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Comparison of short carbon fibre surface treatments on epoxy composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 64 (2004) 2021–2029 www.elsevier.com/locate/compscitech
Comparison of short carbon fibre surface treatments on epoxy composites I. Enhancement of the mechanical properties Hui Zhang, Zhong Zhang *, Claudia Breidt Institute for Composite Materials, University of Kaiserslautern, Erwin Schr€odinger Strasse 58, D-67663 Kaiserslautern, Germany Received 7 August 2003; accepted 22 February 2004 Available online 12 April 2004
Abstract Pitch-based short carbon fibres were treated by both a gaseous oxidation and a cryogenic treatment approach. It was found by scanning electron microscopy that the fibre surface roughness was increased by various oxidative conditions, whereas the fibre diameter was reduced by the cryogenic treatment. In both cases, appropriate treatments could effectively improve the mechanical properties in their epoxy composites due to the enhanced fibre–matrix interfacial bonding. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Short carbon fibres; Fibre treatment; Gaseous oxidation; Cryogenic treatment; Epoxy; Mechanical properties
1. Introduction It is well known that the fibre/matrix adhesion strength plays an important role on the mechanical properties of fibre reinforced polymer composites [1–8]. When load is applied to composites, it will be distributed and transferred through fibre/matrix interfaces. A strong bonding promotes a better involvement of more fibres, accordingly increases the strength of composites. However, carbon fibres usually perform a poor bonding behaviour to polymer matrix due to their nature of smoothness and chemical inertness. In order to improve the bonding properties of carbon fibres, various approaches can be applied, which were classified into oxidative and non-oxidative treatments by Park and Kim [9]. Oxidation treatments involve gas-phase, liquidphase and anodic oxidations, whereas the non-oxidative ones include plasma treatment, deposition of more active forms of carbon, or grafting of the carbon fibre surface with polymers [10]. Oxidation treatment of car*
Corresponding author. Tel.: +49-631-2017213; fax: +49-6312017196. E-mail address: [email protected] (Z. Zhang). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.02.009
bon fibres in hot air is one of the commonly used approaches owing to several advantages, e.g. easy operation, low cost and lack of pollution [11]. On the other hand, cryogenic treatment of carbon fibres, which could increase the fibre strength by clearing the weak layer of amorphous carbon, seems to be an interesting and relatively novel method. Few about this approach were reported. Rashkovan and Korabelnikov [12] treated high-tenacity long carbon fibres in liquid nitrogen for 30 s, in order to improve the mechanical properties in composites. However, the effect on short carbon fibres with an extended treatment time, and the performance on mechanical properties of their composites are not yet fully understood. In the present work, two different approaches, i.e. air oxidation and cryogenic treatment, were applied to treat pitch-based short carbon fibres at various conditions. The configurations of treated carbon fibres and the fractography were studied by using scanning electron microscopy (SEM). The improvement on the mechanical properties of treated carbon fibres reinforced epoxy is reported. Further improvement of the tribological properties will be presented in the subsequent part of this paper [13].
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
average value was reported, with an error scattering of the maximum absolute error.
2. Experimental 2.1. Materials and compounding
2.3. SEM observation Pitch-based short carbon fibres (M-2007S) were supplied by Kureha Co. without treatment, which have a density of 1.6 g/cm3 , and average diameter and length of 14.5 and 90 lm, respectively. They are graphite grade short fibres with excellent mechanical properties, e.g. tensile strength of 800 MPa, modulus of elasticity of 35 GPa, and elongation of 2.3%. For the oxidative treatment, the carbon fibres were oxidized in a muffle furnace at various temperatures for 1 h. The air vent was always applied to keep the air inside the furnace flowing during processing. In the case of cryogenic treatment, short carbon fibres containing in a ferrous vessel were immersed into liquid nitrogen ()196 °C) for various minutes. All treatment details are summarized in Table 1. The matrix used in this work was a bisphenol-A type resin (DER331, Dow) hardened by an amine curing agent (HY2954, Dow), with densities of 1.16 and 0.95 g/ cm3 , respectively. The composites were prepared in a vacuum dissolver by mixing the epoxy resin with 15 vol.% untreated and treated short carbon fibres as described in Table 1. The epoxy resin and the curing agent were preheated at 70 °C in an oven for at least 4 h before use. The mixing process was carried out at 70 °C with stirring speed of 2000 rpm for 30 min under vacuum, subsequently adding definite amount of curing agent, the mixture was stirred at 60 °C with stirring speed of 200 rpm for 15 min and then was poured into a rectangular aluminium mould for curing. The applied gel temperature was 70 °C for 8 h, followed by a curing stage at 122 °C for 16 h. 2.2. Mechanical properties A Zwick universal testing machine was applied to investigate the flexural modulus and strength under a threepoint-bending approach according to DIN-ISO-178. Specimens were cut at a dimension of 100 10 4 mm3 . The test speed was kept constant at 1 mm/min. Five specimens of each composition were measured and an
The configuration of as-received, treated carbon fibres as well as fracture surfaces of epoxy composites were examined using a JEOL-5400 SEM. The diameters of carbon fibres before and after liquid nitrogen treatment were also determined by SEM. In the latter case, the sample holder of SEM was adjusted to zero degree and all samples were coated with gold films previously. An average value of diameters was calculated from at least 200 individual carbon fibres.
