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Influence of composite processing on the properties of CNT grown on carbon surfaces
Accepted Manuscript Title: Influence of composite processing on the properties of CNT grown on carbon surfaces Authors: Claire Guignier, Marie-Ange Bueno, Brigitte Camillieri, Bernard Durand PII: DOI: Reference:
S0169-4332(17)32869-6 https://doi.org/10.1016/j.apsusc.2017.09.221 APSUSC 37300
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-8-2017 25-9-2017 26-9-2017
Please cite this article as: Claire Guignier, Marie-Ange Bueno, Brigitte Camillieri, Bernard Durand, Influence of composite processing on the properties of CNT grown on carbon surfaces, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.09.221 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of composite processing on the properties of CNT grown on carbon surfaces Claire GUIGNIER, Marie-Ange BUENO,* Brigitte CAMILLIERI, Bernard DURAND University of Haute-Alsace, Laboratoire de Physique et Mécanique Textiles, Ecole Nationale Supérieure d’Ingénieurs Sud-Alsace, 11 rue Alfred Werner, 68093 Mulhouse, France Graphical Abstract
Highlights
The influence of catalyst used for CNT growth and abrasive wear on the tribological behaviour of the surface The influence of catalyst used for CNT growth on the adhesion of CNTs on the surface The influence of catalyst used for CNT growth and abrasive wear on the wettability of the surface by epoxy resin The influence of catalysts used for CNT growth and abrasive wear on the surface with CNTs physical and chemical structure.
Abstract *Corresponding author. Tel: +33 (0)3 89 33 60 41. E-mail address: [email protected]
Carbon nanotubes (CNT) grafted on carbon fibres (CF) are the subject of more and more studies on the reinforcement of composite materials thanks to the CNT’ mechanical properties. This study concerns the growth of CNT directly on CF by the flame method, which is an assembly-line process. However the industrial-scale use of this method and of the composite processing leads to stresses on the CNT-grafted fabrics, such as friction and pulling-out. The aim of this study is to determine the behaviour of the CNT under these kinds of stresses and to study theirs consequences in composite processing. For this purpose, adhesion tests and friction tests were performed as well as analysis of the surface by Scanning Electron Microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDX). In friction tests, CNT formed a transfer film, and its effect on the wettability of the fabric with epoxy resin is determined. Finally, the wear of the CNT does not influence the wettability of the fabric. Furthermore, it is proven that the nature of the catalyst needed to grow the CNT modifies the behaviour of the surface. Keywords: Carbon fibres; Carbon nanotubes; Friction; Wear; Wettability; Adhesion 1. Introduction The mechanical properties of composite materials depend on the quality of the interface formed between the reinforcement fibres and the matrix. That is why various techniques exist to reinforce the interface, and more and more studies of carbon nanotubes (CNT) are being conducted for this purpose, thanks to the CNT excellent mechanical properties. A hierarchical composite structure is created by grafting CNT directly on the surface of fibres by various techniques [1]. CNT can be chemically grafted on the surface of the fibre by oxidation [2-4] or the use of dendrimers [5-7], grafted by electrophoresis [8-11], or added during the sizing of the fibres [12-14]. However, all these techniques require many steps, and the realisation of the process becomes long and complicated. That
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is why some methods were developed in order to realise the growth of CNT directly on the surface of the fibres through chemical vapour deposition (CVD) [15-17] and derived processes [18, 19] or by the flame method [20-23]. The CVD processes have the advantage of controlling the morphology of the produced CNT, but the process of growing the CNT takes a long time. The flame method can be realised in an assemblyline process, with a high speed of production, so it can be easily scaled up to industrial level. In this study, the flame method is used to realise the growth of CNT on the surface of carbon fibres (CF) with metallic catalyst. However, considering the industrialisation of the process, the CNT-grafted surfaces will be wound on a roll and stock. Then, for composite manufacturing, the CNT-grafted fabric must be unwound, guided, and positioned with a cylinder in the composite mould (see Figure 1(a)). This causes stress on the fabric, particularly on the CNT, such as the pulling out of CNT, and friction between the CNT and the metallic cylinders (see Figure 1(a)). Then, during manufacture of the composite (see Figure 1(b)), the wettability of the resin on the fabric is one of the most important parameters. Thus this study aims to determine the consequences of the industrialisation of the composite manufacture on the CNT-grafted fabrics. It was decided to simulate the pulling-out of the CNT in order to determine their adhesive resistance, to perform the friction of the fabric against the metallic piece, and to simulate the layer guidance. Analysis of the surfaces before and after friction was performed to observe the consequences of the friction on the CNT. Finally, the consequences of the friction stresses on the wettability behaviour of the CNT-grafted fabrics were enhanced. Moreover, the conditions of growth of CNT were considered in the entire study in order to see the influence of the condition parameters.
