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polyarylacetylene resin composites
Applied Surface Science 254 (2008) 5342–5347
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of interphase modification on carbon fiber/polyarylacetylene resin composites L. Liu *, Y.J. Song, H.J. Fu, Z.X. Jiang, X.Z. Zhang, L.N. Wu, Y.D. Huang Department of Applied Chemistry, Faculty of Science, Harbin Institute of Technology, PO Box 410#, Harbin 150001, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Received 3 November 2007 Received in revised form 17 February 2008 Accepted 19 February 2008 Available online 29 February 2008
Interfacial modification for carbon fiber (CF) reinforced polyarylacetylene (PAA) resin, a kind of nonpolar, was investigated. The high carbon phenolic resin was used as coating to treat the surface of CF after oxidation. Atomic force microscopy (AFM) with force modulation mode was used to analyze the interphase of composite. The interlaminar shear strength (ILSS) and mechanical properties of CF/PAA composites were also measured. It was found that the CF/PAA composites treated with oxidation and coating after oxidation had transition area between carbon fiber and PAA resin. The existence of transition area led to the improvement of interfacial performance of composites. Specially, the thickness and stiffness of interphase of composite treated with coating after oxidation were more suitable for CF/ PAA composites. Thus, the composite treated with coating after oxidation had the highest value of ILSS and the best mechanical properties. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Atomic force microscopy Interphase Polyarylacetylene resin Coating
1. Introduction Polyarylacetylene (PAA), a new kind of high performance resin, is developed increasingly as the matrix for high temperature composites of next generation in carbon–carbon composites, ablation materials and structural materials owing to its outstanding heat resistance and excellent process properties [1–3]. Resulting mainly from the diethynylbenzene (DEB), PAA is a highly cross-linked aromatic polymer that contains only carbon and hydrogen when it is cured by means of addition polymerization, which makes it own excellent thermal stability and oxidation. When the PAA is heated to high temperatures in an inert environment, only about 10 wt% is volatilized, while the remaining 90 wt% is carbon char, which means far less volatile material is generated and minimal shrinkage is associated with pyrolysis [4]. Furthermore, PAA resin is liquid or solvable and fusible solid at room temperature so that it provides better processing flexibility, being applicable for conventional curing processes like compression molding process, vacuum bagging method, resin transfer molding [5]. However, the bad wettability between carbon fiber (CF) and PAA resin from the non-polar structure of PAA and chemical
* Corresponding author. Tel.: +86 451 86414806; fax: +86 451 86413711. E-mail address: [email protected] (L. Liu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.078
inertness of carbon fiber causes the weak interfacial adhesion between fiber and non-polar PAA resin. At the same time, the content of aromatic rings and the degree of cross-linking to PAA is so high that makes the resin fragile after being cured, which results in the adhesion between carbon fiber and PAA resin can hardly come to an ideal prospective aiming. So it is quite necessary to improve the interfacial performance of CF/PAA composites. Usually surface treatment of carbon fiber is an effective way to improve interfacial adhesion of its composites. A variety of surface treatment have been applied to carbon fiber to increase the interaction with resin matrix, including oxidation methods (gas oxidation, chemical oxidation, electrolytic oxidation, plasma treatment, etc.) [6–12] and non-oxidation methods (coating method, coupling agent method and vapor deposition method) [13–18]. All these methods are following the principles such as the similar polarity, the interphase acid and alkali match, providing possibilities and qualifications for the chemical bonding in interphase and introducing a plastic boundary layer. But all the methods, which improve the interfacial adhesion of composites, were developed according to the resin with polar structure, such as epoxy and phenolic resin. While there are few modifying methods that are related to non-polar resin, such as PAA resin. So in this study, in order to obtain the more ideal interfacial properties of the composites and then to discuss the mechanism of improving the fiber–matrix interfacial adhesion, high carbon phenolic coating were applied into the surface of CF oxidized by ozone.
L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347
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Table 1 The comparisons of ablation char yield and curing temperature of these resins Performances Ablation char yield (%) Curing temperature (8C)
Ba-phenolic resin
High carbon phenolic resin
Benzoxazine resin
50–55 170–180
70–80 190–200
60–70 180–200
2. Experimental
PAA 80–90 190–200
strength were tested according to standard of GB/T14209-1993, GB-1447-83 and GB-1449-83, respectively.
