15

20

(a)

18 16

Intensity (10 CPS)

14 12

10

3

3

Intensity (10 CPS)

847

(b)

10 8

(b)

6 4 2

(a)

0 292

290

288

286

284

5

0 292

282

290

Binding energy (eV)

288

286

284

282

Binding energy (eV)

Fig. 5. (Left) Comparison of resin C 1s XPS spectra before (a) and after (b) atmospheric plasma treatment. (Right) Comparison of resin C 1s XPS spectra after treatment at normal (line) and 60◦ (dots) incidence.

Table 2 Relative proportions of carbon bonds. C C; C C Non treated Treated

71% 42%

15

(a)

XPS data data fit (1) C=C / C-C - 284.8 eV (2) C-O /C-N - 286.2 eV

(1)

3

Intensity (10 CPS)

20

10

5

(2) 0 14 12

3

3.2.2. Activation and adhesion strength Fig. 7 shows the effect of plasma activation on the surface free energies. Once activated, all samples have significantly higher surface free energies of approximately 70 mJ/m2 whatever the surface roughness of the substrate. The increase in the surface free energy is only due to the polar component. The activation also results in

much larger adhesion strengths (Fig. 8) than those of the untreated surfaces for all our experiments (Table 3). The adhesion strengths of the aluminium coating on the activated substrates were 7 MPa and 3 MPa for composite A and composite B, respectively. On the very flat resin surface, the effect of the pretreatment is relatively greater than that for the composites; the pretreated coating is able to withstand 1.2 MPa, while the untreated coating had an adhesion strength near 0 MPa. Atmospheric pressure plasma treatments of polymers have been reported to modify the surface roughness on the scale of nanometres [16,40]. In our study, the surface roughness of the composites,

Intensity (10 CPS)

which are in good agreement with the bulk chemical formulae of epoxy resins. The plasma-treated surface shows a significant increase in the O/C ratio from 0.13 to 0.44, indicating that the surface is enriched in oxygen. However, the amount of nitrogen remains approximately constant after treatment, confirming that the surface is mainly modified by oxygen in the plasma process. The detailed C 1s spectra given in Fig. 5a show considerable increases in the carbon binding energies after plasma treatment. The initial peak at 284.8 eV, which includes C C (285 eV) and C C (284.5 eV) bonds [25–27], is significantly reduced, and a new structure appears near 289 eV. This shift to a higher binding energy is consistent with the increase in the amount of oxygen at the surface and indicates the formation of carbon–oxygen bonds. Normalised C 1s spectra recorded at angles of incidence of 0◦ and 60◦ (Fig. 5b) show the lack of angular dependence of the peak profile, indicating the treatment effects extend over the full depth probed by XPS (≈5 nm). The relative weight of each type of carbon is extracted from the C 1s peak decomposition reported in Fig. 6 before (a) and after (b) atmospheric plasma treatment. Quantitative analysis gave the relative composition shown in Table 2. Before plasma treatment of the resin, the majority (71%) of the surface groups are C C and C C bonds, and a smaller fraction (24%) of C O and C N bonds are present, in agreement with the RTM6 chemical formulae (77% and 23%, respectively). The small component observed at 288.2 eV is attributed to functional additives included in the resin. After plasma treatment of the resin, new components appear at 289.3 eV and 288.2 eV corresponding to CO O and C O bonds, respectively [39]. The former accounts for 20% of the carbon bonds, while the latter accounts for 15%. At the same time, the number of carbon–carbon bonds decreases, indicating that the CO O and C O groups are formed by a reaction with the resin carbon chains. Almost 60% of the carbon bonds involve an oxygen atom, which confirms that oxygen is strongly enriched at the resin surface.

10

XPS data data fit (1) C=C / C-C - 284.8 eV (2) C-O /C-N - 286.2 eV (3) C=O - 288.2 eV (4) CO-0 - 289.2 eV

(b)

(1)

8 6

(4)

4 2

(2) (3)

0 292 C O; C N 24% 23%

C O 5% 15%

CO O – 20%

290

288

286

284

282

Binding energy (eV) Fig. 6. RTM6 C 1s XPS spectra before (a) and after (b) atmospheric plasma treatment.