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2. Materials and methods 2.1. Fabrication and characterisation of carbon nanotubes The raw material is a woven plain fabric composed of 6K multifilament carbon warp (the carbon filaments have a diameter of 7 µm and 6000 filaments are assembled in each multifilament) and E-glass weft. The raw material is treated by sulfuric acid in order to eliminate the sizing. Multi-walled carbon nanotubes (MWCNT) are produced directly on the carbon fabric by the flame method. This method consists in spray-coating the fabric with a metallic catalyst and then inserting the fabric, for a few seconds, into a flame formed by the combustion of acetylene gas [22, 23]. This step allows the formation of MWCNT on the carbon fabric surface. The chemical nature of the catalyst and the conditions of growth influence the MWCNT produced, particularly their morphology, density, purity, and growth mechanism. In this study, two ferrous catalysts have been chosen, namely Fe3O4 and Ferrofluid, allowing optimal MWCNT to be grown under the same conditions. Scanning Electron Microscopy (SEM) observations (Hitachi S2360-N) were performed in order to characterise the produced MWCNT in terms of diameter, length, and density. The determination of the chemical composition of the CNT-grafted surface is performed by X-ray fluorescence using microprobe energy-dispersive X-ray spectroscopy (EDX) coupled with SEM (Philips XL30). The detection of elements is carried out from boron and semi-quantitative analysis is possible from sodium. Elements with contents below 0.1% are not detected. Raman spectroscopy was performed on a Horiba Labram BX40 with a laser wavelength of 532 nm and a power of 1 mW measured on the sample with a scan time of 180 s. Nine spectra were collected on each surface. Peak fitting was performed using
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Matlab. Raman spectroscopy allows the determination of the purity of the produced MWCNT. 2.2. Adhesion tests The adhesion tests are used to determine which of the catalysts creates the most resistant CNT/CF interface under a stress representing the unwinding of the fabric from the storage roller (using as storage after the flame process and before the composite manufacturing). The measurement method was developed in a previous study [24] and consists in indentation of the surface with a spherical indenter covered by adhesive tape to reach a fixed normal load. Then, during the unloading step, the adhesion of the CNT can be observed, as shown in Figure 2. The test parameters were fixed at 1 N for the applied normal load and with a speed of 10 µm/s to reach the normal load. These test conditions made it possible to avoid different disruptive phenomenon (breaking of the adhesive tape, taking out of the filament …) while the breaking appears in the CNT layer or between the CNT and the fibres. The displacement of the indenter and the normal load were recorded during the tests, and SEM observations were realised on the surface before and after the tests as well as on the adhesive tape after the tests. 2.3. Friction tests Friction tests are realised with a linear reciprocating tribometer, described elsewhere [25] (see Figure 3), which is constituted of a tangential force sensor and allows different geometries of the slider. The normal load is applied by means of dead weights, and the sliding distance, speed, and acceleration can be chosen. First of all, friction tests were conducted in order to investigate the rolling/sliding between the fabric and a metallic piece. Indeed, in industrial process (Fig. 1), whatever the roller is motorised or not, the main friction stress is due to sliding. The slider was a
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stainless steel cylinder with a diameter of 20 mm under a normal load of 1 N. The normal force was chosen in order to represent the yarn tension on a machine, i.e. 1 N on each yarn, 7 yarns for 10 mm width, and the wrapping angle (90°) applied when unwinding the fabric [26]. The sliding distance was fixed at 20 mm and the sliding speed at 20 mm/s. During the tests, SEM observations were conducted in order to see the evolution of the surface. 2.4.Wettability measurement During the manufacturing process of composite materials, the fibre–matrix adhesion and therefore the wettability of the reinforcement are among the most important properties influencing the mechanical properties of the composite [27]. In order to study the influence of the wear of CNT on the wettability of the surface, wettability tests were performed with the complex epoxy Epolam 2020 and an amine as hardener (Axson Technologies). In order to free the measurement of the viscosity of the epoxy complex, a repeatable test method is carried out: droplets of 15 µL are placed on the surface and a camera is used to record the video of the spreading of the droplet on the surface. Contact angle measurements are carried out with the DropSnake plugin on Image J. Ten droplets are placed on each surface in order to have a representative evolution of the contact angle over time. 3. Results and discussion 3.1. Characterisation of the CNT First of all, the CNT produced on the carbon fibres have been characterised. Figure 4 shows SEM pictures of the CNT-grafted CF obtained with both catalysts. The measurement of the outer diameter of the CNT shows that they present a diameter of around 50 ± 20 nm for Fe3O4 catalyst and 30 ± 10 nm for Ferrofluid. This means that the
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nature of the catalyst influences the geometry of the CNT, and in this way, the structural properties of the CNT (number of walls), because if the diameter is bigger, this can mean that the number of the walls is higher. Indeed, in theory, the inner diameter of CNT is always constant (at around 2 nm) and the mean distance between two walls is around 0.34 nm [28], so if the diameter of the CNT increases, then the number of walls of the CNT increases. Figure 5 presents the Raman spectra obtained with the two catalysts. They look like MWCNT spectra [29, 30], so it can deduced that the measurement was realised on the CNT and not on the carbon fibres. The spectra present two characteristic bands, labelled D and G, which represent respectively the disorder in the carbon structure and the crystalline part of the structure. The ratio between the intensity of these two bands (ID/IG) allows the quantity of structural defects in the CNT structure to be estimated. The value of the ratio is about 1.2 for the CNT produced with Fe3O4 catalyst and 1.02 for the CNT produced with the Ferrofluid catalyst. So there are fewer defects in the CNT with smaller diameter. A defect in the CNT structure can be the formation of a pentagon instead of a hexagon on the graphene structure or missing atoms, which will cause a failure in the structure and chemical heterogeneity of the CNT surface. Therefore, the probability of having a defect is smaller with a smaller number of walls. EDX analysis was performed in order to determine the chemical composition of the surface, and the results are presented in Figure 6. The two surfaces are mainly composed of carbon from the CNT, oxygen and iron from the catalyst, and phosphorous and sulfur, which are residues of the flame method. The only difference between the two catalysts is the presence of aluminium with the Ferrofluid catalyst. So finally the two surfaces are
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almost identical, but the presence of aluminium can change the chemical and physical properties of the surface. 3.2.Adhesion Adhesion tests are processed in order to determine which of the catalysts produces the strongest interface between the CNT and the fibres. Figure 7 presents the evolution of the normal force function of the driving in of the surface by the adhesive indenter. This curve is composed of different steps. The first step is the loading step, when the normal load increases rapidly as the driving in decreases. Then the second step is the unloading, when the normal load decreases rapidly with the increase of the driving in. As soon as the normal load passes below zero, this is the adhesion step, which represents the force needed to separate the surfaces. When the force returns to zero, the contact between the two surfaces is broken. In this study, two parameters are obtained from this curve, the adhesion force, represented by the minimal value of the force during the adhesion step, and the energy of adhesion, represented by the area of the negative part of the curve. The values obtained for the adhesion force and the adhesion energy are given in Figure 8, and it is clear that they are much more important for the Ferrofluid than the Fe3O4 catalyst. However, the force needed to tear the CNT depends on the quantity of CNT in contact and on the quantity of torn CNT, which is why the surface of the torn CNT has been measured by of binarisation of the picture [24]. Figure 8(c) presents the results obtained for both catalysts and it appeared that Fe3O4 catalyst gives more CNT fragments than Ferrofluid. At the end, it is the value of the adhesive force needed to tear the CNT per unit surface area that has the greatest significance. This value is of 9.99 mN/mm2 for Fe3O4 catalyst and 21.07 mN/mm2 for Ferrofluid catalyst, so it represents an increase of 111% between the two catalysts. This means that it is more