2.1. Materials 2.4. Atomic force microscopy test in force modulation mode PAA resin was supplied by Aerospace Research Institute of Material & Processing Technology (Beijing, China). Polyacrylonitrile (PAN) carbon fiber (3 K) was obtained from Jilin Carbon Co. (Jilin, China). High carbon phenolic resin was also supplied by Beijing Aerospace Research Institute of Material and Processing Technology. The reason that high carbon phenolic resin was chose as coating was the results of comparison of three resin whose performances, such as carbon content and curing temperature, etc., were similar to PAA resin. The three resins were Ba-phenolic resin, high carbon phenolic resin and benzoxazine resin. The molecular formula and performances of these resins were shown in Table 1, respectively. From the comparison of structures and characteristics of the three resins with the PAA resin, it is found that the high carbon phenolic resin is the most similar to PAA resin. Thus, we chose high carbon phenolic resin as coating resin The similar structure can ensure that high carbon phenolic resin and PAA resin are compatible, while high carbon content can ensure that the ablation ability of CF/PAA composite did not change much as the introduction of interphase layer. 2.2. Carbon fiber surface treatment and preparation of CF/PAA composites Before the treatment, with flowing N2 protection, the sizing on the carbon fiber was removed through thermal decomposition method. The treatment of carbon fibers was carried out in an experimental system mainly composed of an ozone generator, a treating stove furnished by temperature control system, and the system of tail gas treatment. From the generator, the mixture of ozone and oxygen was get, and the concentration of ozone could be adjusted by flow controlling of gas and changing the electric voltage added on the generator. First, the CF were treated by ozone oxidation, and then immersed into the 1 wt% ethanol solutions of high carbon phenolics coating. The unidirectional long carbon fiber reinforced PAA composites were made with both untreated and different coating treated carbon fiber. Curing was performed in a compression moulding machine by the method of compression moulding, and the content of the resin in composites was controlled at about 35 wt%. The curing process was shown as follows, 120 8C for 2 h, 140 8C for 2 h, 180 8C for 2 h, 200 8C for 2 h, and 250 8C for 0.5 h. During the curing process, the pressure was 2 MPa which was loaded after the temperature being increased to 120 8C and kept for 20 min. When the curing process had finished, the mould was cooled to room temperature with the pressure being maintained. All composite samples were about 6 mm in width and 2 mm in thickness. 2.3. The characterization of the interfacial and mechanical properties of the CF/PAA composite Interfacial and mechanical properties of the CF/PAA composites were all tested on an universal testing machine (WO-5, Changchun, China). ILSS of CF/PAA composites were tested using a three point short beam bending test method according to standard of GB3357-82. Compression strength, tensile strength and flexural
Atomic force microscope (AFM) has the ability to probe local mechanical properties of surfaces or interphase on a nanometer scale, as demonstrated by the force–distance curve, and the load– indentation curve obtained at individual points. These micromechanical properties are important for the interfacial properties of fiber reinforced composites. They may provide further insight into the other related properties and formation mechanisms of interphase in composites. As the instrumentation of AFM becomes more sophisticated, new modes of operation arise, providing new means of surface or interphase investigation. One of these is the force modulation mode, in which the vertical vibration amplitude of the tip, as affected by the local surface features of the sample, is recorded and used to construct images. The most important theoretical advantage of force modulation code AFM, compared with the normal contact mode AFM and tapping mode AFM, is the provision of local contrast in the mechanical properties of the sample surface in addition to topography. Furthermore, the mechanical properties are probed continuously over extended areas instead of at scattered individual points as in force–distance and load–indentation measurements. In this paper, studies of CF/PAA composites in force modulation mode. AFM were presented, together with discussion of the significance of the results, in terms of topography, micro-mechanical properties which was stiffness here, and the underlying mechanism of interfacial adhesion between carbon fiber and PAA resin. The unidirectional CF/PAA composites at most 4 mm length were cut and the cross-section of the samples was washed with acetone after being polished. The polish program is #320 paper for 1 min, #600 paper for 1 min, #1200 paper for 2 min and 1 mm Al2O3 1 min. AFM experiments were carried out on a Solver P47 AFM/STM system (NT-MDT Co., Russia). Force modulation mode was adopted to study the cross-section surface of unidirectional CF/PAA composites and the relative stiffness of various phases, including carbon fiber, interphase and resin. In the force modulation mode, modulation of the contact force between the sample and the tip was achieved by oscillating the tip and the supporting base assembly through a piezoelectric bimorph that was built into a special tip holder. The oscillation frequency and amplitude were controlled by a driving sinusoidal voltage wave applied to the bimorph. The resonance frequency of the bimorph is in the range 5–30 kHz. The oscillation frequency used was selected through the ‘‘tip-tuning’’ function of the soft ware, and was one of the resonance frequencies of the bimorph (not the silicon tip), typically around 7 kHz. Probability histogram and line distribution of CF/PAA cross-section surface relative stiffness, obtained from the statistical analysis of relative stiffness image, were used to compare and study the interfacial properties of composites modified with different treatment. Parameter of average roughness (Ra) was used to determine the roughness of sample surface. Average roughness Ra is defined as Ny Ny Nx X Nx X 1 X 1 X z z Ra ¼ (1) i j i j Nx Ny i¼1 j¼1 Nx Ny i¼1 j¼1 where Nx, Ny is the number of points along axis X and Y.