3. Results and discussion 3.1. SEM observation The SEM configurations of as received, oxygen treated and cryogenic treated short carbon fibres are given in Fig. 1. Remarkable differences in micrographs can be observed on untreated and treated carbon fibres. The surface of as-received fibre (Fig. 1(a)) seems to be relatively smooth and there are some small impurities absorbed on the fibre surface, which were introduced during the fibre manufacturing process. Under a treatment temperature of 450 °C the surface does not look appreciably different when compared to untreated ones, which suggests that the treatment was mild and did not cause essential morphological changes on the fibre surface [14]. However, by using the gas physical adsorption technique, Wan et al. [11] reported that after treatment under a similar condition mentioned above, the total carbon fibre surface area was doubled. Their result indicated that some changes did take place on carbon fibre surface at 450 °C, although it was difficult to be recognized by SEM. Once the treatment temperature was further increased, the changes on the surface could be easily observed. At a treatment temperature of 500 °C for 1 h (Fig. 1(b)), the fibre surface became relatively rough and more pieces of tiny fragments stuck to the fibre surface, which suggested
Table 1 Details of specimens and treatment conditions Treatment approach
Sample No.
CF volume content
Treatment details
–
CF_0
15%
As-received
Oxidation (in air)
CF_450 CF_500 CF_550 CF_600
15% 15% 15% 15%
450 500 550 600
Cryogenic treatment (in liquid nitrogen)
CF_1 CF_5 CF_10 CF_20
15% 15% 15% 15%
1 min 5 min 10 min 20 min
°C/1 °C/1 °C/1 °C/1
h h h h
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
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Fig. 1. SEM configurations of carbon fibres treated under different conditions: (a) as-received; air-oxidized at (b) 500 °C/1 h, and (c) 600 °C/1 h; cryogenic treated in liquid nitrogen for (d) 1 min, and (e) 10 min.
that the absorbability of treated carbon fibres might be enhanced to some extent. A great number of micro-pits could be found on the fibre surface on account of a violent oxygen etching at the highest oxidative temperature of
600 °C in our case (Fig. 1(c)). Those pits with the size in a range of several hundred nanometres had irregular shapes and distributed uniformly over the fibre surface. In some places they were linked together and formed a ditch-like
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029 Table 2 Fibre diameter distribution before and after cryogenic treatment
Fig. 2. Diagram of the mechanism of the cryogenic treatment on carbon fibres.
structure. Obviously, this kind of treatment destroys the original structure of carbon fibres, therefore, leading to a loss of mechanical properties of the carbon fibres somehow, as a consequence to that of the composites as well.
as-received
12
av=14.6µm 10
Fraction [%]
8 6 4 2 0 0
10
(a)
15
20
Peak Peak Peak centre (lm) width (lm) height (%)
As-received 5 min 20 min
14.6 14.2 13.9
14.48 14.47 14.12
6.67 5.66 4.83
10.72 13.38 15.21
In the case of cryogenic treatment, an appreciable characteristic is that only a very short treatment time (1 min, Fig. 1(d)) could make a difference in the morphology of carbon fibres. The surface roughness of the carbon fibre might, after a cooling treatment for 1 min, nearly attain such a level that was reached by the air oxidative treatment at much longer time (1 h at a treatment temperature of 550 °C). When extending the treatment time up to 10 min, the carbon fibre surface became very rough, and lots of tiny fragments and some striations along the fibre axis could be observed (Fig. 1(e)). At the same time it led to a reduction of the fibre diameter on account of the removing of the amorphous carbon layer, which will be further discussed later. So far as the enhancement of the surface roughness and the treatment time are concerned, the cryogenic treatment seems to be more efficient than the air oxidative one. Fig. 2 illustrates a diagram of the cryogenic treatment for carbon fibres. As pointed out by Rashkovan and Korabelnikov [12], a carbon fibre is usually composed of two parts, i.e. the proper carbon fibre, and a layer of amorphous carbon deposited on it, so-called disordered structure (cf. [15]). At low temperatures, due to the difference of the coefficients of linear thermal expansion (CTE) between these two parts, the carbon fibre is exposed to a contraction of the amorphous carbon and an
20min av=13.9µm
16
14.7 14.6 14.5
12
14.4
Diameter [µm]
14
10 8 6
14.3 14.2 14.1
4
14.0
2
13.9
0
13.8 0
(b)
Average diameter (lm)
Diameter [µm] 18
Fraction [%]
25
Short carbon fibre
10
15
20
25
Diameter [µm]
Fig. 3. Typical diameter distribution of carbon fibres before and after cryogenic treatment.
0
5
10
15
20
Treatment Time [min] Fig. 4. Dependence of the fibre average diameter on the cryogenic treatment time.
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
axial expansion of the proper fibre, while the proper fibre has a negative CTE. Shear stresses arise on the interface of the two parts and finally exceed the shear strength between the proper fibre and the amorphous carbon layer. Therefore the latter would easily shell off from the fibre surface. Due to its heterogeneity, the amorphous carbon layer shells off along the fibre to various extents resulting in an increase of the fibre surface roughness, as confirmed by SEM micrographs. 3.2. Fibre diameter distribution As mentioned above, the amorphous carbon layer can partly shell off from the fibre after the cryogenic treatment. Accordingly, the diameter of the carbon fibre is reduced to some degree. Fig. 3 shows the typical
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curves of the diameter distribution for as-received and treated carbon fibres, which were fitted by Gaussian distributions, and the parameters were summarized in Table 2. It can be found that the fitting results of cryogenic treated fibres shifted slightly to lower values and the width of the curve also became narrow. This implicates that the amount of fibres with large diameters decreased, possibly due to the fact that they possess thicker amorphous carbon layers, which are easily to be shelled off after cryogenic treatment. When the treatment time reached up to 20 min, the average diameter decreased about 4.8%. Fig. 4 demonstrates that at the beginning of the treatment, the rate of ‘‘shelling off’’ of the amorphous carbon layer was higher. With an increasing treatment time, the rate ran slowly down. Even with only three points measured in Fig. 4, the average
3.9
4.0
3.5
3.5
3.5 3.4
3.5
3.0
(a)
CF_0
CF_450 CF_500 CF_550 CF_600
Flexural Modulus [GPa]
Flexural Modulus [GPa]
4.5
5.0 4.5 4.4
4.5 4.0
4.0
CF_1
CF_5
4.0 3.5 3.5 3.0
(d)
CF_0
CF_10
CF_20
107.7 110 98.1 100 90.5 88.9 90
80
(b)
CF_0
120
109.7
Flexural Strength [MPa]
Flexural Strength [MPa]
120
CF_450 CF_500 CF_550 CF_600
112.2
110
105.0 97.3
100
99.9
90.5 90 80
(e)
CF_0
CF_1
CF_5
CF_10
CF_20
(c)
5 4
3.36
3.33
3.32
3.2
3.47
3 2 1 0 CF_0
CF_450 CF_500 CF_550 CF_600
Elongation at Break [%]
Elongation at Break [%]
5
(f)
4
3.36
3.15
3.25
3.12
2.98
CF_0
CF_1
CF_5
CF_10
CF_20
3 2 1 0
Fig. 5. Flexural properties of CF/epoxy composites: for air oxidation, (a) flexural modulus, (b) flexural strength, and (c) elongation at break; and for cryogenic treatment, (d) flexural modulus, (e) flexural strength, and (f) elongation at break.
