Carbon fiber composite performance can be enhanced by applying an optimum level of fiber surface treatment such as whiskerization. The main objectives of this study were to conduct whiskerization through carbon nanotube (CNT)-coating of carbon fiber via chemical vapour deposition (CVD) at various conditions (temperature and hydrogen flow rate) and to investigate the enhancement in flexural strength of composites fabricated from these CNT-coated carbon fibers. The results indicated that CNTs were able to grow onto the carbon fibres with the highest amount of whiskerization occurring for samples nearest the reactant gas inlet of the CVD Rig. Various whiskerization behaviours were observed at different reaction temperatures and flow rates. From flexural tests, it was found that whiskerization treatment on carbon fibers increases the flexural strength of its composites between 44-122%

Abdul Rashid, Suraya

Shamsudin, Suryani

Vargis, Christina

Yunus, Robiah

Universiti Putra Malaysia Institutional Repository

ISSN: 0128-7680Pertanika J. Sci. & Technol. 16 (1): 73 - 81 (2008) © Universiti Putra Malaysia PressCVD Whiskerization Treatment Process for the Enhancement ofCarbon Fiber Composite Flexural StrengthSuraya Abdul Rashid*1, Christina Vargis1, Robiah Yunus1 and Suryani Shamsudin21Chemical and Environmental Engineering Department, Faculty of Engineering,Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia2Advanced Material Research Centre (AMREC) SIRIM Berhad,Lot 34, Jalan Hi-Tech 2/3, Kulim Hi-Tech Park,09000 Kulim, Kedah, Malaysia*E-mail: suraya@eng.upm.edu.myABSTRACTCarbon fiber composite performance can be enhanced by applying an optimum level offiber surface treatment such as whiskerization. The main objectives of this study were toconduct whiskerization through carbon nanotube (CNT)-coating of carbon fiber viachemical vapour deposition (CVD) at various conditions (temperature and hydrogen flowrate) and to investigate the enhancement in flexural strength of composites fabricatedfrom these CNT-coated carbon fibers. The results indicated that CNTs were able to growonto the carbon fibres with the highest amount of whiskerization occurring for samplesnearest the reactant gas inlet of the CVD Rig. Various whiskerization behaviours wereobserved at different reaction temperatures and flow rates. From flexural tests, it wasfound that whiskerization treatment on carbon fibers increases the flexural strength of itscomposites between 44-122%.Keywords: Carbon fibers, carbon nanotubes, composites, flexural strengthINTRODUCTIONCarbon fibers are the main reinforcing fibers used in high performance polymer matrixcomposites. Carbon fiber which has a very smooth surface causes the bond between fiberand matrix to be the weakest feature of the combination when laminated. Therefore,though carbon fiber possesses superior modulus as compared to glass fiber, it bondspoorly with matrix materials (Milewski et al., 1971). Consequently carbon fibers wereconsiderably inferior to boron and glass fibers that were being used in structuralmaterials. These observations led investigators to develop various surface treatments thatcould improve the fiber-matrix interfacial bonding (Donnet et al., 1998).These surface treatments may be classified into oxidative treatments and non-oxidative treatments. Non-oxidative treatments that improve the fiber-resin bondinginvolves the deposition of more active forms of carbon on the carbon fiber surface, suchas the growth of carbon whiskers (on the fiber surface) in a process called whiskerization(Donnet et al., 1998). It is believed that these carbon whiskers are either carbonnanotubes (Thostenson et al., 2001) or carbon nanofibers which are known for theirsuperior strength (Downs and Baker, 1995).Fiber bundles or fibrous textures (fabrics) can be coated using vapor-phase processes.A process of this kind which is well suited for coating fiber bundles is the method knownas chemical vapor deposition, or CVD (Huber and Schmaderer, 1992). CVD of films and* Corresponding Author8.-JST57_2008 30/3/09, 11:17 AM73Suraya Abdul Rashid, Christina Vargis, Robiah Yunus and Suryani ShamsudinPertanika J. Sci. & Technol. Vol. 16 (1) 200874coatings involve the chemical reaction of gaseous reactants on or near the vicinity of aheated substrate surface (Choy, 2003). In this study, carbon nanotubes (CNTs) weregrown on carbon fibers through a CVD process known as whiskerization treatment.Milewski et al. (1971), Downs and Baker (1995) and Kowbel et al. (1997) found thatwhiskerization provided immense improvement in the surface bond by 500%, 475% and250% respectively. Donnet et al. (1998) reported that whiskerization contributes immenseimprovement in inter-laminar shear strength (ILSS). A good measure of increment