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Table 3 Comparison of surface energies and adhesion strength after activation treatment for the 3 studied materials. Dispersive component Sd

Sample Composite A Composite B Pure resin

Untreated Plasma treated Untreated Plasma treated Untreated Plasma treated

2

34.6 mJ/m 30.7 mJ/m2 33.2 mJ/m2 23.3 mJ/m2 30.2 mJ/m2 31.6 mJ/m2

p

Polar component S 2

3.2 mJ/m 41 mJ/m2 5.2 mJ/m2 45.5 mJ/m2 10.7 mJ/m2 38.7 mJ/m2

Total surface energy  S

Adhesion strength

37.4 mJ/m2 71.7 mJ/m2 38.4 mJ/m2 68.8 mJ/m2 40.9 mJ/m2 70.3 mJ/m2

4.9 7.1 1.9 3.07 0.06 1.17

± ± ± ± ± ±

0.2 MPa 1.9 MPa 0.2 MPa 0.05 MPa 0.05 MPa 0.04 MPa

Evoluon of adhesion strength with plasma treatment Adhesion strength (MPa)

8 7

Adhesion strength (MPa) without treatment Adhesion strength (MPa) with plasma treatment

6 5 4 3 2 1 0

Fig. 7. Enhancement of surface energy for composites A, B and resin with surface activation as compared to untreated materials. Standard deviation for the surface energies was 2 mJ/m2 .

which is already on the order of micrometres before treatment, does not exhibit any measurable difference after treatment. AFM images of the pure resin (Fig. 9) give an arithmetic Ra of 10 nm, which is in agreement with the surface roughness measured with the profilometre (Table 1). When activated, the surface appears smoother (Ra of about 4 nm) because the plasma treatment might have relaxed the thermal stresses. However, applying a stronger treatment (longer exposure to plasma and torch closer to the surface compared to our optimised conditions) leads to roughening and texturing of the surface (Fig. 9c). For all these reasons, we consider our normal activation conditions create no roughness increase on our samples. The adhesion enhancement brought by plasma activation is quite similar for all the samples (Fig. 8), independently of the surface roughness. This effect is only due to the chemistry modification

Composite A

Composite B

Resin

Fig. 8. Enhancement of adhesion strength of an aluminium coating for composites A, B and resin with surface activation as compared to untreated materials. Standard deviation for adhesion strength was less than 0.2 MPa.

induced by plasma treatment. The level of practical adhesion for the flat resin can be significantly improved either with roughness or with activation. Activation on rough surfaces still increase the adhesion level. As presented in Section 3.2.1, the surface activation generates carbon-oxygen reactive species, which increase both the wettability and chemical reactivity towards metal. These carbonyl and hydrophilic groups contribute to an increase in the polar component of the surface free energies for polymer surface treatments [18,41]. Moreover, the improvement in the adhesion between the substrate and metallic coating has been assigned to be caused by the formation of oxygen–metal complexes, which should be necessary [13,42], especially in the case of aluminium [12].

Fig. 9. AFM images of (a) untreated resin surfaces and (b) and (c) plasma treated resin surfaces.

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Fig. 10. Fracture surface types between aluminium coating and composite surface without (left) and with (right) plasma activation.

Fig. 11. Duration of atmospheric plasma treatment effect. Evolution of total surface energy (open symbols) and dispersive component (filled symbols) versus time for composite A (♦), composite B () and resin ().

The wettability is the first concern for coatings in liquid or viscous states. However, for more energetic coating processes such as physical vapour deposition, creating the effective chemical bonds should be the first aim to enhance metal adhesion on the polymer substrate. This change in surface chemistry will thereby affect the wettability. Thus, we conclude that the wettability may not be the sole criteria to study in order to enhance the adhesion of the aluminium coating. 3.2.3. Adhesion failure To identify the failure modes, the fracture surfaces of the adhesion test samples were examined. The fractures between the aluminium and resin were entirely adhesive for all untreated surfaces irrespective of the roughness as shown in Fig. 10 (left). This fracture was purely interfacial, which is uncommon in adhesion science [29] because it means that the coating and the substrate did not interact chemically. In this case, pure mechanical adhesion is observed. For the plasma-treated surfaces, the failure seems to occur at a different place between the aluminium and epoxy resin as shown in Fig. 10 (right). The failure was not cohesive (inside the resin) because the uncoated composite samples can withstand a tensile strength of 12.5 MPa, which is higher than the strengths of the coated and plasma-treated composites (Table 3). This fracture mode reveals that plasma treatment has created an interaction layer, identified as an interphase between the aluminium coating and plasma-treated epoxy resin. This interphase might be composed of C–O–metal bonds formed during the plasma activation. The mechanical resistance of this interphase might be lower than that of the untreated surface. This conclusion is in accordance with observations previously reported for an interphase formed from a large amount of reactive groups on a thermoplastic polymer [43]. 3.2.4. Ageing For all the previous experiments, the aluminium thin films were deposited just after the activation.