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thickness of the amorphous carbon layer can be roughly estimated as approximately 0.7 lm. Relative Improvement
The effects of the air oxidative treatment on the flexural strength, the modulus and the elongation of CF/ EP composites are given in Fig. 5. The flexural moduli of treated samples increase slightly in the range of treatment temperature between 450 and 550 °C. The modulus decrease at a treatment temperature of 600 °C, which may due to the loss of the distinct fibre strength (Fig. 5(a)). A maximum modulus value can be found at a temperature of 550 °C, i.e. 3.9 GPa, about 10% improvement compared with that of untreated one. On the other hand, the flexural strength of treated samples enhances monotonically with an increased oxidation level (Fig. 5(b)). A maximum strength value can be observed at the treatment temperature of 600 °C, i.e. 109.7 MPa, about 21% improvement compared with that of untreated one. Although under these conditions the proper carbon fibres were damaged, the strong interfacial bonding may contribute these improvements on both strength and modulus. To summarize the above results, the flexural strength of the composite is more sensitive to the change of fibre–matrix interface than the modulus. The latter is largely dominated by the stiffness of the fibres. In term of the cryogenic treatment, the flexural moduli and the strength of treated samples also increased effectively comparing to untreated ones (Fig. 5(d) and (e)). The flexural modulus and strength reach their maximum value at a treatment time of 10 min, i.e. 4.5 GPa and 112.2 MPa, respectively, which were about 29% and 24% improvement comparing with that of untreated one. However, these effects tend to decrease with an increased treatment time. It is considered that there is an optimum treatment condition for the carbon fibres and their composites. After too long treatment time, too much amorphous carbon shelled off from the fibre, corresponding to a decrease in the amounts of interlocking sites. For both treatments, the elongations of the samples appeared to no obvious changes compared to that of the untreated ones (Fig. 5(c) and (f)), indicating that the elongation is a dominate of the matrix. Fig. 6 demonstrates the relative improvements on mechanical properties of epoxy composites for both treatment approaches. The main difference consists in the changes of flexural moduli of CF/EP composites. For the air oxidative treatment (Fig. 6(a)) the flexural moduli of their epoxy composites increased slightly, since the mechanical properties of the carbon fibres did not change at mild treatment conditions or rather decreased to some degree due to the excessive oxygen etching. In term of the cryogenic treatment in Fig. 6(b),
flexural modulus flexural strength elongation
1.3 1.2 1.1 1.0 0.9 0.8 CF_0
CF_450 CF_500 CF_550 CF_600
(a)
Air Oxidation 1.4
Relative Improvement
3.3. Mechanical properties
1.4
flexural modulus flexural strength elongation
1.3 1.2 1.1 1.0 0.9 0.8 CF_0
(b)
CF_1
CF_5
CF_10
CF_20
Cryogenic Treatment
Fig. 6. Relative improvements on the mechanical properties of (a) air-oxidation, and (b) cryogenic treatment.
it is opposing. It is worth to mention that the maximum values for both modulus and strength of the cryogenic treated samples are higher than those of air-treated results. 3.4. SEM fractography The improvement on flexural properties of CE/EP composites could be attributed to the enhancement of the interfacial adhesion strength of the fibre and matrix after fibre surface treatments. There are several mechanisms for the fibre–matrix bonding, which involve mechanical interlocking, adsorption interaction, electrostatic interaction, and diffusion of polymer chain segments [16]. As far as air-treated samples are concerned, the mechanical interlocking could be the most important factor. As shown in SEM micrographs (Fig. 1), with increased treatment temperatures the fibre surfaces become rougher, i.e. the fibres possessed a larger surface area, which could contact with matrix and thus formed more mechanical interlocking sites. In
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
addition, the enhancement of the surface roughness reduces the contact angle between fibre and matrix and hence increases the wettability [17] as well. SEM fractographies of three-point-bending tests are given in Fig. 7. It is easily recognized in Fig. 7(a) that untreated
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carbon fibres performed very poor interface bonding with epoxy matrix. After oxidation treatment shown in Fig. 7(b) and (c), strong interlocking of fibre–matrix bonding can be observed as marked by arrows in Fig. 7(c), which was termed as ‘‘mechanical anchor’’ by
Fig. 7. SEM fractographies of CF/epoxy composites: (a) as-received; oxidized at (b) 500 °C/1 h, (c) 600 °C/1 h; cryogenic treated in liquid nitrogen for (d) 10 min, and (e) 20 min.
