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carbon composites with flexible towpreg carbon fiber
Materials Science and Engineering A 485 (2008) 481–486
Preparation of PAN/phenolic-based carbon/carbon composites with flexible towpreg carbon fiber Wei Li ∗ , Zhenhua Chen, Jin Li, Xianhong Chen, Hao Xuan, Xiaoyi Wang College of Materials Science and Engineering, Hunan University, Changsha 410082, PR China Received 20 March 2007; received in revised form 4 August 2007; accepted 8 August 2007
Abstract Carbon/carbon composites made with flexible towpreg carbon fiber as reinforcement and phenolic resins as matrix precursor were impregnated with pitch during re-carbonization process. The structural characteristics of the composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), three-point bending tests, Archimedes’ method and water adsorption. Results showed that the density of the carbon/carbon composites increases from 1.45 to 1.54 g/cm3 with the cycles of pitch impregnated and re-carbonization. Open porosity measurement indicated that the increase of porosity resulted from the decomposition of phenolic resin matrix, and the open porosity of the composite gradually decreased after the impregnation and re-carbonization process. These composites also exhibited an improvement in flexural strength with increasing number of densification cycles. From SEM morphological observation, it was concluded that few cracks appeared in the surfaces and a few smaller pores with a diameter <1 m could be observed. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Carbon/carbon composites; Flexible towpreg; Pyrolysis
1. Introduction Carbon fiber reinforced carbon matrix (carbon/carbon) composites are primarily developed and designed for high temperature structural applications due to the maintenance of their strength and modulus at high temperature [1,2]. Carbon/carbon composites (C–Cs) are usually formed by pyrolysis of the fiber/precursor composite at high temperature that decomposes the precursor. Degradation of polymer precursor is connected with shrinkage and consequently causes cracking of C–Cs. Scientists usually use very expensive high modulus carbon fibers and HIP, CVD, etc., technologies to make carbon/carbon composites [3–5]. In order to cut down the carbon/carbon composites manufacturing costs and time to make it more affordable to the general industries, the flexible towpreg carbon fiber provides a good solution. A flexible towpreg carbon fiber [6] essentially consisting of a bundle of fibers coated with a discontinuous sheath of a matrix resin thermally fuses to the bundle. The matrix resin thermally fuses in a discontinuous manner only to the fibers on the exterior of the bundle, while fibers in the inte-
∗
Corresponding author. Tel.: +86 731 8821614, fax: +86 731 8821614. E-mail address: [email protected] (W. Li).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.08.017
rior of the bundle remain substantially free of the matrix resin [7,8]. Carbon/carbon composites are formed after carbonization with the flexible towpreg carbon fiber/phenolic resin composites. The formed voids, pores and cracks can be filled by means of liquid infiltration. Impregnation with mesophase pitch followed by curing and re-carbonization appears to be the perspective method for the filling of open volume in C–Cs [9]. Granda et al. [10] studied the densification of undirectural pitch-based C/C composites by multiple melted pitch impregnation under vacuum. Densification of two-dimensional phenolic resin-based C/C composites were investigated by Tzeng et al. [1] using a uniaxial mechanical pressure impregnation. To fill effective open volume by impregnation/re-carbonization process, following characteristics of precursor would appear to be desirable: high carbon yield, ability to infiltrate into voids, pores and cracks and controlled rheology of precursor during impregnation and pyrolysis [11]. In this paper, a kind of carbon/carbon composite was prepared by using the flexible towpreg carbon fiber/phenolic resin. In the processing of C–Cs using polymer resin as the matrix precursor, it was inevitable that the porous C–Cs were formed after carbonization. But after the precursor was carbonized, the C–Cs were impregnated with mesophase pitch under a mechanical pressure followed by re-carbonization. Therefore, the aim
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Table 1 Properties of carbon fibers, FB resins and mesophase pitch used in the carbon/carbon composites Materials
Tensile strength (MPa)
Tensile modulus (GPa)
Density (g/cm3 )
Composition (wt%)
Softening point (◦ C)
Diameter (m)
Carbon fiber FB resins Pitch
≥2800 ≥71 –
≥200 ≥3.6 –
1.76 1.28 1.74
C ≥ 92 C ≥ 69 C ≥ 86
– 168 250
5–7 3–20 –