To evaluate the duration of this pretreatment effect, the surface free energies of the 3 composites were measured over 7 days, and the results are shown in Fig. 11. After the activation (at T0 ), the surface energies of approximately 70 mJ/m2 demonstrate that the surface is activated by increasing the polar component. Later, the surface free energy decreases in the same manner for the composites and resin samples. After 7 days, the surface free energy returns to its original value. This change in surface energy is mainly due to the loss of the polar component. Within that time period, the loss occurs quickly starting the day after the treatment. We focused on the first 24 h as described in Fig. 12 for composite A. The surface energy decreases exponentially over the first 24 h. The surface energy is quite high (60 mJ/m2 ) after 9 h, but it decreases to 50 mJ/m2 after 12 h. This trend is only due to the attenuation of the polar component. Data fit with an exponential decay indicates that the treatment half-life is 20 h; therefore, we recommend that the aluminium film be deposited within 10 h of the plasma pretreatment.

Fig. 12. Ageing of atmospheric plasma treatment effect on composite A, during 24 h.

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4. Conclusions In this paper, the pull-off test has been adapted to practical adhesion strength measurements for 3 epoxy-based composites with different surface roughnesses that were coated with a 1 ␮m-thick evaporated aluminium film. The surface free energies were calculated and revealed that all untreated samples exhibited a moderate wettability behaviour. Adhesion of the aluminium coating on our samples was highly improved (×75) with higher surface roughness values (×300). Our results showed that the adhesion was mainly governed by interlocking phenomena. Atmospheric pressure plasma treatments resulted in the creation of reactive carbonyl groups on the composite surfaces as demonstrated by XPS. This activation increased the surface free energies and the adhesion forces over those of the untreated samples. The adhesion strengths were highly improved after plasma activation for all surface roughnesses studied. In our study, improving the adhesion was obtained either by an increase in the roughness or by the formation of metal–carbonyl bonds, which contributions have been discussed separately. Both effects can add up as plasma activation on rough surfaces still increases the adhesion level. To enhance the adhesion of aluminium coating on composite surfaces via physical vapour deposition, we noticed that the wettability may not be the sole criteria to study. The key mechanism involved in plasma activation is the formation of C–O–metal chemical bonds. Acknowledgements The authors thank C. Baudet for her contribution to these experiments. The XPS experiments were performed at the Institut de Physique de Rennes, and the AFM images were obtained by the IETR microelectronics group from the University of Rennes 1. References [1] R. Stewart, Carbon fibre composites poised for dramatic growth, Reinforced Plastics May (2009) 16–21. [2] D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, Journal of Applied Polymer Science 13 (1969) 1741–1747. [3] F. Awaja, M. Gilbert, G. Kelly, B. Fox, P.J. Pigram, Adhesion of polymers, Progress in Polymer Science 34 (2009) 948–968. [4] F. Poncin-Epaillard, G. Legeay, Why is the modification of polymer surfaces with electromagnetic discharge so advantageous, Annales de Chimie – Science des Matériaux 28 (2003) 55–66. [5] K. Navaneetha Pandiyaraj, V. Selvarajan, R.R. Deshmukh, C. Gao, Adhesive properties of polypropylene (PP) and polyethylene terephthalate (PET) film surfaces treated by DC glow discharge plasma, Vacuum 83 (2008) 332–339. [6] L. Bárdos, H. Baránková, Cold atmospheric plasma: sources, processes, and applications, Thin Solid Films 518 (2010) 6705–6713. [7] V. Andri, F. Arefi, J. Amouroux, P. Puydt Y. d. Bertrand, G. Lorang, et al., Metallization of polypropylène films pretreated in a nitrogen or ammonia low pressure plasma, Thin Solid Films 181 (1989) 451–460. [8] M.J. Shenton, et al., Adhesion enhancement of polymer surfaces by atmospheric plasma treatment, Journal of Physics D: Applied Physics 34 (2001) 2754. [9] U.M. Rashed, H. Ahmed, A. Al-Halwagy, A.A. Garamoon, Surface characteristics and printing properties of PET fabric treated by atmospheric dielectric barrier discharge plasma, The European Physical Journal – Applied Physics 45 (2009) 11001. [10] J.K. Kim, H.S. Kim, D.G. Lee, Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints, Journal of Adhesion Science and Technology 17 (2003) 329–352. [11] G.C. Basak, A. Bandyopadhyay, S. Neogi, A.K. Bhowmick, Surface modification of argon/oxygen plasma treated vulcanized ethylene propylene diene polymethylene surfaces for improved adhesion with natural rubber, Applied Surface Science 257 (2011) 2891–2904. [12] C. Bichler, H.-C. Langowski, U. Moosheimer, B. Seifert, Adhesion mechanism of aluminum, aluminum oxide, and silicon oxide on biaxially oriented polypropylene (BOPP), poly(ethyleneterephthalate) (PET), and poly(vinylchloride) (PVC), Journal of Adhesion Science and Technology 11 (1997) 233–246.

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