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Kalantar and Drzal [16]. The mechanical interlocking provides a strong interface bonding even though other effects are weak [18]. Another possible reason for the enhancement on interfacial adhesion is chemical interaction. Many researchers [19–22] have analysed the O1s /C1s ratio of carbon fibre surface by means of a Xray photoelectron spectroscopy (XPS), and reported that gaseous oxidative treatment could increase the oxygen concentration on carbon fibre surface, which means that the amount of functional groups could be increased, e.g. hydroxyl (–OH), carbonyl (–C@O) and carboxyl (–COOH) groups. These functional groups may improve the wettability by enhancing the surface energy. Therefore chemical reactions can take place with epoxy group forming covalent bonding at the interface. In the case of cryogenic treatment, similar mechanical interlocking was also observed which plays an important role to the improved interfacial adhesion, as shown in Fig. 7(d) marked by arrows. It is interesting that even some broken fibres can be found (Fig. 7(e)) which indicates the mechanical interlocking in some regions was very strong. Since the average length of carbon fibres applied in this study is much shorter compared to the fibre broken critical length as reported in literatures of several hundreds microns [23,24], fibre pull-out and matrix fracture should be the main failure modes in our case. An improved fibre–matrix bonding with fibre broken involving will definitely improve the strength of composites. Another reason for the enhancement of the flexural properties is the strengthening effect of carbon fibres of the cryogenic treatment [12]. After treatment the average strength of the carbon fibres usually increased due to the removal of the weak layer and attached particles, which act as concentrators and decrease the fibre strength to some extent. On the other hand, it should be mention that not all the carbon fibres had the similar obvious changes after cryogenic treatment. Some carbon fibres still kept much smoother surface with poor bonding to the matrix as shown in Fig. 7(e) as well. The possible reasons of this phenomenon could be: (i) the heterogeneity of fibres, while some carbon fibres, which may possess very thin amorphous carbon layer, do not form a rough surface after treatment; and (ii) the treatment process, while some fibres were not effectively treated due to the thermal conduction. An optimum treatment process is still a task to fully exert the potential of this approach on the enhancement of carbon fibre–matrix bonding of polymer composites.
4. Conclusions Based on this work devoted to studying the effect of surface treatment on mechanical properties of short
carbon fibre reinforced epoxy composites, the following conclusions can be drawn: 1. Both oxidative and cryogenic treatments could significantly increase the surface roughness of carbon fibres, accordingly to improve the interfacial adhesion strength of fibre and epoxy bonding due to the mechanical interlocking. 2. The cryogenic treatment has advantages of very short treatment time and environment-friendly media, as well as higher improvement on both modulus and strength compared to the oxidative one. 3. The carbon fibres used in this study were relatively short; therefore the improvement on mechanical properties was limited. Further enhancement of the tribological properties will be concentrated on the subsequent part of this paper [13].
Acknowledgements Z. Zhang is grateful to the Alexander von Humboldt Foundation for his Sofja Kovalevskaja Award, financed by the German Federal Ministry of Education and Research (BMBF) within the German Government’s ‘‘ZIP’’ program for investment in the future. The authors appreciate Prof. Dr.-Ing. Dr. h.c. K. Friedrich, IVW, for his valuable discussions during the course of this work and the preparation of this paper.
References [1] Fitzer E, Heine M. Carbon fiber manufacture and surface treatment. In: Bunsell AR, editor. Fibre reinforcements for composite materials. Amsterdam, The Netherlands: Elsevier; 1988. p. 124–35 [chapter 3]. [2] Dong S, Gauvin R. Application of dynamic mechanical analysis for the study of the interfacial region in carbon fiber/epoxy composite materials. Polym Compos 1993;14:414–20. [3] Ibarra L, Panos D. Dynamic properties of thermoplastic butadiene-styrene (SBS) and oxidized short carbon fiber composite materials. J Appl Polym Sci 1998;67:1819–26. [4] Madhukar MS, Drzal LT. Fiber-matrix adhesion and its effect on composite mechanical properties. I. Inplane and interlaminar shear behavior of graphite/epoxy composites. J Compos Mater 1991;25:932–57. [5] Park BY, Kim SC. A study of the interlaminar fracture toughness of a carbon-fiber/epoxy composite containing surfacemodified short Kevlar fiber. Compos Sci Technol 1998;58:1599– 606. [6] Fu X, Chung DDL. Improving the bond strength between carbon fiber and cement by fiber surface treatment and polymer addition to cement mix. Cement Concrete Res 1995;25:1391–6. [7] Blackketter DM, Upadhyaya D, King TR, King JA. Evaluation of fiber surface treatment and sizing on the shear and transverse tensile strengths of carbon fiber-reinforced thermoset and thermoplastic matrix composites. Polym Compos 1993;14: 430–6.
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029 [8] Hughe JDH. The carbon fiber/epoxy interface – a review. Compos Sci Technol 1991;41:13–45. [9] Park S-J, Kim M-H. Effect of acidic anode treatment on carbon fibers for increasing fiber-matrix adhesion and its relationship to interlaminar shear strength of composites. J Mater Sci 2000;35:1901–5. [10] Chand S. Review carbon fibers for composites. J Mater Sci 2000;35:1303–13. [11] Wan YZ, Wang YL, Zhou FG, Chang GX, Han KY. Three-dimensionally braided carbon fiber-epoxy composites a new type of materials for osteosynthesis device. II. Influence of fiber surface treatment. J Appl Polym Sci 2002;85:1040– 6. [12] Rashkovan IA, Korabelnikov YuG. The effect of fiber surface treatment on its strength and adhesion to the matrix. Compos Sci Technol 1997;57:1017–22. [13] Zhang H, Zhang Z. Comparison of short carbon fibre surface treatments on epoxy composites. II. Enhancement of the wear resistance. Compos Sci Technol 2004;64. [14] Hoffman WP, Hurley WC, Owens TW, Phan HT. Advantage of the scanning tunnelling microscope in documenting changes in carbon fiber surface morphology brought about by various surface treatments. J Mater Sci 1991;26:4545– 53. [15] Yumitori S, Nakanishi Y. Effect of anodic oxidation of coal tar pitch-based carbon fiber on adhesion in epoxy matrix. Part 1. Comparison between H2 SO4 and NaOH solutions. Composites Part A 1996;27:1051–8.