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difficult to separate the two surfaces with Ferrofluid than with Fe3O4, so the interface between CNT and CF is stronger with Ferrofluid catalyst than with Fe3O4. However, the CNT produced with Ferrofluid catalyst present fewer defects, particularly structural ones, so the mechanical properties of these CNT are higher than the those of the CNT produced with Fe3O4. That is why the breaking of the interface or of the CNT occurs more often for the Fe3O4 catalyst. Moreover, the interface between the fibre and the catalyst particles can be weaker because of the chemical composition of the catalyst and due to the particle size distribution. In fact, with Ferrofluid catalyst, particles are smaller, so they have a greater specific surface and can be better linked to the surface. 3.3. Friction 3.3.1. Evolution of friction at industrial scale Friction tests were realised in order to simulate industrial friction, which corresponds to no more than 20 cycles of friction. That is why 20 cycles of friction have been processed in order to see the behaviour of the surface. Figure 9 presents the evolution of the obtained mean of the friction coefficient in function of the number of cycles for the sized carbon fibres and the CNT-grafted CF obtained with both catalysts. First of all, a small decrease in the friction coefficient between the first and second cycles of friction can be noticed. This is due to the rearrangement of the textile structure, which allows relative movement between the filaments inside the yarn. Then, the presence of the CNT drastically increases the friction coefficient of 70% for Fe3O4 catalyst and 150% for Ferrofluid catalyst. Indeed, the presence of the CNT on the surface induces a modification of the surface state, where the CNT act like hairs, creating a rough surface and causing an increase in the friction
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coefficient. The friction force is probably the total of the bending force applied on the CNT to them lay down and the adhesion force between the slider and the CNT [31]. Then, it can be observed that the friction coefficient is always higher for the CNTgrafted CF obtained with the Fe3O4 catalyst compared to those obtained with the Ferrofluid catalyst. The friction coefficient increases of 47% for the first cycle of friction and 21% in the 20th cycle of friction. However, because CNT obtained with Fe3O4 catalyst have a bigger diameter, they have a higher number of walls. Moreover, a study showed that the bending rigidity of MWCNT increases with the number of walls [32]. Therefore, higher force is needed to bend CNT obtained with Fe3O4 compared to those obtained with Ferrofluid catalyst.. This can explain the higher values of the friction coefficient for the CNT obtained with Fe3O4 compared to those obtained with Ferrofluid. 3.3.2. Analysis of the CNT’ surface-wear mechanism SEM observations were conducted in order to determine the effect of the friction on the surface of the CNT. Figure 10 presents the evolution of the surface state during the first 10 cycles of friction. It is clear that before the friction, the CNT are totally and randomly entangled at the surface (see Figure 10(a)). As soon as the friction starts (see Figure 10(b)), the CNT are aligned in the direction friction, that is, toward the fibre axis. This behaviour was already shown in a previous study [33]. As the friction continues (see Figure 10(c)), the CNT are crushed and form a transfer film on the surface. This behaviour, already highlighted with CNT on various substrates [34, 35], is shown for the two catalysts, and no difference was found. This means, that, after industrial friction stresses, that is, after passing over the cylindrical rolls, the CNT will not be entangled on the surface but will form a transfer film. That is why the transfer film has been analysed in order to enhance any difference with the initial surfaces.
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First of all, Figure 11 presents the transfer film for the two surfaces obtained with Fe3O4 and Ferrofluid catalysts. No distinct difference is observed. The transfer film is continuous and compact on the surface of the fibres. The Raman spectra obtained from the transfer film are presented in Figure 12, and they have the same appearance as the spectra of the initial surface (Figure 5), which means that the transfer film is always composed of CNT and that the structure of their cylindrical layer is still the same. The determination of the quantity of defects gave values of 1.06 and 0.91 for CNT obtained with the Fe3O4 and Ferrofluid catalysts, respectively. The quantity of defects is always smaller for those obtained with Ferrofluid catalyst, which means that even if the surface is stressed, the difference between the two surfaces does not change. Moreover, it can be noticed a decrease in the quantity of defects of 12% for Fe3O4 catalyst and 11% for Ferrofluid catalyst in comparison with the initial surface. These are unexpected results, because they mean that after a friction stress the CNT have a greater purity than the initial CNT. However, this result can be explained by the fact that there is some amorphous carbon at the surface of the CNT, particularly around the CNT, and with the friction, this amorphous carbon tends to leave the contact and so more CNT are visible for the measurement of the quantity of defects. However, the presence of amorphous carbon is present in surfaces obtained with both catalysts, because it is only due to the flame method. Thereafter, the chemical composition of the transfer film was determined, and the obtained EDX spectra are presented in Figure 13. The chemical difference between the surfaces obtained with the two catalysts is always remarkable due to the presence of aluminium when the Ferrofluid catalyst is used. However, there is no difference in
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chemical composition of the surface before and after friction, which means that during the friction tests, there is no material transfer from the slider to the CNT. To briefly summarise regarding the friction and wear of the CNT, as noticed above, the friction stresses are composed of a bending process of the CNT on the surface, and the CNT formed a transfer film on the surfaces obtained with both catalysts. However the transfer film is always composed of cylindrical CNT, which means that the friction stresses are not high enough to crush the CNT’ structure. Moreover the friction tests do not change the chemical composition of the surface. 3.3.3. Evolution of friction and wear of transfer film relative to the sliding cycle number In the previous study, it was shown that the friction stresses lead to the formation of a transfer film composed of CNT for surfaces obtained with both catalysts. In this section, the influence of the number of friction cycles on the stability of the transfer film has been studied. For this purpose, the friction tests were realised under a normal load of 1 N and 2000 cycles of friction were performed. Figure 14 presents the evolution of the mean of the friction coefficient of the sized carbon surface and the CNT-grafted carbon surfaces obtained with both catalysts. The decrease in the first cycles of friction is still noticeable, due to the textile rearrangement. Then the friction coefficient is higher for the CNT-grafted surface than for the sized carbon surface, which means that the presence of the CNT increases the friction coefficient, even if they are in transfer film state. Moreover, it can be observed that the friction coefficient of the CNT-grafted carbon surface decreases progressively with the number of cycles, so it can concluded that a different phenomenon appears on the surface, and previous studies show that the decrease of the friction coefficient is due the apparition
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of a wear mechanism [26]. The wear mechanism occurs with the formation of the transfer film and then the appearance of some cracks in the film, finally reaching the breaking of the film and leaving the carbon fibres with only a few pieces of CNT. In this study, the stability of the transfer film is determined for surfaces obtained with both catalysts under the same conditions. That is why SEM observations have been done during the friction tests of 2000 cycles. Figure 14 presents the schematisation of the state of the CNT observed during the friction tests for both catalysts. It appears that the decrease of the friction coefficient goes hand in hand with the advancement of the wear mechanism. Indeed, for CNT obtained with Fe3O4 catalyst, the transfer film is present below 500 friction cycles, while after 800 friction cycles the transfer film breaks progressively and the friction coefficient decreases progressively. However, for the CNT obtained with Ferrofluid catalyst, the decrease of the friction coefficient continued during the first 300 cycles of friction and it appears that after that, the transfer film is already broken, which means that the whole of the wear mechanism occurs continuously. So it can concluded that the Fe3O4 catalyst makes it possible to achieve a greater friction resistance of the transfer film. 3.4.Wettability by epoxy resin During the composite processing, the wettability of the fabric with the resin is one of the most important parameters. However, in the previous sections it is highlighted that the friction stresses modify the surface state of the CNT, and it is essential to determine the influence of the wear of the CNT on the wettability. During the wettability tests, it appears that the drops spread over all of the studied surface, which is why we decided to measure the contact angle as a function of time to quantify the impregnation.