of the present investigation was to study the structural characteristics of this carbon/carbon composites carbonized from carbon fiber/phenolic resin composites during the process of pyrolysis. 2. Experimental procedures 2.1. Materials The reinforcements for carbon/carbon composites in our experiments were untreated and unsized 12 K poly-acrylonitrile (PAN)-based high strength fibers, manufactured by Shanghai Carbon Factory, Jilin Carbon Group, China. For the matrix materials, phenolic resins adopted in this study was FB resins (Bengbu High-Temperature Resistant Resin Factory, China), which was yellow powder with the gel time 70–100 s at 200 ◦ C. For the densification of carbon/carbon composites, mesophase pitch was used as the impregnant. The properties of the asreceived materials are listed in Table 1. 2.2. Preparation of samples The carbon fiber reinforced phenolic resin composites were fabricated with flexible towpreg carbon fibers by hot pressing technique. The weight ratio of carbon fiber and phenolic resin was 1:1. The flexible towpreg carbon fibers was placed in an mold, then the mold was placed in a hot press and consolidated at a pressure of 30 MPa and the temperature about 200 ◦ C, forming a composite 70.0 mm × 12.5 mm × 2.2 mm. The carbon fiber reinforced phenolic resin composites were then pyrolyzed to convert into C–Cs. Carbonization of the carbon fiber reinforced phenolic resin composites was performed at a heating rate of 2 ◦ C/min up to the desired temperatures 1000 ◦ C in a nitrogen atmosphere. The hold time at 1000 ◦ C was 1 h. Then C–Cs were impregnated with mesophase pitch by curing and recarbonization at a heating rate of 10 ◦ C/min up to 1700 ◦ C in a nitrogen atmosphere for the filling of open volume. The used references for designating the composites with the different process stages are listed in Table 2. Table 2 The used references for designating the composites with the different process stages Process stage
References
Carbonization at 1000 ◦ C Carbonization at 1700 ◦ C First densification/carbonization Second densification/carbonization Three densification/carbonization
C1000 C1700 D1 D2 D3
2.3. Measurement The development of cracks and voids of carbon/carbon composites after carbonization and re-carbonization with mesophase pitch impregnated were analyzed by scanning electron microscope (SEM, JSM-6700F JEOL) operated at 20 kV and 20 mA. X-ray diffraction (XRD) was used to analyze the structural evolution of carbon fiber reinforced carbon matrix composites after pyrolysis at different temperatures. A Siemens D5000 Xray diffractometer was used in the X-ray analysis with Cu K␣ radiation and Ni filter. The operating power was 35 kV and 20 mA. Three-point bending tests with a span to depth ratio of 20 and a crosshead speed of 1.0 mm/min were performed in a Computer Controlled Electronic Universal Testing Machine (WDW-100). Flexural strengths of the fabricated C–Cs were measured according to the GB3356-82 method. The fracture surfaces were observed by SEM after flexural test. The analytical balance with an accuracy of 0.1 mg was used to measure the weight variation of the specimens before and after pyrolysis. The actual densities of the specimens were measured by the Archimedes’ method according to ASTM D792. The open porosity of carbon/carbon composites was measured by means of water adsorption in the water according to ASTM D570. The densities and the open porosity of samples were obtained by Eqs. (1) and (2), respectively [12]: mρ0 ρ= (1) m1 − m 2 m1 − m P= (2) m1 − m 2 where ρ is the real density of sample (g/cm3 ), ρ0 the density of water (g/cm3 ), m the sample weight in the air (g), m1 the sample weight measured after being immersed in the water for 24 h and then wiped (g) and m2 is the sample weight in distilled water (g). For all the tests, five or six specimens were tested for each batch of composites. 3. Results and discussion 3.1. XRD diffraction study Fig. 1 shows the XRD patterns of the phenolic resins after pyrolysis at different temperatures. In Fig. 1 the diffraction patterns of carbon fiber reinforced carbon matrix composites show a broad peak (0 0 2) at 2θ ≈ 25◦ , which is due to adjacent chains of linear phenolic resins. Carbonization of carbon fiber reinforced matrix composites results in a glassy carbon and the diffraction patterns showing two broad maxima, corresponding to the
W. Li et al. / Materials Science and Engineering A 485 (2008) 481–486
Fig. 1. XRD patterns of carbon fiber reinforced carbon matrix composites heat treated at different temperatures.
(0 0 2) reflections of graphite structure and (10) reflections of a turbostratic carbon structure. It is clear that in addition to the shift of the (0 0 2) peak to the right, the peak also became sharper with the heat treatment temperature being increased. And above 1700 ◦ C another peak (0 0 4) at 2θ ≈ 54◦ is observed, appeared and intensified with increasing temperature, it is obvious that the main component of pyrolytic carbon is graphite structure and a well-defined crystallinity.