insurface bonding is by measuring the flexural strength of composites which correlates wellwith ILSS (Jang and Yang, 2000).The objectives of this study were (i) to carry out whiskerization treatment by coatinguntreated PAN-based carbon fiber with CNTs at various reaction temperatures (800-1000oC) and hydrogen flow rates (100-500ml/min) conditions and to characterize theCNT-coated fibers at these conditions and (ii) to investigate the effect of these varyingwhiskerization treatment conditions on the flexural strength of carbon fiber-epoxy composite.MATERIALS AND METHODSWhiskerization TreatmentIn this study, the whiskerization treatment was carried out using a custom-built CVD Rig.A description of the apparatus has been reported by Suraya et al. (2006). Whiskerizationtreatment was conducted on carbon fibers by bubbling hydrogen gas into benzene andthen flowing this reaction gas into the quartz tube where ferrocene and carbon fiber towswere positioned. Ferrocene decomposes under high temperature and expels organiccompounds whilst the Fe ions are left to diffuse into exposed pores on the carbon fibers.Benzene vapour which enters the furnace decomposes into carbon atoms. These carbonatoms are attracted to the heated Fe ions which act as catalysts, thus stimulating whiskernucleation and growth. This treatment was conducted at reaction temperatures of 800oC,900oC and 1000oC at a heating rate of 5oC/min. At each of the reaction temperature, thehydrogen flow rate was varied at 100ml/min, 300ml/min and 500ml/min. The reactorwas cooled under argon gas before collecting the treated fiber.Fiber CharacterizationThe treated fiber samples from each treatment condition were then analyzed usingscanning electron microscopy (SEM) (JEOL JSM-6400 model). The CNTs grown on thecarbon fiber surfaces were characterized using HRTEM model Philip Tecnai 20.Composite ProcessingComposites were then fabricated from carbon fiber and epoxy resin using the hand layupmethod before being subjected to flexural test according to ASTM D790 using InstronUniversal Testing Machine. A SEM, model JEOL JSM-6400, was then used to characterizethe fracture surface of these composites.RESULTS AND DISCUSSIONCharacterization of CNT-coated FiberA total of nine sets of treatment conditions were experimented, and the details of thedesignations used for these treatment conditions are given in Table 1.8.-JST57_2008 30/3/09, 11:17 AM7475Pertanika J. Sci. & Technol. Vol. 16 (1) 2008CVD Whiskerization Treatment Process for the Enhancement of Carbon Fiber CompositeFor each treatment, CNT-coated fiber samples were taken from three main regionsalong the reaction tube to examine the pattern of CNT growth on the carbon fibersalong the reaction tube. Fig. 1 illustrates these three regions in the quartz tube denotedas regions 1, 2 and 3.TABLE 1Description of the treatment condition designations usedDesignation Hydrogen Flow Rate Reaction Temperature (ml/min) (ºC)A 100 800B 300 800C 500 800D 100 900E 300 900F 500 900G 100 1000H 300 1000I 500 1000Fig. 1: Positions of the three regions in the reaction tube where CNT-coated fibersamples were taken to examine the whiskerization patternFig. 2 depicts the SEM micrographs of CNT-coated carbon fibers for condition A.CNTs tend to grow from carbon fibers only at region 1. This pattern was evident for alltreatment conditions. CNTs are likely to form at the inlet region of a CVD reactor wherethe ferrocene vapour is fed because the Fe particles (resulting from ferrocenedecomposition) will increase in diameter with an increase in the axial distance and onlythe catalyst particles at the inlet with relatively smaller diameters are capable of performingas the growth seed for CNT growth. As the larger Fe particles form towards the reactorexit, they become too large and inactive for CNT growth (Moisala et al. 2006; Kuwana andSaito, 2005).Figs. 3 to 5 depict the SEM micrographs of CNT-coated carbon fibers for conditionsA to I at region 1.  CNT-coated fibers consist mainly of relatively short CNT length at aInletGasRegion 1 Region 2 Region 3FurnaceEffective heating lengthGasExitQuartzTube8.-JST57_2008 30/3/09, 11:17 AM75Suraya Abdul Rashid, Christina Vargis, Robiah Yunus and Suryani ShamsudinPertanika J. Sci. & Technol. Vol. 16 (1) 200876reaction temperature of 800oC whereas at higher reaction temperatures (900oC and1000oC), longer strand-like CNT is noticeable on CNT-coated fibers. Zhao et al. (2005)reported similar findings where low temperature CVD processes produced short MWCNTswhile high temperature CVD produces long MWCNTs. HRTEM analysis was carried outin order to examine the alignment of parallel graphitic sheets of individual CNTs (grownon carbon fibers) at 800, 900 and 1000oC, and the images are shown in Fig. 6 (a, b, c).The alignment of graphitic sheets parallel to the tube axis is evident in all the figures,with the parallel graphitic sheet of CNTs being more aligned and distinct as thetemperature increases. This is because as the temperature increases, the carbon diffusionrate (from benzene decomposition) increases. Consequently, the growth rate of CNTsincreases and the graphitic sheets build up with a less defect (Lee et al., 2003).Fig. 2: SEM micrograph of CNT-coated carbon fiber at a reaction temperature of 800ºC and 100ml/minof hydrogen gas flow rate, A, at (a) region 1, (b) region 2 and (c) region 3 along the reaction tubeFig. 