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Figure 15 presents the evolution of the contact angle as a function of time for all of the studied surfaces. It can be clearly observed that the presence of CNT increases the contact angle of 24% for the Fe3O4 catalyst and 27% for Ferrofluid catalyst. As CNT create a roughness on the surface of the fibre, their presence modifies the porosity of the surface, which influences the impregnation of the resin. The influence of the wear of the CNT can be seen from the difference between the dotted line and the solid line for each catalyst. For CNT obtained with both catalysts, no significant difference between the evolution of the contact angle for the worn and unworn surfaces can be seen. This means that the impregnation of the surface does not depend on the presence of vertical and entangled CNT or horizontal and aligned CNT on the surface, that is, on the roughness of the surface. However, 30 seconds after the beginning of the wettability, the contact angle of the surface with CNT has almost the same value as that of the sized surface, which means that the impregnation is slower, but at the end, the results are the same. Then, the surfaces obtained with the two catalysts can be compared. The difference between the two curves is exactly the same for the unworn surface as well as the worn surface. However, the roughness of the surface does not change the wettability, otherwise there will be a difference between the worn and unworn surfaces, which means that the geometry of the CNT cannot be the reason for this difference. Finally, this difference can arise from the chemical composition of the surface, particularly the presence of aluminium on the surface obtained with Ferrofluid catalyst. 4. Conclusion
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In this study, it is shown that many stresses will be applied to the fabric after the growth of the CNT and that theses stresses have an impact on the CNT and on the properties of the surface, particularly the state of the surface. Moreover, it has been determined that the nature of the catalyst, which changes the geometrical, structural, and chemical properties of the CNT, also changes the properties of the surface. Indeed, the friction stresses are divided into bending and shear stresses on the CNT. Therefore, friction is higher (+47%) when the diameter of the CNT is large. Moreover, the adhesion of the CNT on the fibre is higher (+111%) if they present few defects in their structure. The study has highlighted the formation of a transfer film during friction but it is always composed of CNT and there is no chemical modification of the surface. Furthermore, the resistance of the transfer film under continuous friction stresses also depends on the mechanical properties of the CNT, depending on their geometry. Finally, the wettability of fabrics after the growth of CNT was measured and revealed that the chemical composition of the surface changes the kinetics of the drop spreading. It appears that the presence of the transfer film, due to wear of the CNT does not change the behaviour of the surface. This result is important because the worn and unworn surfaces have the same impregnation behaviour, so similar properties of the composite materials can be expected. Acknowledgements The authors would like to express their gratitude to Dr Florence Biguenet, Assistant Professor, and Professor Dominique Dupuis from the Laboratoire de Physique et Mécanique Textiles of the University of Haute Alsace for their help and advice regarding the wetting investigation.
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Figure captions Figure 1: Schematisation of the different stresses applied to the fabric after the growth of the CNT: (a) during the unwinding and the layer positioning and (b) during the composite manufacturing. Figure 2: Schematisation of the principle of adhesion tests and the obtained curve. Figure 3: Reciprocating tribometer: (a) global picture and (b) picture of the linear contact obtained with the cylindrical slider. Figure 4: SEM pictures of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 5: Raman spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 6: EDX spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 7: Adhesion curve obtained for surfaces obtained with the two catalysts under a normal load of 1 N. Figure 8: Histograms of the evolution of the (a) adhesion force, (b) adhesion energy, (c) area of torn CNT, and (d) adhesion force per unit surface area of torn CNT. Figure 9: Evolution of the friction coefficient for 20 cycles of friction for three surfaces: sized CF and CF grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid. Figure 10: Evolution of the CNT during the first cycles of friction: (a) before friction, (b) after the first cycle of friction, (c) after 10 cycles of friction. Figure 11: SEM pictures of the transfer film obtained with (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
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Figure 12: Raman spectra of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst, after 10 cycles of friction under 1 N. Figure 13: EDX spectra of the CNT grafted on CF; (a) Fe3O4 catalyst and (b) Ferrofluid catalyst after 10 cycles of friction under 1 N. Figure 14: Evolution of the friction coefficient for 2000 cycles of friction for three surfaces: sized CF and CF fibres grafted with CNT obtained with two catalysts: Fe 3O4 and Ferrofluid. Figure 15: Evolution of the wettability of the different fabrics with different surface states with epoxy resin.
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Figure 1: Schematisation of the different stresses applied to the fabric after the growth of the CNT: (a) during the unwinding and the layer positioning and (b) during the composite manufacturing.
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Figure 2: Schematisation of the principle of adhesion tests and the obtained curve.
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Figure 3: Reciprocating tribometer: (a) global picture and (b) picture of the linear contact obtained with the cylindrical slider.
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Figure 4: SEM pictures of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
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Figure 5: Raman spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
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Figure 6: EDX spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
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Figure 7: Adhesion curve obtained for surfaces obtained with the two catalysts under a normal load of 1 N.
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Figure 8: Histograms of the evolution of the (a) adhesion force, (b) adhesion energy, (c) area of torn CNT, and (d) adhesion force per unit surface area of torn CNT. Adhesion force (mN)
Adhesion energy (N.m)
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Figure 9: Evolution of the friction coefficient for 20 cycles of friction for three surfaces: sized CF and CF grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid.
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Figure 10: Evolution of the CNT during the first cycles of friction: (a) before friction, (b) after the first cycle of friction, (c) after 10 cycles of friction.
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Figure 11: SEM pictures of the transfer film obtained with (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
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Figure 12: Raman spectra of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst, after 10 cycles of friction under 1 N.
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Figure 13: EDX spectra of the CNT grafted on CF; (a) Fe3O4 catalyst and (b) Ferrofluid catalyst after 10 cycles of friction under 1 N.
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Figure 14: Evolution of the friction coefficient for 2000 cycles of friction for three surfaces: sized CF and CF fibres grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid.
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Figure 15: Evolution of the wettability of the different fabrics with different surface states with epoxy resin.