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Fig. 1. Schematic diagram of atom force microscopy in force modulation mode.
2.5. XPS analysis XPS analysis was carried out on a FEI Sirion 200 spectrometer (Royal Dutch Philips Electronics Ltd., Netherlands), using monochromatic Al Ka radiation at 12.5 keV and 300 W. A pass energy of 300 eV was used for the survey spectra. The slit width of the electron analyzer was set at 1.9 mm. Take-off-angles relative to sample surface of 458 was employed. Charge compensation was accomplished with an electron flood gun operating at electron energy of 4–6 eV. 3. Results and discussion 3.1. Micro-topography and rigidity distribution analysis of composites Fig. 1 shows the mechanics of force modulation mode AFM. In force modulation mode AFM, the degree of indentation of the tip into sample surface depends on the geometry and mechanical properties of both the tip and the sample surface, and the experimental conditions, such as driving amplitude and set point of the feedback control. On very soft areas of the sample, the tip indents comparatively deep into the surface, resulting in a very small reduction in its oscillating amplitude; whereas at very hard areas, the tip indents little into the sample, and the reduction in its oscillating amplitude is more significant.
The software that appends in AFM was used to make the statistic analysis for the distributing of relative stiffness. Thus, the stiffness image and the line distributing of relative stiffness for the cross-section surface of the composites is obtained, as shown in Fig. 2. The relative stiffness is indirectly indicated by the detection of magnitude (MAG, nA) of the flexural quantum of the microcantilever. It can be seen from the topography in Fig. 2(a) that fiber, matrix and interphase can be observed clearly and the part of carbon fiber is higher than that of PAA resin, which is more distinct in relative stiffness figure (as shown in Fig. 2(b)). Observations of AFM in the force modulation mode are describing of relative stiffness of each phase in composites. The difference of the stiffness between carbon fiber and PAA in relative stiffness figure is more remarkable than the difference of them in the topographies, so the figure of relative stiffness was chosen to observe the distribution of the phases in CF/PAA composites, including carbon fiber, PAA resin and interphase. However, the distribution shows that there are no transition layers between fiber and matrix and the change of stiffness from matrix to carbon fiber is acute. The stiffness topography and the line stiffness distribution of the cross-section surface of CF/PAA composites treated with ozone oxidation are shown in Fig. 3. The interphase in Fig. 3(a) of relative stiffness is clearer than that shown in Fig. 3(a). It seems that there are now complicated phase around the fiber. Appropriate scope near carbon fibers surface (the line in Fig. 3(a)) was chosen to be analyzed statistically and the result of the line distributing of relative stiffness is shown in Fig. 3(b). There is a transition layer between carbon fibers and PAA resin and the change of stiffness is not acute. The thickness of the transitional layer between carbon fiber and PAA resin is more or less 100 nm. The stiffness image and the stiffness distribution of the crosssection surface of CF/PAA composites treated with oxidation and high carbon phenolic resin coating are shown in Fig. 4. It seems that there are obvious transition layer between carbon fibers and PAA resin (Fig. 4(a)) and an obvious stiffness peak between carbon fiber and PAA resin (Fig. 4(b)). Following the arrow (Fig. 4(b)), the stiffness line distribution from resin to fiber is achieved, which indicate the moderate boundary between the carbon fiber and resin is obviously appeared. The thickness of the moderate boundary is more than 200 nm. All of above analysis reveals that the high carbon phenolic resin coating has formed a transition layer between carbon fiber and PAA resin. The stiffness of the interphase is in between the carbon fiber and PAA resin, and the thickness of the layer is higher than composite treated with oxidation. The increase of thickness and appropriate stiffness result in the formation of more suitable interphase. The existence of suitable interphase in composite can effectively transmit load and absorb energy when the composite is loaded.
Fig. 2. Force modulation mode microscopy images of untreated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347
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Fig. 3. Topographic and relative stiffness images of oxidation treated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
Fig. 4. Topographic and relative stiffness images of oxidation and coating treated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
Results of ILSS of the CF/PAA composites are shown in Fig. 5. It indicates that the treatments have great effect on ILSS of the CF/ PAA composites. The ILSS of CF controlled/PAA composites is only 36 MPa, while the ILSS of treated sample is 45 MPa and 56 MPa, respectively. The values of ILSS (as shown in Fig. 5) of the CF/PAA composites show that the oxidation treatments as well as coating treatment after oxidation have a marked effect on the interfacial properties of the composites. Especially, compare with untreated one, the ILSS of CF with coating after oxidation/PAA composite has an increment of 55%. The mechanical properties of composites untreated and treated also show the same tendency, as shown in
Fig. 6. The mechanical properties of composites are improved when fiber is treated. Furthermore, the composite treated with coating after oxidation gives the best mechanical performances. The increment of ILSS and mechanical properties of composites are attributed to the formation of transition area between the carbon fiber and PAA resin after treatment. From the analysis of Section 3.1, it can be found that the interphase of untreated CF/PAA composite is not almost existed, and there is no transition area between the carbon fiber and PAA resin. The smooth surface of carbon fiber and non-polar structure of the PAA resin result in the happen of above situation. These make the interphase between the fiber and resin easy to destroy when the composite is loaded, so the ILSS of composite is low. After oxidation treatment, the
Fig. 5. Effect of treatment on ILSS of samples.