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Fig. 3. Photo of carbon/carbon composites made with flexible towpreg carbon fiber: (a) and (b) pictures of the specimens; (c) and (d) pictures of bending failed specimens.
Fig. 2 shows the flexural strength of the carbon/carbon composites derived from carbon fiber reinforced carbon matrix composites in different process stages. Compared with the cured composite, the strengths of both composites largely decreased after carbonization. Many shrinkage cracks and pores are created during the carbonization. Since the carbonized carbon/carbon composites showed brittle fracture, it is possible that the strength decreases after carbonization of the ductile polymer matrix composites. The pictures of untested and failed specimens are shown in Fig. 3. After two or three densification/carbonization cycles,
as expected, the strengths of both composites largely increased. However, quite unexpectedly, the strength of the composite did not increase with the increasing carbonization cycles after three cycles. The reason for this is possibly because of the very low densification efficiency due to voids closure as discussed previously. The average flexural strength of flexible towpreg carbon fiber/phenolic resins composites was higher than the strength of impregnant carbon fiber/phenolic resins composites. As can be seen from Fig. 7(d), the cracks of flexible towpreg carbon fiber/phenolic resins composites caused by shrinkage stresses during heat treatment developed primarily in the matrix and not at the fiber/matrix interface, with the cracks following paralleled to the basal planes of the matrix. This is a result of flexible towpreg carbon fiber/matrix delamination, which absorbs crack energy and finally results in a further increase of the flexural strength. This effect became even more dramatic when the carbon/carbon composites had not been disposed by mesophase pitch impregnant and re-carbonization. The flexural strength of carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins carbonized composites was
Fig. 2. Flexural strengths of composites under different process stage with different treatments of carbon fiber.
Fig. 4. Bulk density of phenolic resin-based carbon/carbon composites after pyrolysis at different temperatures without pitch impregnation.
3.2. Flexural properties
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Fig. 5. Change in bulk density of carbon/carbon composites with the pith impregnation after different densification cycles.
101 MPa. Those values for carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins composites and impregnant carbon fiber/phenolic resins composites were 101–134 MPa and 85–114 MPa. The increase in strength was 33 and 34%, respectively.
Fig. 6. Open porosity values of composite under different process stages with different treatments of carbon fiber.
3.3. Density and open porosity The variation in density of carbon/carbon composites at various heat treatment temperatures are shown in Fig. 4. There are two factors affecting carbonization of composites. One is chem-
Fig. 7. SEM photomicrographs of the surface of carbon/carbon composites: (a) and (b) the flexible towpreg carbon fiber/phenolic resins composites carbonized directly; (c) and (d) carbon/carbon composites impregnated with pitch and re-carbonization.
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Fig. 8. SEM photomicrographs of fracture surfaces of carbon/carbon composites: (a) and (b) the flexible towpreg carbon fiber/phenolic resins composites carbonized directly; (c–e), carbon/carbon composites impregnated with pitch and re-carbonization.
ical densification resulting from the evolution of gases as well as change of chemical structure during the process of pyrolysis, which leads to denser structure. The other is the formation of pores, associated to evolution of gases and the rearrangement of structures. In the process of carbonization, the polymers degrade and generate gaseous products, and there is a significant decrease in density. For the composites system, the density decreases
from 1.65 to 1.45 g/cm3 . However, as the densification cycles increases from 0 to 4, the densities of the carbon/carbon composites also increase from 1.45 to 1.54 g/cm3 (Fig. 5). According to the changes of the density, it can be concluded that the enhancement of densities of the carbon/carbon composites is the cause that mesophase pitch infiltrated into formed voids, pores and cracks during impregnation and re-carbonization pro-
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cess. Matzinos et al. [13] pointed out that when 1D and 2D carbon/carbon composites are densified at low pressure, the densification depends only the impregnation efficiency, pitch expulsion during pyrolysis and the carbon yield of the pitch. As indicated in Fig. 6, during the carbonization of carbon fiber reinforced carbon matrix composites, the open porosity of the composite largely increased, while in the impregnation and re-carbonization process the open porosity of the composite gradually decreased. Two possible reasons could be attributed to the change of the open porosity: first, the matrix resins were degraded and generated gaseous products, which increased the open porosity. Second, mesophase pitch by curing and recarbonization was filled into the open volume. 