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[16] Kalantar J, Drzal LT. The bonding mechanism of aramid fibers to epoxy matrices. Part 1. A review of the literature. J Mater Sci 1990;25:4186–93. [17] Jang J, Yang H. The effects of surface treatment on the performance improvement of carbon fiber/polybenzoxazine composites. J Mater Sci 2000;35:2297–303. [18] Harris B, Braddell OG, Almond DP. Study of carbon fiber surface treatment by dynamic mechanical analysis. J Mater Sci 1993;28:3353–66. [19] Boudou JP, Paredes JI, Cuesta A, M-Alonso A, Tascon JM. Oxygen plasma modification of pitch-based isotropic carbon fibers. Carbon 2003;41:41–56. [20] Lee J-S, Kang TJ. Changes in physic-chemical and morphological properties of carbon fiber by surface treatment. Carbon 1997;35:209–16. [21] Fu R, Liu L, Huang W, Sun P-C. Studies on the structure of activated carbon fibers activated by phosphoric acid. J Appl Polym Sci 2003;87:2253–61. [22] Shin S, Jang J, Yoon S-H, Mochida I. A study on the effect of heat treatment on functional groups of pitch based activated carbon fiber using FTIR. Carbon 1997;35:1743–93. [23] Netravali AN, Li Z-F, Sachse W, Wu HF. Determination of fiber/ matrix interfacial shear strength by an acoustic emission technique. J Mater Sci 1991;26:6631–8. [24] Fjeldly A, Olsen T, Rysjedal JH, Berg JE. Influence of the fiber surface treatment and hot-wet environment on the mechanical behaviour of carbon/epoxy composites. Composites Part A 2001;32:373–8.
Comparison of short carbon fibre surface treatments on epoxy composites I. Enhancement of the mechanical properties Hui Zhang, Zhong Zhang *, Claudia Breidt Institute for Composite Materials, University of Kaiserslautern, Erwin Schr€odinger Strasse 58, D-67663 Kaiserslautern, Germany Received 7 August 2003; accepted 22 February 2004 Available online 12 April 2004
Abstract Pitch-based short carbon fibres were treated by both a gaseous oxidation and a cryogenic treatment approach. It was found by scanning electron microscopy that the fibre surface roughness was increased by various oxidative conditions, whereas the fibre diameter was reduced by the cryogenic treatment. In both cases, appropriate treatments could effectively improve the mechanical properties in their epoxy composites due to the enhanced fibre–matrix interfacial bonding. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Short carbon fibres; Fibre treatment; Gaseous oxidation; Cryogenic treatment; Epoxy; Mechanical properties
1. Introduction It is well known that the fibre/matrix adhesion strength plays an important role on the mechanical properties of fibre reinforced polymer composites [1–8]. When load is applied to composites, it will be distributed and transferred through fibre/matrix interfaces. A strong bonding promotes a better involvement of more fibres, accordingly increases the strength of composites. However, carbon fibres usually perform a poor bonding behaviour to polymer matrix due to their nature of smoothness and chemical inertness. In order to improve the bonding properties of carbon fibres, various approaches can be applied, which were classified into oxidative and non-oxidative treatments by Park and Kim [9]. Oxidation treatments involve gas-phase, liquidphase and anodic oxidations, whereas the non-oxidative ones include plasma treatment, deposition of more active forms of carbon, or grafting of the carbon fibre surface with polymers [10]. Oxidation treatment of car*
Corresponding author. Tel.: +49-631-2017213; fax: +49-6312017196. E-mail address: [email protected] (Z. Zhang). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.02.009
bon fibres in hot air is one of the commonly used approaches owing to several advantages, e.g. easy operation, low cost and lack of pollution [11]. On the other hand, cryogenic treatment of carbon fibres, which could increase the fibre strength by clearing the weak layer of amorphous carbon, seems to be an interesting and relatively novel method. Few about this approach were reported. Rashkovan and Korabelnikov [12] treated high-tenacity long carbon fibres in liquid nitrogen for 30 s, in order to improve the mechanical properties in composites. However, the effect on short carbon fibres with an extended treatment time, and the performance on mechanical properties of their composites are not yet fully understood. In the present work, two different approaches, i.e. air oxidation and cryogenic treatment, were applied to treat pitch-based short carbon fibres at various conditions. The configurations of treated carbon fibres and the fractography were studied by using scanning electron microscopy (SEM). The improvement on the mechanical properties of treated carbon fibres reinforced epoxy is reported. Further improvement of the tribological properties will be presented in the subsequent part of this paper [13].
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
average value was reported, with an error scattering of the maximum absolute error.