Fig. 6. Effect of treatment on mechanical properties of samples.
3.2. Mechanical properties of composites
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L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347 Table 2 Relative amounts of the elements on carbon fibers surface Treated methods
Element content (%)
O/C
C
O
N
Untreated Oxidation treated
87.92 86.67
11.06 12.21
0.96 0.98
0.126 0.141
Table 3 Main roughness parameters of the carbon fiber with different treatments measured by AFM on images of 6 mm 6 mm
Fig. 7. XPS total spectra of CF before and after oxidation treatment. (a) Untreated; (b) oxidation treated.
transition area of composite begins to form. The formation of transition area is attributed to the oxidized and etching effect of oxidation treatment which improves the interfacial adhesion of composite [19,20]. Fig. 7 shows the XPS total spectra of CF before and after oxidation treatment, and Table 2 shows the relative amounts of the elements of CF surface which from Fig. 7. From the value of O/C in Table 2, it can clearly see the existence of oxidized effect when the fiber is treated with oxidation. The oxidized effect increases the polar functional groups on the surface of carbon fiber, which can improve the wettability between fiber and resin.
Treated methods
Ra (nm)
Standard deviation (nm)
Untreated Oxidation treated
45.49 50.37
2.14 3.62
Surface topographies of the carbon fiber before and after oxidation treatment were characterized by AFM. Resultant AFM images of 6 mm 6 mm are shown in Fig. 8. Like other PAN-based carbon fiber [21–23], the entire fiber surface has clear ridges and striations running along the axis of the fiber. Furthermore, it can be seen from Fig. 8 that the longitudinal ridges and the striations of treated carbon fiber are clearer than that of untreated. This presents that original features of the surface topography are changes after oxidation treatment. These assertions obtained from Fig. 8 are corroborated with the roughness parameters in Table 3. Table 3 summarizes the results of the roughness analysis of carbon fiber as obtained from AFM. Comparison the roughness values of the untreated carbon fiber with treated ones, it can be found that the treated fiber surface roughness is higher than that of untreated. These confirm the existence of etching effect after oxidation treatment. The etching effect can make the carbon fiber surface more roughness, which can enhance the mechanical interlocking between fiber and resin. The formation of transition area between the fiber and resin can transmit load when composite is loaded, and the transition area also can prevent the spread of crack. Thus, the interfacial performance of composite treated with oxidation is improved and the adhesion between fiber and resin is also enhanced. Moreover, when the carbon fiber is treated with coating after oxidation, the transition area is also formed and the value of thickness of interphase is more than that of the composite treated with oxidation. The introduction of coating after oxidation strengthens the effectiveness of oxidation treatment, and increases
Fig. 8. Three-dimensional AFM micrographs of carbon fiber surface. (a) Untreated; (b) treated by oxidation.
L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347
the thickness of interphase. In addition, the stiffness of interphase of composite treated with coating after oxidation is in between the fiber and resin. All of these lead to the formation of appropriate interphase between the carbon fiber and the PAA resin, which improves of overall mechanical performance of composite. So the composite treated with coating after oxidation has the highest ILSS value. 4. Conclusions The carbon fiber was treated by oxidation and high carbon phenolic resin coating after oxidation, and it is found that interfacial and mechanical properties of the CF/PAA composite treated by coating after oxidation is best. Furthermore, it can be concluded that the interphase between non-polar PAA resin and carbon fiber treated with oxidation and coating is suitable for CF/ PAA composite. The reason was explained using AFM analysis. With the aid of AFM, it found that the transition area was formed between carbon fiber and PAA resin in composite which was treated with oxidation as well as oxidation and high carbon phenolic resin coating. The existence of transition area led to the improvement of interfacial performance of composites. In addition, the matching extent between the carbon fiber, PAA resin and the interphase in composite treated with oxidation and coating is more than that composite treated with oxidation. Thus, the composite treated with oxidation and coating has the highest ILSS value and best mechanical performances.