3.4. Morphological properties SEM investigations were adopted to examine the morphology of the carbon/carbon composites. Fig. 7 shows the surface morphology of carbon/carbon composites derived from the flexible towpreg carbon fiber/phenolic resins composites with carbonization up to 1000 ◦ C and pitch impregnation and re-carbonization at designed temperature. Fig. 7(a) and (c) shows the large cracks and the development of angle cracks run at an angle with the longitudinal fiber bundle. These cracks as indicated in Fig. 7(a) result from the tensile stress caused by the carbonization shrinkage of phenolic resin matrix. But after the pitch impregnant entered into the composites these cracks as shown in decrease clearly in the SEM. Fig. 7(b) and (d) shows the size of the pores in the carbon/carbon composites. The big pores as indicated in Fig. 7(c) which are a result of decomposition of the phenolic resin matrix after carbonization can be seen more clearly in the SEM photograph. It demonstrates the efficiency of pitch impregnation in filling up of cracks throughout the body of the material. The width of the cracks and the size of the pores decreased with the pitch impregnant entering into the composites during the densification/re-carbonization as shown in Fig. 7(d). Relatively larger coplanar and bundle cracks, small inter-fiber pores, formation of which is a result of decomposition of the phenolic resin matrix after carbonization, were also observed and presented in the SEM picture. The porous carbon matrix in Fig. 8 also indicated that few fracture occurred at the matrix and the fiber–matrix interface due to strong fiber/matrix interfacial bonding. Many smaller inter-fiber pores with a diameter <1 m can also be observed in Fig. 8 as black dots with a higher magnification. Fig. 8(a) and (b) shows many smaller and a few bigger pores, these pores due to pyrolysis of the resin are created. After an efficient impregnation, the closed pores are uniformly distributed in the matrix. Interfacial and fiber/matrix debonding at the fiber/matrix interface, which are an indication of a strong fiber/matrix bonding, and could allow an efficient load transfer from the matrix to the fibers. Fig. 8(c)–(e) shows the size of the pores and the porosity of the carbon/carbon composites reduced after the pitch impregnation. Finally, healing of shrinkage bigger cracks and pores are demonstrated in the pitch impregnation and re-carbonization process. Gupta and Harrison [14] found macroporosity of tens to several hundreds of microns
developed primarily in the curing stage of the resin, at temperatures lower than 200 ◦ C. As HTT was raised to 450 ◦ C, there was an increased pore size and small pores coalesced to form larger pores. They also found a higher heating rate yielded a more random distribution of small pores in the bulk. In comparison with the pores in the radius range of 2.44–122.19 m occupied 81% of the pore volume after pyrolysis at 900 ◦ C reported by Zhang et al. [12]. And that of carbonization rate effected the pores level described by Kuo et al. [15]. When the resin was carbonized at 1 ◦ C/min, there were only a few large cracks and small (2–5 m) pores. As the carbonization rate increased to 100 ◦ C/min, pores of 10–100 m could be found in the cross-sectional micrograph. In this study, the results of SEM, density and open porosity indicate that the carbon/carbon composites fabricated from the flexible towpreg carbon fiber/phenolic resins composites have fewer cracks, smaller pores and higher density than other process of prepared C–Cs. 4. Conclusions (1) The carbon fiber reinforced carbon matrix composites were prepared by flexible towpreg carbon fiber/phenolic resins compacted by hot pressing. After re-carbonization process, the density of the carbon/carbon composites was increased from 1.45 to 1.54 g/cm3 and the open porosity was decreased from 16.5 to 9.9%. (2) The flexural strength values for carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins composites were higher than those of impregnant carbon fiber/phenolic resins composites. Furthermore, the values were increased with the cycles of the impregnation and re-carbonization process. (3) The microstucture of the carbon/carbon composites carbonized from flexible towpreg carbon fiber/phenolic resins was studied. Few cracks appeared in the surfaces and a few smaller pores with a diameter <1 m could be observed. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
S.-S. Tzeng, J.-H. Pan, Mater. Sci. Eng. A 316 (2001) 127–134. S.-S. Tzeng, Y.-G. Chr, Mater. Chem. Phys. 73 (2002) 162–169. T.H. Ko, Polym. Composite. 14 (3) (1993) 247–256. L.M. Mannocha, H. Bhatt, S. Mannocha, Carbon 34 (1996) 841–849. T.-H. Ko, W.-S. Kuo, S.-S. Tzeng, Y.-H. Chang, Compos. Sci. Technol. 63 (2003) 1965–1969. A.H. Miller, N. Dodds, J.M. Hale, A.G. Gibson, Compos. A 29 (1998) 773–782. J.W. Klett, D.D. Edie, Carbon 33 (10) (1995) 1485–1503. C. Narasinha, K.R. Parasnis, Compos. A 27 (1996) 567–574. M. Kluc´akov´a, Compos. Sci. Technol. 64 (2004) 1041–1047. M. Granda, J.W. Patrick, A. Walker, E. Casal, J. Bermejo, R. Menendez, Carbon 36 (1998) 943. M. Kluc´akov´a, Ceramics-Silik´aty 40 (2) (1996) 77–80. Y. Zhang, Y. Xu, L. Gao, L. Zhang, Mater. Sci. Eng. A 430 (2006) 9–14. P.D. Matzinos, J.W. Patrick, A. Walker, Carbon 38 (2000) 1123. A. Gupta, I.R. Harrison, Carbon 32 (1994) 953–960. H.H. Kuo, J.H. Chern Lin, C.P. Ju, Carbon 43 (2005) 229–239.