2. Experimental 2.1. Materials and compounding
2.3. SEM observation Pitch-based short carbon fibres (M-2007S) were supplied by Kureha Co. without treatment, which have a density of 1.6 g/cm3 , and average diameter and length of 14.5 and 90 lm, respectively. They are graphite grade short fibres with excellent mechanical properties, e.g. tensile strength of 800 MPa, modulus of elasticity of 35 GPa, and elongation of 2.3%. For the oxidative treatment, the carbon fibres were oxidized in a muffle furnace at various temperatures for 1 h. The air vent was always applied to keep the air inside the furnace flowing during processing. In the case of cryogenic treatment, short carbon fibres containing in a ferrous vessel were immersed into liquid nitrogen ()196 °C) for various minutes. All treatment details are summarized in Table 1. The matrix used in this work was a bisphenol-A type resin (DER331, Dow) hardened by an amine curing agent (HY2954, Dow), with densities of 1.16 and 0.95 g/ cm3 , respectively. The composites were prepared in a vacuum dissolver by mixing the epoxy resin with 15 vol.% untreated and treated short carbon fibres as described in Table 1. The epoxy resin and the curing agent were preheated at 70 °C in an oven for at least 4 h before use. The mixing process was carried out at 70 °C with stirring speed of 2000 rpm for 30 min under vacuum, subsequently adding definite amount of curing agent, the mixture was stirred at 60 °C with stirring speed of 200 rpm for 15 min and then was poured into a rectangular aluminium mould for curing. The applied gel temperature was 70 °C for 8 h, followed by a curing stage at 122 °C for 16 h. 2.2. Mechanical properties A Zwick universal testing machine was applied to investigate the flexural modulus and strength under a threepoint-bending approach according to DIN-ISO-178. Specimens were cut at a dimension of 100 10 4 mm3 . The test speed was kept constant at 1 mm/min. Five specimens of each composition were measured and an
The configuration of as-received, treated carbon fibres as well as fracture surfaces of epoxy composites were examined using a JEOL-5400 SEM. The diameters of carbon fibres before and after liquid nitrogen treatment were also determined by SEM. In the latter case, the sample holder of SEM was adjusted to zero degree and all samples were coated with gold films previously. An average value of diameters was calculated from at least 200 individual carbon fibres.
3. Results and discussion 3.1. SEM observation The SEM configurations of as received, oxygen treated and cryogenic treated short carbon fibres are given in Fig. 1. Remarkable differences in micrographs can be observed on untreated and treated carbon fibres. The surface of as-received fibre (Fig. 1(a)) seems to be relatively smooth and there are some small impurities absorbed on the fibre surface, which were introduced during the fibre manufacturing process. Under a treatment temperature of 450 °C the surface does not look appreciably different when compared to untreated ones, which suggests that the treatment was mild and did not cause essential morphological changes on the fibre surface [14]. However, by using the gas physical adsorption technique, Wan et al. [11] reported that after treatment under a similar condition mentioned above, the total carbon fibre surface area was doubled. Their result indicated that some changes did take place on carbon fibre surface at 450 °C, although it was difficult to be recognized by SEM. Once the treatment temperature was further increased, the changes on the surface could be easily observed. At a treatment temperature of 500 °C for 1 h (Fig. 1(b)), the fibre surface became relatively rough and more pieces of tiny fragments stuck to the fibre surface, which suggested
Table 1 Details of specimens and treatment conditions Treatment approach
Sample No.
CF volume content
Treatment details
–
CF_0
15%
As-received
Oxidation (in air)
CF_450 CF_500 CF_550 CF_600
15% 15% 15% 15%
450 500 550 600
Cryogenic treatment (in liquid nitrogen)
CF_1 CF_5 CF_10 CF_20
15% 15% 15% 15%
1 min 5 min 10 min 20 min
°C/1 °C/1 °C/1 °C/1
h h h h
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
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Fig. 1. SEM configurations of carbon fibres treated under different conditions: (a) as-received; air-oxidized at (b) 500 °C/1 h, and (c) 600 °C/1 h; cryogenic treated in liquid nitrogen for (d) 1 min, and (e) 10 min.
that the absorbability of treated carbon fibres might be enhanced to some extent. A great number of micro-pits could be found on the fibre surface on account of a violent oxygen etching at the highest oxidative temperature of
600 °C in our case (Fig. 1(c)). Those pits with the size in a range of several hundred nanometres had irregular shapes and distributed uniformly over the fibre surface. In some places they were linked together and formed a ditch-like
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029 Table 2 Fibre diameter distribution before and after cryogenic treatment
Fig. 2. Diagram of the mechanism of the cryogenic treatment on carbon fibres.
structure. Obviously, this kind of treatment destroys the original structure of carbon fibres, therefore, leading to a loss of mechanical properties of the carbon fibres somehow, as a consequence to that of the composites as well.
as-received
12
av=14.6µm 10
Fraction [%]
8 6 4 2 0 0
10
(a)
15
20
Peak Peak Peak centre (lm) width (lm) height (%)
As-received 5 min 20 min
14.6 14.2 13.9
14.48 14.47 14.12
6.67 5.66 4.83
10.72 13.38 15.21
In the case of cryogenic treatment, an appreciable characteristic is that only a very short treatment time (1 min, Fig. 1(d)) could make a difference in the morphology of carbon fibres. The surface roughness of the carbon fibre might, after a cooling treatment for 1 min, nearly attain such a level that was reached by the air oxidative treatment at much longer time (1 h at a treatment temperature of 550 °C). When extending the treatment time up to 10 min, the carbon fibre surface became very rough, and lots of tiny fragments and some striations along the fibre axis could be observed (Fig. 1(e)). At the same time it led to a reduction of the fibre diameter on account of the removing of the amorphous carbon layer, which will be further discussed later. So far as the enhancement of the surface roughness and the treatment time are concerned, the cryogenic treatment seems to be more efficient than the air oxidative one. Fig. 2 illustrates a diagram of the cryogenic treatment for carbon fibres. As pointed out by Rashkovan and Korabelnikov [12], a carbon fibre is usually composed of two parts, i.e. the proper carbon fibre, and a layer of amorphous carbon deposited on it, so-called disordered structure (cf. [15]). At low temperatures, due to the difference of the coefficients of linear thermal expansion (CTE) between these two parts, the carbon fibre is exposed to a contraction of the amorphous carbon and an
20min av=13.9µm
16
14.7 14.6 14.5
12
14.4
Diameter [µm]
14
10 8 6
14.3 14.2 14.1
4
14.0
2
13.9
0
13.8 0
(b)
Average diameter (lm)
Diameter [µm] 18
Fraction [%]
25
Short carbon fibre
10
15
20
25
Diameter [µm]
Fig. 3. Typical diameter distribution of carbon fibres before and after cryogenic treatment.