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Acknowledgement The authors would like to thank the National Natural Science Foundation of China (No. 50603004) for financial supports.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of interphase modification on carbon fiber/polyarylacetylene resin composites L. Liu *, Y.J. Song, H.J. Fu, Z.X. Jiang, X.Z. Zhang, L.N. Wu, Y.D. Huang Department of Applied Chemistry, Faculty of Science, Harbin Institute of Technology, PO Box 410#, Harbin 150001, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Received 3 November 2007 Received in revised form 17 February 2008 Accepted 19 February 2008 Available online 29 February 2008
Interfacial modification for carbon fiber (CF) reinforced polyarylacetylene (PAA) resin, a kind of nonpolar, was investigated. The high carbon phenolic resin was used as coating to treat the surface of CF after oxidation. Atomic force microscopy (AFM) with force modulation mode was used to analyze the interphase of composite. The interlaminar shear strength (ILSS) and mechanical properties of CF/PAA composites were also measured. It was found that the CF/PAA composites treated with oxidation and coating after oxidation had transition area between carbon fiber and PAA resin. The existence of transition area led to the improvement of interfacial performance of composites. Specially, the thickness and stiffness of interphase of composite treated with coating after oxidation were more suitable for CF/ PAA composites. Thus, the composite treated with coating after oxidation had the highest value of ILSS and the best mechanical properties. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Atomic force microscopy Interphase Polyarylacetylene resin Coating
1. Introduction Polyarylacetylene (PAA), a new kind of high performance resin, is developed increasingly as the matrix for high temperature composites of next generation in carbon–carbon composites, ablation materials and structural materials owing to its outstanding heat resistance and excellent process properties [1–3]. Resulting mainly from the diethynylbenzene (DEB), PAA is a highly cross-linked aromatic polymer that contains only carbon and hydrogen when it is cured by means of addition polymerization, which makes it own excellent thermal stability and oxidation. When the PAA is heated to high temperatures in an inert environment, only about 10 wt% is volatilized, while the remaining 90 wt% is carbon char, which means far less volatile material is generated and minimal shrinkage is associated with pyrolysis [4]. Furthermore, PAA resin is liquid or solvable and fusible solid at room temperature so that it provides better processing flexibility, being applicable for conventional curing processes like compression molding process, vacuum bagging method, resin transfer molding [5]. However, the bad wettability between carbon fiber (CF) and PAA resin from the non-polar structure of PAA and chemical
* Corresponding author. Tel.: +86 451 86414806; fax: +86 451 86413711. E-mail address: [email protected] (L. Liu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.078
inertness of carbon fiber causes the weak interfacial adhesion between fiber and non-polar PAA resin. At the same time, the content of aromatic rings and the degree of cross-linking to PAA is so high that makes the resin fragile after being cured, which results in the adhesion between carbon fiber and PAA resin can hardly come to an ideal prospective aiming. So it is quite necessary to improve the interfacial performance of CF/PAA composites. Usually surface treatment of carbon fiber is an effective way to improve interfacial adhesion of its composites. A variety of surface treatment have been applied to carbon fiber to increase the interaction with resin matrix, including oxidation methods (gas oxidation, chemical oxidation, electrolytic oxidation, plasma treatment, etc.) [6–12] and non-oxidation methods (coating method, coupling agent method and vapor deposition method) [13–18]. All these methods are following the principles such as the similar polarity, the interphase acid and alkali match, providing possibilities and qualifications for the chemical bonding in interphase and introducing a plastic boundary layer. But all the methods, which improve the interfacial adhesion of composites, were developed according to the resin with polar structure, such as epoxy and phenolic resin. While there are few modifying methods that are related to non-polar resin, such as PAA resin. So in this study, in order to obtain the more ideal interfacial properties of the composites and then to discuss the mechanism of improving the fiber–matrix interfacial adhesion, high carbon phenolic coating were applied into the surface of CF oxidized by ozone.
L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347
5343
Table 1 The comparisons of ablation char yield and curing temperature of these resins Performances Ablation char yield (%) Curing temperature (8C)
Ba-phenolic resin
High carbon phenolic resin
Benzoxazine resin
50–55 170–180
70–80 190–200
60–70 180–200
2. Experimental
PAA 80–90 190–200
strength were tested according to standard of GB/T14209-1993, GB-1447-83 and GB-1449-83, respectively.