Preparation of PAN/phenolic-based carbon/carbon composites with flexible towpreg carbon fiber Wei Li ∗ , Zhenhua Chen, Jin Li, Xianhong Chen, Hao Xuan, Xiaoyi Wang College of Materials Science and Engineering, Hunan University, Changsha 410082, PR China Received 20 March 2007; received in revised form 4 August 2007; accepted 8 August 2007
Abstract Carbon/carbon composites made with flexible towpreg carbon fiber as reinforcement and phenolic resins as matrix precursor were impregnated with pitch during re-carbonization process. The structural characteristics of the composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), three-point bending tests, Archimedes’ method and water adsorption. Results showed that the density of the carbon/carbon composites increases from 1.45 to 1.54 g/cm3 with the cycles of pitch impregnated and re-carbonization. Open porosity measurement indicated that the increase of porosity resulted from the decomposition of phenolic resin matrix, and the open porosity of the composite gradually decreased after the impregnation and re-carbonization process. These composites also exhibited an improvement in flexural strength with increasing number of densification cycles. From SEM morphological observation, it was concluded that few cracks appeared in the surfaces and a few smaller pores with a diameter <1 m could be observed. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Carbon/carbon composites; Flexible towpreg; Pyrolysis
1. Introduction Carbon fiber reinforced carbon matrix (carbon/carbon) composites are primarily developed and designed for high temperature structural applications due to the maintenance of their strength and modulus at high temperature [1,2]. Carbon/carbon composites (C–Cs) are usually formed by pyrolysis of the fiber/precursor composite at high temperature that decomposes the precursor. Degradation of polymer precursor is connected with shrinkage and consequently causes cracking of C–Cs. Scientists usually use very expensive high modulus carbon fibers and HIP, CVD, etc., technologies to make carbon/carbon composites [3–5]. In order to cut down the carbon/carbon composites manufacturing costs and time to make it more affordable to the general industries, the flexible towpreg carbon fiber provides a good solution. A flexible towpreg carbon fiber [6] essentially consisting of a bundle of fibers coated with a discontinuous sheath of a matrix resin thermally fuses to the bundle. The matrix resin thermally fuses in a discontinuous manner only to the fibers on the exterior of the bundle, while fibers in the inte-
∗
Corresponding author. Tel.: +86 731 8821614, fax: +86 731 8821614. E-mail address: [email protected] (W. Li).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.08.017
rior of the bundle remain substantially free of the matrix resin [7,8]. Carbon/carbon composites are formed after carbonization with the flexible towpreg carbon fiber/phenolic resin composites. The formed voids, pores and cracks can be filled by means of liquid infiltration. Impregnation with mesophase pitch followed by curing and re-carbonization appears to be the perspective method for the filling of open volume in C–Cs [9]. Granda et al. [10] studied the densification of undirectural pitch-based C/C composites by multiple melted pitch impregnation under vacuum. Densification of two-dimensional phenolic resin-based C/C composites were investigated by Tzeng et al. [1] using a uniaxial mechanical pressure impregnation. To fill effective open volume by impregnation/re-carbonization process, following characteristics of precursor would appear to be desirable: high carbon yield, ability to infiltrate into voids, pores and cracks and controlled rheology of precursor during impregnation and pyrolysis [11]. In this paper, a kind of carbon/carbon composite was prepared by using the flexible towpreg carbon fiber/phenolic resin. In the processing of C–Cs using polymer resin as the matrix precursor, it was inevitable that the porous C–Cs were formed after carbonization. But after the precursor was carbonized, the C–Cs were impregnated with mesophase pitch under a mechanical pressure followed by re-carbonization. Therefore, the aim
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Table 1 Properties of carbon fibers, FB resins and mesophase pitch used in the carbon/carbon composites Materials
Tensile strength (MPa)
Tensile modulus (GPa)
Density (g/cm3 )
Composition (wt%)
Softening point (◦ C)
Diameter (m)
Carbon fiber FB resins Pitch
≥2800 ≥71 –
≥200 ≥3.6 –
1.76 1.28 1.74
C ≥ 92 C ≥ 69 C ≥ 86