0
5
10
15
20
Treatment Time [min] Fig. 4. Dependence of the fibre average diameter on the cryogenic treatment time.
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
axial expansion of the proper fibre, while the proper fibre has a negative CTE. Shear stresses arise on the interface of the two parts and finally exceed the shear strength between the proper fibre and the amorphous carbon layer. Therefore the latter would easily shell off from the fibre surface. Due to its heterogeneity, the amorphous carbon layer shells off along the fibre to various extents resulting in an increase of the fibre surface roughness, as confirmed by SEM micrographs. 3.2. Fibre diameter distribution As mentioned above, the amorphous carbon layer can partly shell off from the fibre after the cryogenic treatment. Accordingly, the diameter of the carbon fibre is reduced to some degree. Fig. 3 shows the typical
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curves of the diameter distribution for as-received and treated carbon fibres, which were fitted by Gaussian distributions, and the parameters were summarized in Table 2. It can be found that the fitting results of cryogenic treated fibres shifted slightly to lower values and the width of the curve also became narrow. This implicates that the amount of fibres with large diameters decreased, possibly due to the fact that they possess thicker amorphous carbon layers, which are easily to be shelled off after cryogenic treatment. When the treatment time reached up to 20 min, the average diameter decreased about 4.8%. Fig. 4 demonstrates that at the beginning of the treatment, the rate of ‘‘shelling off’’ of the amorphous carbon layer was higher. With an increasing treatment time, the rate ran slowly down. Even with only three points measured in Fig. 4, the average
3.9
4.0
3.5
3.5
3.5 3.4
3.5
3.0
(a)
CF_0
CF_450 CF_500 CF_550 CF_600
Flexural Modulus [GPa]
Flexural Modulus [GPa]
4.5
5.0 4.5 4.4
4.5 4.0
4.0
CF_1
CF_5
4.0 3.5 3.5 3.0
(d)
CF_0
CF_10
CF_20
107.7 110 98.1 100 90.5 88.9 90
80
(b)
CF_0
120
109.7
Flexural Strength [MPa]
Flexural Strength [MPa]
120
CF_450 CF_500 CF_550 CF_600
112.2
110
105.0 97.3
100
99.9
90.5 90 80
(e)
CF_0
CF_1
CF_5
CF_10
CF_20
(c)
5 4
3.36
3.33
3.32
3.2
3.47
3 2 1 0 CF_0
CF_450 CF_500 CF_550 CF_600
Elongation at Break [%]
Elongation at Break [%]
5
(f)
4
3.36
3.15
3.25
3.12
2.98
CF_0
CF_1
CF_5
CF_10
CF_20
3 2 1 0
Fig. 5. Flexural properties of CF/epoxy composites: for air oxidation, (a) flexural modulus, (b) flexural strength, and (c) elongation at break; and for cryogenic treatment, (d) flexural modulus, (e) flexural strength, and (f) elongation at break.
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
thickness of the amorphous carbon layer can be roughly estimated as approximately 0.7 lm. Relative Improvement
The effects of the air oxidative treatment on the flexural strength, the modulus and the elongation of CF/ EP composites are given in Fig. 5. The flexural moduli of treated samples increase slightly in the range of treatment temperature between 450 and 550 °C. The modulus decrease at a treatment temperature of 600 °C, which may due to the loss of the distinct fibre strength (Fig. 5(a)). A maximum modulus value can be found at a temperature of 550 °C, i.e. 3.9 GPa, about 10% improvement compared with that of untreated one. On the other hand, the flexural strength of treated samples enhances monotonically with an increased oxidation level (Fig. 5(b)). A maximum strength value can be observed at the treatment temperature of 600 °C, i.e. 109.7 MPa, about 21% improvement compared with that of untreated one. Although under these conditions the proper carbon fibres were damaged, the strong interfacial bonding may contribute these improvements on both strength and modulus. To summarize the above results, the flexural strength of the composite is more sensitive to the change of fibre–matrix interface than the modulus. The latter is largely dominated by the stiffness of the fibres. In term of the cryogenic treatment, the flexural moduli and the strength of treated samples also increased effectively comparing to untreated ones (Fig. 5(d) and (e)). The flexural modulus and strength reach their maximum value at a treatment time of 10 min, i.e. 4.5 GPa and 112.2 MPa, respectively, which were about 29% and 24% improvement comparing with that of untreated one. However, these effects tend to decrease with an increased treatment time. It is considered that there is an optimum treatment condition for the carbon fibres and their composites. After too long treatment time, too much amorphous carbon shelled off from the fibre, corresponding to a decrease in the amounts of interlocking sites. For both treatments, the elongations of the samples appeared to no obvious changes compared to that of the untreated ones (Fig. 5(c) and (f)), indicating that the elongation is a dominate of the matrix. Fig. 6 demonstrates the relative improvements on mechanical properties of epoxy composites for both treatment approaches. The main difference consists in the changes of flexural moduli of CF/EP composites. For the air oxidative treatment (Fig. 6(a)) the flexural moduli of their epoxy composites increased slightly, since the mechanical properties of the carbon fibres did not change at mild treatment conditions or rather decreased to some degree due to the excessive oxygen etching. In term of the cryogenic treatment in Fig. 6(b),
flexural modulus flexural strength elongation
1.3 1.2 1.1 1.0 0.9 0.8 CF_0
CF_450 CF_500 CF_550 CF_600
(a)
Air Oxidation 1.4
Relative Improvement
3.3. Mechanical properties
1.4
flexural modulus flexural strength elongation
1.3 1.2 1.1 1.0 0.9 0.8 CF_0
(b)
CF_1
CF_5
CF_10
CF_20
Cryogenic Treatment
Fig. 6. Relative improvements on the mechanical properties of (a) air-oxidation, and (b) cryogenic treatment.