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S0169-4332(17)32869-6 https://doi.org/10.1016/j.apsusc.2017.09.221 APSUSC 37300
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-8-2017 25-9-2017 26-9-2017
Please cite this article as: Claire Guignier, Marie-Ange Bueno, Brigitte Camillieri, Bernard Durand, Influence of composite processing on the properties of CNT grown on carbon surfaces, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.09.221 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of composite processing on the properties of CNT grown on carbon surfaces Claire GUIGNIER, Marie-Ange BUENO,* Brigitte CAMILLIERI, Bernard DURAND University of Haute-Alsace, Laboratoire de Physique et Mécanique Textiles, Ecole Nationale Supérieure d’Ingénieurs Sud-Alsace, 11 rue Alfred Werner, 68093 Mulhouse, France Graphical Abstract
Highlights
The influence of catalyst used for CNT growth and abrasive wear on the tribological behaviour of the surface The influence of catalyst used for CNT growth on the adhesion of CNTs on the surface The influence of catalyst used for CNT growth and abrasive wear on the wettability of the surface by epoxy resin The influence of catalysts used for CNT growth and abrasive wear on the surface with CNTs physical and chemical structure.
Abstract *Corresponding author. Tel: +33 (0)3 89 33 60 41. E-mail address: [email protected]
Carbon nanotubes (CNT) grafted on carbon fibres (CF) are the subject of more and more studies on the reinforcement of composite materials thanks to the CNT’ mechanical properties. This study concerns the growth of CNT directly on CF by the flame method, which is an assembly-line process. However the industrial-scale use of this method and of the composite processing leads to stresses on the CNT-grafted fabrics, such as friction and pulling-out. The aim of this study is to determine the behaviour of the CNT under these kinds of stresses and to study theirs consequences in composite processing. For this purpose, adhesion tests and friction tests were performed as well as analysis of the surface by Scanning Electron Microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDX). In friction tests, CNT formed a transfer film, and its effect on the wettability of the fabric with epoxy resin is determined. Finally, the wear of the CNT does not influence the wettability of the fabric. Furthermore, it is proven that the nature of the catalyst needed to grow the CNT modifies the behaviour of the surface. Keywords: Carbon fibres; Carbon nanotubes; Friction; Wear; Wettability; Adhesion 1. Introduction The mechanical properties of composite materials depend on the quality of the interface formed between the reinforcement fibres and the matrix. That is why various techniques exist to reinforce the interface, and more and more studies of carbon nanotubes (CNT) are being conducted for this purpose, thanks to the CNT excellent mechanical properties. A hierarchical composite structure is created by grafting CNT directly on the surface of fibres by various techniques [1]. CNT can be chemically grafted on the surface of the fibre by oxidation [2-4] or the use of dendrimers [5-7], grafted by electrophoresis [8-11], or added during the sizing of the fibres [12-14]. However, all these techniques require many steps, and the realisation of the process becomes long and complicated. That
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is why some methods were developed in order to realise the growth of CNT directly on the surface of the fibres through chemical vapour deposition (CVD) [15-17] and derived processes [18, 19] or by the flame method [20-23]. The CVD processes have the advantage of controlling the morphology of the produced CNT, but the process of growing the CNT takes a long time. The flame method can be realised in an assemblyline process, with a high speed of production, so it can be easily scaled up to industrial level. In this study, the flame method is used to realise the growth of CNT on the surface of carbon fibres (CF) with metallic catalyst. However, considering the industrialisation of the process, the CNT-grafted surfaces will be wound on a roll and stock. Then, for composite manufacturing, the CNT-grafted fabric must be unwound, guided, and positioned with a cylinder in the composite mould (see Figure 1(a)). This causes stress on the fabric, particularly on the CNT, such as the pulling out of CNT, and friction between the CNT and the metallic cylinders (see Figure 1(a)). Then, during manufacture of the composite (see Figure 1(b)), the wettability of the resin on the fabric is one of the most important parameters. Thus this study aims to determine the consequences of the industrialisation of the composite manufacture on the CNT-grafted fabrics. It was decided to simulate the pulling-out of the CNT in order to determine their adhesive resistance, to perform the friction of the fabric against the metallic piece, and to simulate the layer guidance. Analysis of the surfaces before and after friction was performed to observe the consequences of the friction on the CNT. Finally, the consequences of the friction stresses on the wettability behaviour of the CNT-grafted fabrics were enhanced. Moreover, the conditions of growth of CNT were considered in the entire study in order to see the influence of the condition parameters.
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2. Materials and methods 2.1. Fabrication and characterisation of carbon nanotubes The raw material is a woven plain fabric composed of 6K multifilament carbon warp (the carbon filaments have a diameter of 7 µm and 6000 filaments are assembled in each multifilament) and E-glass weft. The raw material is treated by sulfuric acid in order to eliminate the sizing. Multi-walled carbon nanotubes (MWCNT) are produced directly on the carbon fabric by the flame method. This method consists in spray-coating the fabric with a metallic catalyst and then inserting the fabric, for a few seconds, into a flame formed by the combustion of acetylene gas [22, 23]. This step allows the formation of MWCNT on the carbon fabric surface. The chemical nature of the catalyst and the conditions of growth influence the MWCNT produced, particularly their morphology, density, purity, and growth mechanism. In this study, two ferrous catalysts have been chosen, namely Fe3O4 and Ferrofluid, allowing optimal MWCNT to be grown under the same conditions. Scanning Electron Microscopy (SEM) observations (Hitachi S2360-N) were performed in order to characterise the produced MWCNT in terms of diameter, length, and density. The determination of the chemical composition of the CNT-grafted surface is performed by X-ray fluorescence using microprobe energy-dispersive X-ray spectroscopy (EDX) coupled with SEM (Philips XL30). The detection of elements is carried out from boron and semi-quantitative analysis is possible from sodium. Elements with contents below 0.1% are not detected. Raman spectroscopy was performed on a Horiba Labram BX40 with a laser wavelength of 532 nm and a power of 1 mW measured on the sample with a scan time of 180 s. Nine spectra were collected on each surface. Peak fitting was performed using
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Matlab. Raman spectroscopy allows the determination of the purity of the produced MWCNT. 2.2. Adhesion tests The adhesion tests are used to determine which of the catalysts creates the most resistant CNT/CF interface under a stress representing the unwinding of the fabric from the storage roller (using as storage after the flame process and before the composite manufacturing). The measurement method was developed in a previous study [24] and consists in indentation of the surface with a spherical indenter covered by adhesive tape to reach a fixed normal load. Then, during the unloading step, the adhesion of the CNT can be observed, as shown in Figure 2. The test parameters were fixed at 1 N for the applied normal load and with a speed of 10 µm/s to reach the normal load. These test conditions made it possible to avoid different disruptive phenomenon (breaking of the adhesive tape, taking out of the filament …) while the breaking appears in the CNT layer or between the CNT and the fibres. The displacement of the indenter and the normal load were recorded during the tests, and SEM observations were realised on the surface before and after the tests as well as on the adhesive tape after the tests. 2.3. Friction tests Friction tests are realised with a linear reciprocating tribometer, described elsewhere [25] (see Figure 3), which is constituted of a tangential force sensor and allows different geometries of the slider. The normal load is applied by means of dead weights, and the sliding distance, speed, and acceleration can be chosen. First of all, friction tests were conducted in order to investigate the rolling/sliding between the fabric and a metallic piece. Indeed, in industrial process (Fig. 1), whatever the roller is motorised or not, the main friction stress is due to sliding. The slider was a