– 168 250
5–7 3–20 –
of the present investigation was to study the structural characteristics of this carbon/carbon composites carbonized from carbon fiber/phenolic resin composites during the process of pyrolysis. 2. Experimental procedures 2.1. Materials The reinforcements for carbon/carbon composites in our experiments were untreated and unsized 12 K poly-acrylonitrile (PAN)-based high strength fibers, manufactured by Shanghai Carbon Factory, Jilin Carbon Group, China. For the matrix materials, phenolic resins adopted in this study was FB resins (Bengbu High-Temperature Resistant Resin Factory, China), which was yellow powder with the gel time 70–100 s at 200 ◦ C. For the densification of carbon/carbon composites, mesophase pitch was used as the impregnant. The properties of the asreceived materials are listed in Table 1. 2.2. Preparation of samples The carbon fiber reinforced phenolic resin composites were fabricated with flexible towpreg carbon fibers by hot pressing technique. The weight ratio of carbon fiber and phenolic resin was 1:1. The flexible towpreg carbon fibers was placed in an mold, then the mold was placed in a hot press and consolidated at a pressure of 30 MPa and the temperature about 200 ◦ C, forming a composite 70.0 mm × 12.5 mm × 2.2 mm. The carbon fiber reinforced phenolic resin composites were then pyrolyzed to convert into C–Cs. Carbonization of the carbon fiber reinforced phenolic resin composites was performed at a heating rate of 2 ◦ C/min up to the desired temperatures 1000 ◦ C in a nitrogen atmosphere. The hold time at 1000 ◦ C was 1 h. Then C–Cs were impregnated with mesophase pitch by curing and recarbonization at a heating rate of 10 ◦ C/min up to 1700 ◦ C in a nitrogen atmosphere for the filling of open volume. The used references for designating the composites with the different process stages are listed in Table 2. Table 2 The used references for designating the composites with the different process stages Process stage
References
Carbonization at 1000 ◦ C Carbonization at 1700 ◦ C First densification/carbonization Second densification/carbonization Three densification/carbonization
C1000 C1700 D1 D2 D3
2.3. Measurement The development of cracks and voids of carbon/carbon composites after carbonization and re-carbonization with mesophase pitch impregnated were analyzed by scanning electron microscope (SEM, JSM-6700F JEOL) operated at 20 kV and 20 mA. X-ray diffraction (XRD) was used to analyze the structural evolution of carbon fiber reinforced carbon matrix composites after pyrolysis at different temperatures. A Siemens D5000 Xray diffractometer was used in the X-ray analysis with Cu K␣ radiation and Ni filter. The operating power was 35 kV and 20 mA. Three-point bending tests with a span to depth ratio of 20 and a crosshead speed of 1.0 mm/min were performed in a Computer Controlled Electronic Universal Testing Machine (WDW-100). Flexural strengths of the fabricated C–Cs were measured according to the GB3356-82 method. The fracture surfaces were observed by SEM after flexural test. The analytical balance with an accuracy of 0.1 mg was used to measure the weight variation of the specimens before and after pyrolysis. The actual densities of the specimens were measured by the Archimedes’ method according to ASTM D792. The open porosity of carbon/carbon composites was measured by means of water adsorption in the water according to ASTM D570. The densities and the open porosity of samples were obtained by Eqs. (1) and (2), respectively [12]: mρ0 ρ= (1) m1 − m 2 m1 − m P= (2) m1 − m 2 where ρ is the real density of sample (g/cm3 ), ρ0 the density of water (g/cm3 ), m the sample weight in the air (g), m1 the sample weight measured after being immersed in the water for 24 h and then wiped (g) and m2 is the sample weight in distilled water (g). For all the tests, five or six specimens were tested for each batch of composites. 3. Results and discussion 3.1. XRD diffraction study Fig. 1 shows the XRD patterns of the phenolic resins after pyrolysis at different temperatures. In Fig. 1 the diffraction patterns of carbon fiber reinforced carbon matrix composites show a broad peak (0 0 2) at 2θ ≈ 25◦ , which is due to adjacent chains of linear phenolic resins. Carbonization of carbon fiber reinforced matrix composites results in a glassy carbon and the diffraction patterns showing two broad maxima, corresponding to the
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Fig. 1. XRD patterns of carbon fiber reinforced carbon matrix composites heat treated at different temperatures.