2.1. Materials 2.4. Atomic force microscopy test in force modulation mode PAA resin was supplied by Aerospace Research Institute of Material & Processing Technology (Beijing, China). Polyacrylonitrile (PAN) carbon fiber (3 K) was obtained from Jilin Carbon Co. (Jilin, China). High carbon phenolic resin was also supplied by Beijing Aerospace Research Institute of Material and Processing Technology. The reason that high carbon phenolic resin was chose as coating was the results of comparison of three resin whose performances, such as carbon content and curing temperature, etc., were similar to PAA resin. The three resins were Ba-phenolic resin, high carbon phenolic resin and benzoxazine resin. The molecular formula and performances of these resins were shown in Table 1, respectively. From the comparison of structures and characteristics of the three resins with the PAA resin, it is found that the high carbon phenolic resin is the most similar to PAA resin. Thus, we chose high carbon phenolic resin as coating resin The similar structure can ensure that high carbon phenolic resin and PAA resin are compatible, while high carbon content can ensure that the ablation ability of CF/PAA composite did not change much as the introduction of interphase layer. 2.2. Carbon fiber surface treatment and preparation of CF/PAA composites Before the treatment, with flowing N2 protection, the sizing on the carbon fiber was removed through thermal decomposition method. The treatment of carbon fibers was carried out in an experimental system mainly composed of an ozone generator, a treating stove furnished by temperature control system, and the system of tail gas treatment. From the generator, the mixture of ozone and oxygen was get, and the concentration of ozone could be adjusted by flow controlling of gas and changing the electric voltage added on the generator. First, the CF were treated by ozone oxidation, and then immersed into the 1 wt% ethanol solutions of high carbon phenolics coating. The unidirectional long carbon fiber reinforced PAA composites were made with both untreated and different coating treated carbon fiber. Curing was performed in a compression moulding machine by the method of compression moulding, and the content of the resin in composites was controlled at about 35 wt%. The curing process was shown as follows, 120 8C for 2 h, 140 8C for 2 h, 180 8C for 2 h, 200 8C for 2 h, and 250 8C for 0.5 h. During the curing process, the pressure was 2 MPa which was loaded after the temperature being increased to 120 8C and kept for 20 min. When the curing process had finished, the mould was cooled to room temperature with the pressure being maintained. All composite samples were about 6 mm in width and 2 mm in thickness. 2.3. The characterization of the interfacial and mechanical properties of the CF/PAA composite Interfacial and mechanical properties of the CF/PAA composites were all tested on an universal testing machine (WO-5, Changchun, China). ILSS of CF/PAA composites were tested using a three point short beam bending test method according to standard of GB3357-82. Compression strength, tensile strength and flexural
Atomic force microscope (AFM) has the ability to probe local mechanical properties of surfaces or interphase on a nanometer scale, as demonstrated by the force–distance curve, and the load– indentation curve obtained at individual points. These micromechanical properties are important for the interfacial properties of fiber reinforced composites. They may provide further insight into the other related properties and formation mechanisms of interphase in composites. As the instrumentation of AFM becomes more sophisticated, new modes of operation arise, providing new means of surface or interphase investigation. One of these is the force modulation mode, in which the vertical vibration amplitude of the tip, as affected by the local surface features of the sample, is recorded and used to construct images. The most important theoretical advantage of force modulation code AFM, compared with the normal contact mode AFM and tapping mode AFM, is the provision of local contrast in the mechanical properties of the sample surface in addition to topography. Furthermore, the mechanical properties are probed continuously over extended areas instead of at scattered individual points as in force–distance and load–indentation measurements. In this paper, studies of CF/PAA composites in force modulation mode. AFM were presented, together with discussion of the significance of the results, in terms of topography, micro-mechanical properties which was stiffness here, and the underlying mechanism of interfacial adhesion between carbon fiber and PAA resin. The unidirectional CF/PAA composites at most 4 mm length were cut and the cross-section of the samples was washed with acetone after being polished. The polish program is #320 paper for 1 min, #600 paper for 1 min, #1200 paper for 2 min and 1 mm Al2O3 1 min. AFM experiments were carried out on a Solver P47 AFM/STM system (NT-MDT Co., Russia). Force modulation mode was adopted to study the cross-section surface of unidirectional CF/PAA composites and the relative stiffness of various phases, including carbon fiber, interphase and resin. In the force modulation mode, modulation of the contact force between the sample and the tip was achieved by oscillating the tip and the supporting base assembly through a piezoelectric bimorph that was built into a special tip holder. The oscillation frequency and amplitude were controlled by a driving sinusoidal voltage wave applied to the bimorph. The resonance frequency of the bimorph is in the range 5–30 kHz. The oscillation frequency used was selected through the ‘‘tip-tuning’’ function of the soft ware, and was one of the resonance frequencies of the bimorph (not the silicon tip), typically around 7 kHz. Probability histogram and line distribution of CF/PAA cross-section surface relative stiffness, obtained from the statistical analysis of relative stiffness image, were used to compare and study the interfacial properties of composites modified with different treatment. Parameter of average roughness (Ra) was used to determine the roughness of sample surface. Average roughness Ra is defined as Ny Ny Nx X Nx X 1 X 1 X z z Ra ¼ (1) i j i j Nx Ny i¼1 j¼1 Nx Ny i¼1 j¼1 where Nx, Ny is the number of points along axis X and Y.