3: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min, (b) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 800ºC at region 1Fig. 4: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min, (b) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 900ºC at region 1(a) (b) (c)(a) (b) (c)(a) (b) (c)8.-JST57_2008 30/3/09, 11:17 AM7677Pertanika J. Sci. & Technol. Vol. 16 (1) 2008CVD Whiskerization Treatment Process for the Enhancement of Carbon Fiber CompositeFig. 5: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min. (b) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 1000ºC at region 1(a) (b) (c)Fig. 6: HRTEM micrographs on CNT grown at (a) 800ºC, (b) 900ºC(a)(b)(c)8.-JST57_2008 30/3/09, 11:17 AM77Suraya Abdul Rashid, Christina Vargis, Robiah Yunus and Suryani ShamsudinPertanika J. Sci. & Technol. Vol. 16 (1) 200878Figs. 7 to 9 show the SEM micrographs of CNT-coated carbon fibers for conditionsA to I at region 1 at a lower magnification. The micrographs show that most of the CNT-coated fibers at the lower hydrogen flow rate of 100ml/min comprises of fibers withamorphous carbon impurities (indicated by clumps) and relatively uneven CNT coating.At higher hydrogen flow rates of 300 and 500ml/min, the clumps on the CNT-coatedfibers disappeared and the CNT coatings on the fiber were relatively more even. Theseobservations were much more evident at lower temperatures especially at 800oC. Thistrend could be due to the role of hydrogen in hindering or preventing the formation ofsurface carbide (Fe3C) which is the cause for low carbon deposition rates. Yang and Yang(1986) reported that surface carbide is essentially inactive for benzene decomposition.Fig. 7: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min, (c) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 800ºC at region 1Fig. 8: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min, (c) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 900ºC at region 1Fig. 9: SEM micrograph of CNT-coated carbon fibers at (a) 100ml/min, (c) 300ml/min and (c) 500ml/min of hydrogen gas flow rate and a reaction temperature of 1000ºC at region 1(a) (b) (c)(a) (b) (c)(a) (b) (c)8.-JST57_2008 30/3/09, 11:18 AM7879Pertanika J. Sci. & Technol. Vol. 16 (1) 2008CVD Whiskerization Treatment Process for the Enhancement of Carbon Fiber CompositeTABLE 2Flexural strength of composites made from untreated andCNT-coated carbon fibersWhiskerization Flexural Strength (MPa) Increment (%)Treatment ConditionUntreated 1.45 -800oC, 100ml/min 3.22 122800oC, 300ml/min 3.03 109800oC, 500ml/min 2.84 96900oC, 100ml/min 2.62 81900oC, 300ml/min 2.30 59900oC, 500ml/min 2.27 571000oC, 100ml/min 2.25 551000oC, 300ml/min 2.23 541000oC, 500ml/min 2.09 44Effect of Varying Reaction Temperature and Hydrogen Flow RateFig. 10 compares the flexural strength of composites fabricated from CNT-coated carbonfibers at various temperatures for various hydrogen flow rates. As the whiskerizationtreatment reaction temperature increases from 800oC to 1000oC, at 100ml/min ofhydrogen, the flexural strength of composites fabricated at these conditions decreasesbut is still more than composites fabricated from untreated carbon fibers. This samepattern is observed for 300 and 500 ml/min of hydrogen as the reaction temperatureincreases from 800oC to 1000oC. This can be attributed to the increased reactiontemperature causing the surface activity of the fiber surface to decrease (Wang et al.2006), resulting in lesser fiber and epoxy interaction and this in turn lowers the flexuralstrength of the composite. Therefore though whiskerization treatment at highertemperatures (900oC and 1000oC) enhances the flexural properties of composites by 44-Carbide is formed, along with amorphous carbon and graphite, which deactivates the Feactivity for carbon deposition. In the presence of H2, a high activity is obtained probablybecause a metallic surface is maintained, thus sustaining the activity.  