(0 0 2) reflections of graphite structure and (10) reflections of a turbostratic carbon structure. It is clear that in addition to the shift of the (0 0 2) peak to the right, the peak also became sharper with the heat treatment temperature being increased. And above 1700 ◦ C another peak (0 0 4) at 2θ ≈ 54◦ is observed, appeared and intensified with increasing temperature, it is obvious that the main component of pyrolytic carbon is graphite structure and a well-defined crystallinity.
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Fig. 3. Photo of carbon/carbon composites made with flexible towpreg carbon fiber: (a) and (b) pictures of the specimens; (c) and (d) pictures of bending failed specimens.
Fig. 2 shows the flexural strength of the carbon/carbon composites derived from carbon fiber reinforced carbon matrix composites in different process stages. Compared with the cured composite, the strengths of both composites largely decreased after carbonization. Many shrinkage cracks and pores are created during the carbonization. Since the carbonized carbon/carbon composites showed brittle fracture, it is possible that the strength decreases after carbonization of the ductile polymer matrix composites. The pictures of untested and failed specimens are shown in Fig. 3. After two or three densification/carbonization cycles,
as expected, the strengths of both composites largely increased. However, quite unexpectedly, the strength of the composite did not increase with the increasing carbonization cycles after three cycles. The reason for this is possibly because of the very low densification efficiency due to voids closure as discussed previously. The average flexural strength of flexible towpreg carbon fiber/phenolic resins composites was higher than the strength of impregnant carbon fiber/phenolic resins composites. As can be seen from Fig. 7(d), the cracks of flexible towpreg carbon fiber/phenolic resins composites caused by shrinkage stresses during heat treatment developed primarily in the matrix and not at the fiber/matrix interface, with the cracks following paralleled to the basal planes of the matrix. This is a result of flexible towpreg carbon fiber/matrix delamination, which absorbs crack energy and finally results in a further increase of the flexural strength. This effect became even more dramatic when the carbon/carbon composites had not been disposed by mesophase pitch impregnant and re-carbonization. The flexural strength of carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins carbonized composites was
Fig. 2. Flexural strengths of composites under different process stage with different treatments of carbon fiber.
Fig. 4. Bulk density of phenolic resin-based carbon/carbon composites after pyrolysis at different temperatures without pitch impregnation.
3.2. Flexural properties
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Fig. 5. Change in bulk density of carbon/carbon composites with the pith impregnation after different densification cycles.
101 MPa. Those values for carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins composites and impregnant carbon fiber/phenolic resins composites were 101–134 MPa and 85–114 MPa. The increase in strength was 33 and 34%, respectively.
Fig. 6. Open porosity values of composite under different process stages with different treatments of carbon fiber.
3.3. Density and open porosity The variation in density of carbon/carbon composites at various heat treatment temperatures are shown in Fig. 4. There are two factors affecting carbonization of composites. One is chem-
Fig. 7. SEM photomicrographs of the surface of carbon/carbon composites: (a) and (b) the flexible towpreg carbon fiber/phenolic resins composites carbonized directly; (c) and (d) carbon/carbon composites impregnated with pitch and re-carbonization.
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Fig. 8. SEM photomicrographs of fracture surfaces of carbon/carbon composites: (a) and (b) the flexible towpreg carbon fiber/phenolic resins composites carbonized directly; (c–e), carbon/carbon composites impregnated with pitch and re-carbonization.