it is opposing. It is worth to mention that the maximum values for both modulus and strength of the cryogenic treated samples are higher than those of air-treated results. 3.4. SEM fractography The improvement on flexural properties of CE/EP composites could be attributed to the enhancement of the interfacial adhesion strength of the fibre and matrix after fibre surface treatments. There are several mechanisms for the fibre–matrix bonding, which involve mechanical interlocking, adsorption interaction, electrostatic interaction, and diffusion of polymer chain segments [16]. As far as air-treated samples are concerned, the mechanical interlocking could be the most important factor. As shown in SEM micrographs (Fig. 1), with increased treatment temperatures the fibre surfaces become rougher, i.e. the fibres possessed a larger surface area, which could contact with matrix and thus formed more mechanical interlocking sites. In
H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
addition, the enhancement of the surface roughness reduces the contact angle between fibre and matrix and hence increases the wettability [17] as well. SEM fractographies of three-point-bending tests are given in Fig. 7. It is easily recognized in Fig. 7(a) that untreated
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carbon fibres performed very poor interface bonding with epoxy matrix. After oxidation treatment shown in Fig. 7(b) and (c), strong interlocking of fibre–matrix bonding can be observed as marked by arrows in Fig. 7(c), which was termed as ‘‘mechanical anchor’’ by
Fig. 7. SEM fractographies of CF/epoxy composites: (a) as-received; oxidized at (b) 500 °C/1 h, (c) 600 °C/1 h; cryogenic treated in liquid nitrogen for (d) 10 min, and (e) 20 min.
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H. Zhang et al. / Composites Science and Technology 64 (2004) 2021–2029
Kalantar and Drzal [16]. The mechanical interlocking provides a strong interface bonding even though other effects are weak [18]. Another possible reason for the enhancement on interfacial adhesion is chemical interaction. Many researchers [19–22] have analysed the O1s /C1s ratio of carbon fibre surface by means of a Xray photoelectron spectroscopy (XPS), and reported that gaseous oxidative treatment could increase the oxygen concentration on carbon fibre surface, which means that the amount of functional groups could be increased, e.g. hydroxyl (–OH), carbonyl (–C@O) and carboxyl (–COOH) groups. These functional groups may improve the wettability by enhancing the surface energy. Therefore chemical reactions can take place with epoxy group forming covalent bonding at the interface. In the case of cryogenic treatment, similar mechanical interlocking was also observed which plays an important role to the improved interfacial adhesion, as shown in Fig. 7(d) marked by arrows. It is interesting that even some broken fibres can be found (Fig. 7(e)) which indicates the mechanical interlocking in some regions was very strong. Since the average length of carbon fibres applied in this study is much shorter compared to the fibre broken critical length as reported in literatures of several hundreds microns [23,24], fibre pull-out and matrix fracture should be the main failure modes in our case. An improved fibre–matrix bonding with fibre broken involving will definitely improve the strength of composites. Another reason for the enhancement of the flexural properties is the strengthening effect of carbon fibres of the cryogenic treatment [12]. After treatment the average strength of the carbon fibres usually increased due to the removal of the weak layer and attached particles, which act as concentrators and decrease the fibre strength to some extent. On the other hand, it should be mention that not all the carbon fibres had the similar obvious changes after cryogenic treatment. Some carbon fibres still kept much smoother surface with poor bonding to the matrix as shown in Fig. 7(e) as well. The possible reasons of this phenomenon could be: (i) the heterogeneity of fibres, while some carbon fibres, which may possess very thin amorphous carbon layer, do not form a rough surface after treatment; and (ii) the treatment process, while some fibres were not effectively treated due to the thermal conduction. An optimum treatment process is still a task to fully exert the potential of this approach on the enhancement of carbon fibre–matrix bonding of polymer composites.
4. Conclusions Based on this work devoted to studying the effect of surface treatment on mechanical properties of short
carbon fibre reinforced epoxy composites, the following conclusions can be drawn: 1. Both oxidative and cryogenic treatments could significantly increase the surface roughness of carbon fibres, accordingly to improve the interfacial adhesion strength of fibre and epoxy bonding due to the mechanical interlocking. 2. The cryogenic treatment has advantages of very short treatment time and environment-friendly media, as well as higher improvement on both modulus and strength compared to the oxidative one. 3. The carbon fibres used in this study were relatively short; therefore the improvement on mechanical properties was limited. Further enhancement of the tribological properties will be concentrated on the subsequent part of this paper [13].
Acknowledgements Z. Zhang is grateful to the Alexander von Humboldt Foundation for his Sofja Kovalevskaja Award, financed by the German Federal Ministry of Education and Research (BMBF) within the German Government’s ‘‘ZIP’’ program for investment in the future. The authors appreciate Prof. Dr.-Ing. Dr. h.c. K. Friedrich, IVW, for his valuable discussions during the course of this work and the preparation of this paper.
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