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Fig. 1. Schematic diagram of atom force microscopy in force modulation mode.
2.5. XPS analysis XPS analysis was carried out on a FEI Sirion 200 spectrometer (Royal Dutch Philips Electronics Ltd., Netherlands), using monochromatic Al Ka radiation at 12.5 keV and 300 W. A pass energy of 300 eV was used for the survey spectra. The slit width of the electron analyzer was set at 1.9 mm. Take-off-angles relative to sample surface of 458 was employed. Charge compensation was accomplished with an electron flood gun operating at electron energy of 4–6 eV. 3. Results and discussion 3.1. Micro-topography and rigidity distribution analysis of composites Fig. 1 shows the mechanics of force modulation mode AFM. In force modulation mode AFM, the degree of indentation of the tip into sample surface depends on the geometry and mechanical properties of both the tip and the sample surface, and the experimental conditions, such as driving amplitude and set point of the feedback control. On very soft areas of the sample, the tip indents comparatively deep into the surface, resulting in a very small reduction in its oscillating amplitude; whereas at very hard areas, the tip indents little into the sample, and the reduction in its oscillating amplitude is more significant.
The software that appends in AFM was used to make the statistic analysis for the distributing of relative stiffness. Thus, the stiffness image and the line distributing of relative stiffness for the cross-section surface of the composites is obtained, as shown in Fig. 2. The relative stiffness is indirectly indicated by the detection of magnitude (MAG, nA) of the flexural quantum of the microcantilever. It can be seen from the topography in Fig. 2(a) that fiber, matrix and interphase can be observed clearly and the part of carbon fiber is higher than that of PAA resin, which is more distinct in relative stiffness figure (as shown in Fig. 2(b)). Observations of AFM in the force modulation mode are describing of relative stiffness of each phase in composites. The difference of the stiffness between carbon fiber and PAA in relative stiffness figure is more remarkable than the difference of them in the topographies, so the figure of relative stiffness was chosen to observe the distribution of the phases in CF/PAA composites, including carbon fiber, PAA resin and interphase. However, the distribution shows that there are no transition layers between fiber and matrix and the change of stiffness from matrix to carbon fiber is acute. The stiffness topography and the line stiffness distribution of the cross-section surface of CF/PAA composites treated with ozone oxidation are shown in Fig. 3. The interphase in Fig. 3(a) of relative stiffness is clearer than that shown in Fig. 3(a). It seems that there are now complicated phase around the fiber. Appropriate scope near carbon fibers surface (the line in Fig. 3(a)) was chosen to be analyzed statistically and the result of the line distributing of relative stiffness is shown in Fig. 3(b). There is a transition layer between carbon fibers and PAA resin and the change of stiffness is not acute. The thickness of the transitional layer between carbon fiber and PAA resin is more or less 100 nm. The stiffness image and the stiffness distribution of the crosssection surface of CF/PAA composites treated with oxidation and high carbon phenolic resin coating are shown in Fig. 4. It seems that there are obvious transition layer between carbon fibers and PAA resin (Fig. 4(a)) and an obvious stiffness peak between carbon fiber and PAA resin (Fig. 4(b)). Following the arrow (Fig. 4(b)), the stiffness line distribution from resin to fiber is achieved, which indicate the moderate boundary between the carbon fiber and resin is obviously appeared. The thickness of the moderate boundary is more than 200 nm. All of above analysis reveals that the high carbon phenolic resin coating has formed a transition layer between carbon fiber and PAA resin. The stiffness of the interphase is in between the carbon fiber and PAA resin, and the thickness of the layer is higher than composite treated with oxidation. The increase of thickness and appropriate stiffness result in the formation of more suitable interphase. The existence of suitable interphase in composite can effectively transmit load and absorb energy when the composite is loaded.
Fig. 2. Force modulation mode microscopy images of untreated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
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Fig. 3. Topographic and relative stiffness images of oxidation treated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
Fig. 4. Topographic and relative stiffness images of oxidation and coating treated CF/PAA composites surface. (a) Stiffness image; (b) line distributing of relative stiffness.