ical densification resulting from the evolution of gases as well as change of chemical structure during the process of pyrolysis, which leads to denser structure. The other is the formation of pores, associated to evolution of gases and the rearrangement of structures. In the process of carbonization, the polymers degrade and generate gaseous products, and there is a significant decrease in density. For the composites system, the density decreases
from 1.65 to 1.45 g/cm3 . However, as the densification cycles increases from 0 to 4, the densities of the carbon/carbon composites also increase from 1.45 to 1.54 g/cm3 (Fig. 5). According to the changes of the density, it can be concluded that the enhancement of densities of the carbon/carbon composites is the cause that mesophase pitch infiltrated into formed voids, pores and cracks during impregnation and re-carbonization pro-
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cess. Matzinos et al. [13] pointed out that when 1D and 2D carbon/carbon composites are densified at low pressure, the densification depends only the impregnation efficiency, pitch expulsion during pyrolysis and the carbon yield of the pitch. As indicated in Fig. 6, during the carbonization of carbon fiber reinforced carbon matrix composites, the open porosity of the composite largely increased, while in the impregnation and re-carbonization process the open porosity of the composite gradually decreased. Two possible reasons could be attributed to the change of the open porosity: first, the matrix resins were degraded and generated gaseous products, which increased the open porosity. Second, mesophase pitch by curing and recarbonization was filled into the open volume. 3.4. Morphological properties SEM investigations were adopted to examine the morphology of the carbon/carbon composites. Fig. 7 shows the surface morphology of carbon/carbon composites derived from the flexible towpreg carbon fiber/phenolic resins composites with carbonization up to 1000 ◦ C and pitch impregnation and re-carbonization at designed temperature. Fig. 7(a) and (c) shows the large cracks and the development of angle cracks run at an angle with the longitudinal fiber bundle. These cracks as indicated in Fig. 7(a) result from the tensile stress caused by the carbonization shrinkage of phenolic resin matrix. But after the pitch impregnant entered into the composites these cracks as shown in decrease clearly in the SEM. Fig. 7(b) and (d) shows the size of the pores in the carbon/carbon composites. The big pores as indicated in Fig. 7(c) which are a result of decomposition of the phenolic resin matrix after carbonization can be seen more clearly in the SEM photograph. It demonstrates the efficiency of pitch impregnation in filling up of cracks throughout the body of the material. The width of the cracks and the size of the pores decreased with the pitch impregnant entering into the composites during the densification/re-carbonization as shown in Fig. 7(d). Relatively larger coplanar and bundle cracks, small inter-fiber pores, formation of which is a result of decomposition of the phenolic resin matrix after carbonization, were also observed and presented in the SEM picture. The porous carbon matrix in Fig. 8 also indicated that few fracture occurred at the matrix and the fiber–matrix interface due to strong fiber/matrix interfacial bonding. Many smaller inter-fiber pores with a diameter <1 m can also be observed in Fig. 8 as black dots with a higher magnification. Fig. 8(a) and (b) shows many smaller and a few bigger pores, these pores due to pyrolysis of the resin are created. After an efficient impregnation, the closed pores are uniformly distributed in the matrix. Interfacial and fiber/matrix debonding at the fiber/matrix interface, which are an indication of a strong fiber/matrix bonding, and could allow an efficient load transfer from the matrix to the fibers. Fig. 8(c)–(e) shows the size of the pores and the porosity of the carbon/carbon composites reduced after the pitch impregnation. Finally, healing of shrinkage bigger cracks and pores are demonstrated in the pitch impregnation and re-carbonization process. Gupta and Harrison [14] found macroporosity of tens to several hundreds of microns
developed primarily in the curing stage of the resin, at temperatures lower than 200 ◦ C. As HTT was raised to 450 ◦ C, there was an increased pore size and small pores coalesced to form larger pores. They also found a higher heating rate yielded a more random distribution of small pores in the bulk. In comparison with the pores in the radius range of 2.44–122.19 m occupied 81% of the pore volume after pyrolysis at 900 ◦ C reported by Zhang et al. [12]. And that of carbonization rate effected the pores level described by Kuo et al. [15]. When the resin was carbonized at 1 ◦ C/min, there were only a few large cracks and small (2–5 m) pores. As the carbonization rate increased to 100 ◦ C/min, pores of 10–100 m could be found in the cross-sectional micrograph. In this study, the results of SEM, density and open porosity indicate that the carbon/carbon composites fabricated from the flexible towpreg carbon fiber/phenolic resins composites have fewer cracks, smaller pores and higher density than other process of prepared C–Cs. 4. Conclusions (1) The carbon fiber reinforced carbon matrix composites were prepared by flexible towpreg carbon fiber/phenolic resins compacted by hot pressing. After re-carbonization process, the density of the carbon/carbon composites was increased from 1.45 to 1.54 g/cm3 and the open porosity was decreased from 16.5 to 9.9%. (2) The flexural strength values for carbon/carbon composites developed from flexible towpreg carbon fiber/phenolic resins composites were higher than those of impregnant carbon fiber/phenolic resins composites. Furthermore, the values were increased with the cycles of the impregnation and re-carbonization process. (3) The microstucture of the carbon/carbon composites carbonized from flexible towpreg carbon fiber/phenolic resins was studied. Few cracks appeared in the surfaces and a few smaller pores with a diameter <1 m could be observed. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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