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stainless steel cylinder with a diameter of 20 mm under a normal load of 1 N. The normal force was chosen in order to represent the yarn tension on a machine, i.e. 1 N on each yarn, 7 yarns for 10 mm width, and the wrapping angle (90°) applied when unwinding the fabric [26]. The sliding distance was fixed at 20 mm and the sliding speed at 20 mm/s. During the tests, SEM observations were conducted in order to see the evolution of the surface. 2.4.Wettability measurement During the manufacturing process of composite materials, the fibre–matrix adhesion and therefore the wettability of the reinforcement are among the most important properties influencing the mechanical properties of the composite [27]. In order to study the influence of the wear of CNT on the wettability of the surface, wettability tests were performed with the complex epoxy Epolam 2020 and an amine as hardener (Axson Technologies). In order to free the measurement of the viscosity of the epoxy complex, a repeatable test method is carried out: droplets of 15 µL are placed on the surface and a camera is used to record the video of the spreading of the droplet on the surface. Contact angle measurements are carried out with the DropSnake plugin on Image J. Ten droplets are placed on each surface in order to have a representative evolution of the contact angle over time. 3. Results and discussion 3.1. Characterisation of the CNT First of all, the CNT produced on the carbon fibres have been characterised. Figure 4 shows SEM pictures of the CNT-grafted CF obtained with both catalysts. The measurement of the outer diameter of the CNT shows that they present a diameter of around 50 ± 20 nm for Fe3O4 catalyst and 30 ± 10 nm for Ferrofluid. This means that the
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nature of the catalyst influences the geometry of the CNT, and in this way, the structural properties of the CNT (number of walls), because if the diameter is bigger, this can mean that the number of the walls is higher. Indeed, in theory, the inner diameter of CNT is always constant (at around 2 nm) and the mean distance between two walls is around 0.34 nm [28], so if the diameter of the CNT increases, then the number of walls of the CNT increases. Figure 5 presents the Raman spectra obtained with the two catalysts. They look like MWCNT spectra [29, 30], so it can deduced that the measurement was realised on the CNT and not on the carbon fibres. The spectra present two characteristic bands, labelled D and G, which represent respectively the disorder in the carbon structure and the crystalline part of the structure. The ratio between the intensity of these two bands (ID/IG) allows the quantity of structural defects in the CNT structure to be estimated. The value of the ratio is about 1.2 for the CNT produced with Fe3O4 catalyst and 1.02 for the CNT produced with the Ferrofluid catalyst. So there are fewer defects in the CNT with smaller diameter. A defect in the CNT structure can be the formation of a pentagon instead of a hexagon on the graphene structure or missing atoms, which will cause a failure in the structure and chemical heterogeneity of the CNT surface. Therefore, the probability of having a defect is smaller with a smaller number of walls. EDX analysis was performed in order to determine the chemical composition of the surface, and the results are presented in Figure 6. The two surfaces are mainly composed of carbon from the CNT, oxygen and iron from the catalyst, and phosphorous and sulfur, which are residues of the flame method. The only difference between the two catalysts is the presence of aluminium with the Ferrofluid catalyst. So finally the two surfaces are
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almost identical, but the presence of aluminium can change the chemical and physical properties of the surface. 3.2.Adhesion Adhesion tests are processed in order to determine which of the catalysts produces the strongest interface between the CNT and the fibres. Figure 7 presents the evolution of the normal force function of the driving in of the surface by the adhesive indenter. This curve is composed of different steps. The first step is the loading step, when the normal load increases rapidly as the driving in decreases. Then the second step is the unloading, when the normal load decreases rapidly with the increase of the driving in. As soon as the normal load passes below zero, this is the adhesion step, which represents the force needed to separate the surfaces. When the force returns to zero, the contact between the two surfaces is broken. In this study, two parameters are obtained from this curve, the adhesion force, represented by the minimal value of the force during the adhesion step, and the energy of adhesion, represented by the area of the negative part of the curve. The values obtained for the adhesion force and the adhesion energy are given in Figure 8, and it is clear that they are much more important for the Ferrofluid than the Fe3O4 catalyst. However, the force needed to tear the CNT depends on the quantity of CNT in contact and on the quantity of torn CNT, which is why the surface of the torn CNT has been measured by of binarisation of the picture [24]. Figure 8(c) presents the results obtained for both catalysts and it appeared that Fe3O4 catalyst gives more CNT fragments than Ferrofluid. At the end, it is the value of the adhesive force needed to tear the CNT per unit surface area that has the greatest significance. This value is of 9.99 mN/mm2 for Fe3O4 catalyst and 21.07 mN/mm2 for Ferrofluid catalyst, so it represents an increase of 111% between the two catalysts. This means that it is more
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difficult to separate the two surfaces with Ferrofluid than with Fe3O4, so the interface between CNT and CF is stronger with Ferrofluid catalyst than with Fe3O4. However, the CNT produced with Ferrofluid catalyst present fewer defects, particularly structural ones, so the mechanical properties of these CNT are higher than the those of the CNT produced with Fe3O4. That is why the breaking of the interface or of the CNT occurs more often for the Fe3O4 catalyst. Moreover, the interface between the fibre and the catalyst particles can be weaker because of the chemical composition of the catalyst and due to the particle size distribution. In fact, with Ferrofluid catalyst, particles are smaller, so they have a greater specific surface and can be better linked to the surface. 3.3. Friction 3.3.1. Evolution of friction at industrial scale Friction tests were realised in order to simulate industrial friction, which corresponds to no more than 20 cycles of friction. That is why 20 cycles of friction have been processed in order to see the behaviour of the surface. Figure 9 presents the evolution of the obtained mean of the friction coefficient in function of the number of cycles for the sized carbon fibres and the CNT-grafted CF obtained with both catalysts. First of all, a small decrease in the friction coefficient between the first and second cycles of friction can be noticed. This is due to the rearrangement of the textile structure, which allows relative movement between the filaments inside the yarn. Then, the presence of the CNT drastically increases the friction coefficient of 70% for Fe3O4 catalyst and 150% for Ferrofluid catalyst. Indeed, the presence of the CNT on the surface induces a modification of the surface state, where the CNT act like hairs, creating a rough surface and causing an increase in the friction
9
coefficient. The friction force is probably the total of the bending force applied on the CNT to them lay down and the adhesion force between the slider and the CNT [31]. Then, it can be observed that the friction coefficient is always higher for the CNTgrafted CF obtained with the Fe3O4 catalyst compared to those obtained with the Ferrofluid catalyst. The friction coefficient increases of 47% for the first cycle of friction and 21% in the 20th cycle of friction. However, because CNT obtained with Fe3O4 catalyst have a bigger diameter, they have a higher number of walls. Moreover, a study showed that the bending rigidity of MWCNT increases with the number of walls [32]. Therefore, higher force is needed to bend CNT obtained with Fe3O4 compared to those obtained with Ferrofluid catalyst.. This can explain the higher values of the friction coefficient for the CNT obtained with Fe3O4 compared to those obtained with Ferrofluid. 3.3.2. Analysis of the CNT’ surface-wear mechanism SEM observations were conducted in order to determine the effect of the friction on the surface of the CNT. Figure 10 presents the evolution of the surface state during the first 10 cycles of friction. It is clear that before the friction, the CNT are totally and randomly entangled at the surface (see Figure 10(a)). As soon as the friction starts (see Figure 10(b)), the CNT are aligned in the direction friction, that is, toward the fibre axis. This behaviour was already shown in a previous study [33]. As the friction continues (see Figure 10(c)), the CNT are crushed and form a transfer film on the surface. This behaviour, already highlighted with CNT on various substrates [34, 35], is shown for the two catalysts, and no difference was found. This means, that, after industrial friction stresses, that is, after passing over the cylindrical rolls, the CNT will not be entangled on the surface but will form a transfer film. That is why the transfer film has been analysed in order to enhance any difference with the initial surfaces.