This explains thereduced impurities and even coating of CNT at higher hydrogen flow rate.Whiskerization treatment on carbon fibers increases the flexural strength of compositesby 44-122%. The extent of increment in flexural strength varies according to thewhiskerization treatment conditions. The flexural strength of composites made fromuntreated and CNT-coated carbon fibers are listed in Table 2. The carbon nanotubesgrown on carbon fibers improves the surface geometry of fibers by providing greatermechanical grip for matrix material which when combined with the fibers formscomposites. The CNT-coated fibers have a considerable degree of cohesion even beforethey are impregnated with matrix filler material because the CNTs from adjacent fibrilsoverlap, intertwine and occupy all of the interstitial voids located between adjacentbundles of fibers (Milewski et al., 1971). This type of fiber-matrix adhesion mechanismis called mechanical interlocking and results in enormous increase in flexural strengthwhen these fibers are combined with matrix material. The resulting composite behavesin a manner resembling more nearly an isotropic material.8.-JST57_2008 30/3/09, 11:18 AM79Suraya Abdul Rashid, Christina Vargis, Robiah Yunus and Suryani ShamsudinPertanika J. Sci. & Technol. Vol. 16 (1) 20088081%, this enhancement is less compared to flexural strength of composites fabricatedfrom carbon fibers treated at 800oC with corresponding increment of 96-122%.Alternatively, comparing the flexural strength of composites fabricated from CNT-coated carbon fibers at various hydrogen flow rates at the same reaction temperatureindicates an overall decrease in the flexural strength of composites fabricated for allthree reaction temperatures. Nevertheless the difference in flexural strength for thevarious hydrogen flow rates at the same temperature is not very significant with a mere7-13% decrement. Therefore, it can be concluded that the hydrogen gas flow rate (usedduring whiskerization) does not play a major role in flexural strength of these CNT-coated carbon fiber composites.CONCLUSIONSObservation of SEM micrographs of treated carbon fiber samples taken at various pointsin each region showed that CNTs tend to grow from carbon fibers only at region 1 forall treatments. Observation of SEM micrographs of CNT-coated carbon fibers at reactiontemperatures of 800oC, 900oC and 1000oC showed that the CNTs grown on the carbonfibers increased in length and distinctness of parallel graphitic sheets alignment as thereaction temperature increased from 800 to 1000oC. On the other hand, as the hydrogenflow rate increased from 100 to 500 ml/min, SEM micrographs depicted that the CNTcoatings on the fibers were relatively more even. The amorphous carbon impurities(indicated by clumps) on the CNT-coated fibers also disappeared as the hydrogen flowrate increased from 100 to 500 ml/min but this was only observed at a reactiontemperature of 800oC.Whiskerization of carbon fibers increases the flexural strength of composites by 44-122%. Higher reaction temperature and hydrogen flow rate during carbon fiberwhiskerization lowers the flexural strength of its composites with the latter conditionhaving less impact on the strength.Fig. 10: Flexural strength of composites fabricated from untreated andCNT-coated carbon fibers8.-JST57_2008 30/3/09, 11:18 AM8081Pertanika J. Sci. & Technol. Vol. 16 (1) 2008CVD Whiskerization Treatment Process for the Enhancement of Carbon Fiber CompositeACKNOWLEDGEMENTSThe authors would like to acknowledge the financial support provided by the Ministry ofScience, Technology and the Environment, Malaysia (MOSTE) under programme No.03-01-01-0065-PR0072/08-05 and also acknowledge the SEM analysis service of UniversitiPutra Malaysia, Bioscience Institute, Microscopy and Nanoscience Imaging Institute aswell the HRTEM analysis service of Advanced Materials Research Centre (AMREC)SIRIM Berhad.REFERENCESCHOY, K.L. (2003). Chemical vapour deposition of coatings. Progress in Material Science, 48, 57-100.DONNET, J.B., WANG, T.K., PENG, J.C.M. and REBOUILLAT, S. (1998). Carbon Fibers [ (p. 162). MarcelDekker Inc.: New York.DOWNS, W.B. and BAKER, R.T.K. (1995). Modification of the surface properties of carbon fibers viathe catalytic growth of carbon nanofibers. Journal of Materials Research, 10(3), 625-633.HUBER, R. and SCHMADERER, F. (1992). 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Carbon, 24, 687-693.ZHAO, Z-G., CI, L-J., CHENG, H-M. and BAI, J-B. (2005). The growth of multi-walled carbon nanotubeswith different morphologies on carbon fibers. Letters to the Editor / Carbon, 43, 651-673.8.-JST57_2008 30/3/09, 11:18 AM81

CVD whiskerization treatment process for the enhancement of carbon fiber composite flexural strength

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CVD whiskerization treatment process for the enhancement of carbon fiber composite flexural strength

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