Results of ILSS of the CF/PAA composites are shown in Fig. 5. It indicates that the treatments have great effect on ILSS of the CF/ PAA composites. The ILSS of CF controlled/PAA composites is only 36 MPa, while the ILSS of treated sample is 45 MPa and 56 MPa, respectively. The values of ILSS (as shown in Fig. 5) of the CF/PAA composites show that the oxidation treatments as well as coating treatment after oxidation have a marked effect on the interfacial properties of the composites. Especially, compare with untreated one, the ILSS of CF with coating after oxidation/PAA composite has an increment of 55%. The mechanical properties of composites untreated and treated also show the same tendency, as shown in
Fig. 6. The mechanical properties of composites are improved when fiber is treated. Furthermore, the composite treated with coating after oxidation gives the best mechanical performances. The increment of ILSS and mechanical properties of composites are attributed to the formation of transition area between the carbon fiber and PAA resin after treatment. From the analysis of Section 3.1, it can be found that the interphase of untreated CF/PAA composite is not almost existed, and there is no transition area between the carbon fiber and PAA resin. The smooth surface of carbon fiber and non-polar structure of the PAA resin result in the happen of above situation. These make the interphase between the fiber and resin easy to destroy when the composite is loaded, so the ILSS of composite is low. After oxidation treatment, the
Fig. 5. Effect of treatment on ILSS of samples.
Fig. 6. Effect of treatment on mechanical properties of samples.
3.2. Mechanical properties of composites
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L. Liu et al. / Applied Surface Science 254 (2008) 5342–5347 Table 2 Relative amounts of the elements on carbon fibers surface Treated methods
Element content (%)
O/C
C
O
N
Untreated Oxidation treated
87.92 86.67
11.06 12.21
0.96 0.98
0.126 0.141
Table 3 Main roughness parameters of the carbon fiber with different treatments measured by AFM on images of 6 mm 6 mm
Fig. 7. XPS total spectra of CF before and after oxidation treatment. (a) Untreated; (b) oxidation treated.
transition area of composite begins to form. The formation of transition area is attributed to the oxidized and etching effect of oxidation treatment which improves the interfacial adhesion of composite [19,20]. Fig. 7 shows the XPS total spectra of CF before and after oxidation treatment, and Table 2 shows the relative amounts of the elements of CF surface which from Fig. 7. From the value of O/C in Table 2, it can clearly see the existence of oxidized effect when the fiber is treated with oxidation. The oxidized effect increases the polar functional groups on the surface of carbon fiber, which can improve the wettability between fiber and resin.
Treated methods
Ra (nm)
Standard deviation (nm)
Untreated Oxidation treated
45.49 50.37
2.14 3.62
Surface topographies of the carbon fiber before and after oxidation treatment were characterized by AFM. Resultant AFM images of 6 mm 6 mm are shown in Fig. 8. Like other PAN-based carbon fiber [21–23], the entire fiber surface has clear ridges and striations running along the axis of the fiber. Furthermore, it can be seen from Fig. 8 that the longitudinal ridges and the striations of treated carbon fiber are clearer than that of untreated. This presents that original features of the surface topography are changes after oxidation treatment. These assertions obtained from Fig. 8 are corroborated with the roughness parameters in Table 3. Table 3 summarizes the results of the roughness analysis of carbon fiber as obtained from AFM. Comparison the roughness values of the untreated carbon fiber with treated ones, it can be found that the treated fiber surface roughness is higher than that of untreated. These confirm the existence of etching effect after oxidation treatment. The etching effect can make the carbon fiber surface more roughness, which can enhance the mechanical interlocking between fiber and resin. The formation of transition area between the fiber and resin can transmit load when composite is loaded, and the transition area also can prevent the spread of crack. Thus, the interfacial performance of composite treated with oxidation is improved and the adhesion between fiber and resin is also enhanced. Moreover, when the carbon fiber is treated with coating after oxidation, the transition area is also formed and the value of thickness of interphase is more than that of the composite treated with oxidation. The introduction of coating after oxidation strengthens the effectiveness of oxidation treatment, and increases
Fig. 8. Three-dimensional AFM micrographs of carbon fiber surface. (a) Untreated; (b) treated by oxidation.
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the thickness of interphase. In addition, the stiffness of interphase of composite treated with coating after oxidation is in between the fiber and resin. All of these lead to the formation of appropriate interphase between the carbon fiber and the PAA resin, which improves of overall mechanical performance of composite. So the composite treated with coating after oxidation has the highest ILSS value. 4. Conclusions The carbon fiber was treated by oxidation and high carbon phenolic resin coating after oxidation, and it is found that interfacial and mechanical properties of the CF/PAA composite treated by coating after oxidation is best. Furthermore, it can be concluded that the interphase between non-polar PAA resin and carbon fiber treated with oxidation and coating is suitable for CF/ PAA composite. The reason was explained using AFM analysis. With the aid of AFM, it found that the transition area was formed between carbon fiber and PAA resin in composite which was treated with oxidation as well as oxidation and high carbon phenolic resin coating. The existence of transition area led to the improvement of interfacial performance of composites. In addition, the matching extent between the carbon fiber, PAA resin and the interphase in composite treated with oxidation and coating is more than that composite treated with oxidation. Thus, the composite treated with oxidation and coating has the highest ILSS value and best mechanical performances.
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Acknowledgement The authors would like to thank the National Natural Science Foundation of China (No. 50603004) for financial supports.
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