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First of all, Figure 11 presents the transfer film for the two surfaces obtained with Fe3O4 and Ferrofluid catalysts. No distinct difference is observed. The transfer film is continuous and compact on the surface of the fibres. The Raman spectra obtained from the transfer film are presented in Figure 12, and they have the same appearance as the spectra of the initial surface (Figure 5), which means that the transfer film is always composed of CNT and that the structure of their cylindrical layer is still the same. The determination of the quantity of defects gave values of 1.06 and 0.91 for CNT obtained with the Fe3O4 and Ferrofluid catalysts, respectively. The quantity of defects is always smaller for those obtained with Ferrofluid catalyst, which means that even if the surface is stressed, the difference between the two surfaces does not change. Moreover, it can be noticed a decrease in the quantity of defects of 12% for Fe3O4 catalyst and 11% for Ferrofluid catalyst in comparison with the initial surface. These are unexpected results, because they mean that after a friction stress the CNT have a greater purity than the initial CNT. However, this result can be explained by the fact that there is some amorphous carbon at the surface of the CNT, particularly around the CNT, and with the friction, this amorphous carbon tends to leave the contact and so more CNT are visible for the measurement of the quantity of defects. However, the presence of amorphous carbon is present in surfaces obtained with both catalysts, because it is only due to the flame method. Thereafter, the chemical composition of the transfer film was determined, and the obtained EDX spectra are presented in Figure 13. The chemical difference between the surfaces obtained with the two catalysts is always remarkable due to the presence of aluminium when the Ferrofluid catalyst is used. However, there is no difference in
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chemical composition of the surface before and after friction, which means that during the friction tests, there is no material transfer from the slider to the CNT. To briefly summarise regarding the friction and wear of the CNT, as noticed above, the friction stresses are composed of a bending process of the CNT on the surface, and the CNT formed a transfer film on the surfaces obtained with both catalysts. However the transfer film is always composed of cylindrical CNT, which means that the friction stresses are not high enough to crush the CNT’ structure. Moreover the friction tests do not change the chemical composition of the surface. 3.3.3. Evolution of friction and wear of transfer film relative to the sliding cycle number In the previous study, it was shown that the friction stresses lead to the formation of a transfer film composed of CNT for surfaces obtained with both catalysts. In this section, the influence of the number of friction cycles on the stability of the transfer film has been studied. For this purpose, the friction tests were realised under a normal load of 1 N and 2000 cycles of friction were performed. Figure 14 presents the evolution of the mean of the friction coefficient of the sized carbon surface and the CNT-grafted carbon surfaces obtained with both catalysts. The decrease in the first cycles of friction is still noticeable, due to the textile rearrangement. Then the friction coefficient is higher for the CNT-grafted surface than for the sized carbon surface, which means that the presence of the CNT increases the friction coefficient, even if they are in transfer film state. Moreover, it can be observed that the friction coefficient of the CNT-grafted carbon surface decreases progressively with the number of cycles, so it can concluded that a different phenomenon appears on the surface, and previous studies show that the decrease of the friction coefficient is due the apparition
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of a wear mechanism [26]. The wear mechanism occurs with the formation of the transfer film and then the appearance of some cracks in the film, finally reaching the breaking of the film and leaving the carbon fibres with only a few pieces of CNT. In this study, the stability of the transfer film is determined for surfaces obtained with both catalysts under the same conditions. That is why SEM observations have been done during the friction tests of 2000 cycles. Figure 14 presents the schematisation of the state of the CNT observed during the friction tests for both catalysts. It appears that the decrease of the friction coefficient goes hand in hand with the advancement of the wear mechanism. Indeed, for CNT obtained with Fe3O4 catalyst, the transfer film is present below 500 friction cycles, while after 800 friction cycles the transfer film breaks progressively and the friction coefficient decreases progressively. However, for the CNT obtained with Ferrofluid catalyst, the decrease of the friction coefficient continued during the first 300 cycles of friction and it appears that after that, the transfer film is already broken, which means that the whole of the wear mechanism occurs continuously. So it can concluded that the Fe3O4 catalyst makes it possible to achieve a greater friction resistance of the transfer film. 3.4.Wettability by epoxy resin During the composite processing, the wettability of the fabric with the resin is one of the most important parameters. However, in the previous sections it is highlighted that the friction stresses modify the surface state of the CNT, and it is essential to determine the influence of the wear of the CNT on the wettability. During the wettability tests, it appears that the drops spread over all of the studied surface, which is why we decided to measure the contact angle as a function of time to quantify the impregnation.
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Figure 15 presents the evolution of the contact angle as a function of time for all of the studied surfaces. It can be clearly observed that the presence of CNT increases the contact angle of 24% for the Fe3O4 catalyst and 27% for Ferrofluid catalyst. As CNT create a roughness on the surface of the fibre, their presence modifies the porosity of the surface, which influences the impregnation of the resin. The influence of the wear of the CNT can be seen from the difference between the dotted line and the solid line for each catalyst. For CNT obtained with both catalysts, no significant difference between the evolution of the contact angle for the worn and unworn surfaces can be seen. This means that the impregnation of the surface does not depend on the presence of vertical and entangled CNT or horizontal and aligned CNT on the surface, that is, on the roughness of the surface. However, 30 seconds after the beginning of the wettability, the contact angle of the surface with CNT has almost the same value as that of the sized surface, which means that the impregnation is slower, but at the end, the results are the same. Then, the surfaces obtained with the two catalysts can be compared. The difference between the two curves is exactly the same for the unworn surface as well as the worn surface. However, the roughness of the surface does not change the wettability, otherwise there will be a difference between the worn and unworn surfaces, which means that the geometry of the CNT cannot be the reason for this difference. Finally, this difference can arise from the chemical composition of the surface, particularly the presence of aluminium on the surface obtained with Ferrofluid catalyst. 4. Conclusion
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In this study, it is shown that many stresses will be applied to the fabric after the growth of the CNT and that theses stresses have an impact on the CNT and on the properties of the surface, particularly the state of the surface. Moreover, it has been determined that the nature of the catalyst, which changes the geometrical, structural, and chemical properties of the CNT, also changes the properties of the surface. Indeed, the friction stresses are divided into bending and shear stresses on the CNT. Therefore, friction is higher (+47%) when the diameter of the CNT is large. Moreover, the adhesion of the CNT on the fibre is higher (+111%) if they present few defects in their structure. The study has highlighted the formation of a transfer film during friction but it is always composed of CNT and there is no chemical modification of the surface. Furthermore, the resistance of the transfer film under continuous friction stresses also depends on the mechanical properties of the CNT, depending on their geometry. Finally, the wettability of fabrics after the growth of CNT was measured and revealed that the chemical composition of the surface changes the kinetics of the drop spreading. It appears that the presence of the transfer film, due to wear of the CNT does not change the behaviour of the surface. This result is important because the worn and unworn surfaces have the same impregnation behaviour, so similar properties of the composite materials can be expected. Acknowledgements The authors would like to express their gratitude to Dr Florence Biguenet, Assistant Professor, and Professor Dominique Dupuis from the Laboratoire de Physique et Mécanique Textiles of the University of Haute Alsace for their help and advice regarding the wetting investigation.
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20
Figure captions Figure 1: Schematisation of the different stresses applied to the fabric after the growth of the CNT: (a) during the unwinding and the layer positioning and (b) during the composite manufacturing. Figure 2: Schematisation of the principle of adhesion tests and the obtained curve. Figure 3: Reciprocating tribometer: (a) global picture and (b) picture of the linear contact obtained with the cylindrical slider. Figure 4: SEM pictures of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 5: Raman spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 6: EDX spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst. Figure 7: Adhesion curve obtained for surfaces obtained with the two catalysts under a normal load of 1 N. Figure 8: Histograms of the evolution of the (a) adhesion force, (b) adhesion energy, (c) area of torn CNT, and (d) adhesion force per unit surface area of torn CNT. Figure 9: Evolution of the friction coefficient for 20 cycles of friction for three surfaces: sized CF and CF grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid. Figure 10: Evolution of the CNT during the first cycles of friction: (a) before friction, (b) after the first cycle of friction, (c) after 10 cycles of friction. Figure 11: SEM pictures of the transfer film obtained with (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
21
Figure 12: Raman spectra of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst, after 10 cycles of friction under 1 N. Figure 13: EDX spectra of the CNT grafted on CF; (a) Fe3O4 catalyst and (b) Ferrofluid catalyst after 10 cycles of friction under 1 N. Figure 14: Evolution of the friction coefficient for 2000 cycles of friction for three surfaces: sized CF and CF fibres grafted with CNT obtained with two catalysts: Fe 3O4 and Ferrofluid. Figure 15: Evolution of the wettability of the different fabrics with different surface states with epoxy resin.
22
Figure 1: Schematisation of the different stresses applied to the fabric after the growth of the CNT: (a) during the unwinding and the layer positioning and (b) during the composite manufacturing.
(a)
(b)
23
Figure 2: Schematisation of the principle of adhesion tests and the obtained curve.
24
Figure 3: Reciprocating tribometer: (a) global picture and (b) picture of the linear contact obtained with the cylindrical slider.
25
Figure 4: SEM pictures of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
(a)
(b)
26
Figure 5: Raman spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
(a)
(b)
27
Figure 6: EDX spectra of the initial CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
(a)
(b)
28
Figure 7: Adhesion curve obtained for surfaces obtained with the two catalysts under a normal load of 1 N.
29
Figure 8: Histograms of the evolution of the (a) adhesion force, (b) adhesion energy, (c) area of torn CNT, and (d) adhesion force per unit surface area of torn CNT. Adhesion force (mN)
Adhesion energy (N.m)
300
3.0E-04
250
2.5E-04
200
2.0E-04
150
1.5E-04
100
1.0E-04
50
5.0E-05
0
0.0E+00 Fe3O4
Ferrofluid
Fe3O4
(a)
Ferrofluid
(b) Area of teared CNT (mm²)
Adhesion force / Area of teared CNT (mN/mm²)
20 30 15
25 20
10
15 10
5
5 0
0 Fe3O4
Ferrofluid
Fe3O4
(c)
(d)
30
Ferrofluid
Figure 9: Evolution of the friction coefficient for 20 cycles of friction for three surfaces: sized CF and CF grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid.
31
Figure 10: Evolution of the CNT during the first cycles of friction: (a) before friction, (b) after the first cycle of friction, (c) after 10 cycles of friction.
(a)
(b)
32
(c)
33
Figure 11: SEM pictures of the transfer film obtained with (a) Fe3O4 catalyst and (b) Ferrofluid catalyst.
(a)
(b)
34
Figure 12: Raman spectra of the CNT grafted on CF: (a) Fe3O4 catalyst and (b) Ferrofluid catalyst, after 10 cycles of friction under 1 N.
(a)
(b)
35
Figure 13: EDX spectra of the CNT grafted on CF; (a) Fe3O4 catalyst and (b) Ferrofluid catalyst after 10 cycles of friction under 1 N.
(a)
(b)
36
Figure 14: Evolution of the friction coefficient for 2000 cycles of friction for three surfaces: sized CF and CF fibres grafted with CNT obtained with two catalysts: Fe3O4 and Ferrofluid.
37
Figure 15: Evolution of the wettability of the different fabrics with different surface states with epoxy resin.
38