- Email: [email protected]
epoxy resin composites with an interpenetrating network structure derived from natural sponge
CARBON
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation and wear behavior of carbon/epoxy resin composites with an interpenetrating network structure derived from natural sponge Wang Tianchi *, Xiong Dangsheng, Zhou Tianle Department of Material Science and English, Nanjing University of Science and Technology, Nanjing 210094, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Natural sponge was used as the template to produce carbon/epoxy resin composites with
Received 12 November 2009
interpenetrating network structure. Carbon with a network structure was first obtained by
Accepted 9 March 2010
the pyrolysis of natural sponge. The composites were then obtained by injecting epoxy
Available online 12 March 2010
resin into the carbon. Their microstructures and wear properties were analyzed. The results show that the natural structure of sponge controlled the interpenetrating network structures of the carbon/epoxy resin composites. The carbon-network in the composites was shown to stabilize the coefficient of friction of epoxy resin. The carbon also reduces the wear rate of the epoxy resin remarkably. 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Life-forms, after hundreds of millions of years of evolution, possess rational and graceful microstructures that cannot be obtained artificially. In recent years, researchers have used plants as a template to prepare new materials with plant structure. The method has been used by several workers who used plants such as wood, bamboo, and medium-density board to prepare ceramics with plant structure such as SiC, Si/ SiC/C, SiOC/C, Al2O3, TiO2, ZrO2, SnO2 [1–8]. Results show that the natural structure of plants can play an important role in obtaining excellent properties for some new ceramics. These ceramics may be extensively used for molecular screen, filters, catalyst support, thermal insulation materials, gas-sensing materials, electromagnetic shielding materials, far-infrared radiation materials, etc. The method of using life-form to prepare new materials has broken through the traditional concept of material design, reflecting the human desire to learn from nature, and also provides a new research idea for material development. Recently, new developments of polymer matrix composite fabrication have drawn attention in that both phases are
continuous and tri-dimensionally interpenetrating through the microstructure [9–11]. This kind of composite, which mimics many natural structural materials such as bone, bamboo and so on, presents multifunctional characteristics. Each phase contributes its own properties. The most general method of fabricating the composites with an interpenetrating network is to impregnate a desired second phase into a preexisting open-cell porous ceramic or porous metal. This then can produce the requisite connectivity and spatial distribution of the two or more component phases. Usually, the structures of porous preform depend on the artificial fabrication processes, such as foaming. Through these processes, we can determine the diameter of pores, distribution of pores, porosity of porous preform, etc. However, the process of fabricating materials with desired and delicate microstructures remains to be a challenge. Natural sponge is an old cellulous halobios, which is adapted to warm seawater. Sponges are an abundant resource and at present, 5000 species have been identified microscopic investigation reveals a skeleton of interlaced sponge that exhibits a three-dimensional net structure. It possesses a high capacity of absorbing water or grease, and it is often
* Corresponding author: Fax: +86 2584315776. E-mail address: [email protected] (W. Tianchi). 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.03.011
2436
CARBON
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
harvested from the sea for the manufacture of bathing and cleaning products. Its structure can also give us illumination for manufacturing polymer matrix composites with interpenetrating network. In this work, we developed a method to manufacture the interpenetrating polymer matrix composite with microstructures controlled by the natural structures of sponge. We manufactured the carbon/epoxy resin composites through infiltrating the epoxy resin into the netlike carbon preform derived from natural sponge templates. The microstructures, thermal diffusivity, and wear behavior of the resin composites were analyzed.
Table 1 – The codes of the three netlike carbon and their processing parameters.
Volume ration of phenolic resin and sponge (ml/cm3) Sintering temperature (C)
C1
C2
C3
0.4
0.6
0.4
800
800
1400
2.
Experimental
100 g, supplied by Wuxi Resin Co. Then at a pressure of 1 MPa, polymer matrix was injected into the three carbons. After heating at 160 C for 2 h, the matrix cured and the carbon/ epoxy resin composites were obtained.
2.1.
Sample preparation
2.2.
The natural sponge (family of origin: filaceous sponge, producing area: Mediterranean Sea) was selected as the template. Fig. 1 shows its actual photo and the image captured by the scanning electron microscopy (SEM, JSM-6380, JEOL, Japan). It can be seen that skeleton fibers in the sponge link up, interlace and form a complex net structure. Three shares of thermoset phenolic resin (solid content: 45–55%, solvent: ethanol, Tu-4 cup viscosity at 25 C: 640 s) with different volumes were poured into three pieces of this kind of sponge. The volume ratio of the phenolic resin poured and the sponge for one piece was 0.6 ml resin/1 cm3 sponge. The ratios for the other two pieces of sponge were 0.4 ml/cm3. After the phenolic resin was adequately absorbed in the sponges, the sponges were squeezed gently to spread the resin coat evenly on the sponge fibers. The three resin-coated sponges were dried at 80 C for 6 h and heated at 160 C for 2 h for resin cure. Then, one cured piece coated by phenolic resin in 0.4 ml/cm3 ration was sintered at 1400 C for 2 h in the vacuum condition and the other two pieces were sintered at 800 C for 2 h. During sintering, the phenolic resin was pyrolyzed to carbon in situ, whereby the sponge pieces turned to netlike carbon that maintained the structure of template. The codes of C1, C2, and C3 are used to represent these three carbons, and their processing parameters are presented in Table 1. The liquid of polymer matrix can be compounded through mixing the epoxy resin (WSR618), curing agent, and absolute alcohol in a 4:1:1 weight ratio. Here, the epoxy resin is diglycidyl ether of bisphenol A (DGEBA) with the epoxy value of 0.48–0.54 mol/
Microstructures and thermal diffusivity
The microstructures of the materials were observed using optical microscopy (OM). The phases of netlike materials were identified using a X-ray diffractometer (XRD, Cu-Ka, RigakuX 2038, Japan). The bulk density (geometrical density) was determined by measuring the weight and the volume of the cuboid specimens. The thermal diffusivity of the composites and epoxy resin were measured using a laser conductometer (Nanuflashi LFA447, Netzsch, Germany). The shape of thermal diffusivity specimen was a column with size of u12.6 · 1 mm3.
2.3.
Wear tests
Friction and wear tests were conducted with a MM-200 wear tester by a block-on-ring arrangement in air at room temperature without lubricant (see Fig. 2). The sliding ring with a diameter of uouter40–uinner16 · 10 mm was made of GR15 bearing steel (oil hardened to HRC 62). The wide rectangular faces of the block specimens with dimension of 10 · 10 · 30 mm3 were put in line contact with the sliding ring. Prior to testing, the test surfaces of the specimens and ring were ground to a smooth finish on the 1000 grit SiC emery paper. The wear tests were performed at the following conditions: three load levels of 49, 73.5 and 98 N, and one rotational speed of 200 rpm (0.419 m/s). A standard test runs 50 min, which corresponds to a sliding distance of 1256 m. Three specimens of every material were tested for every wearing condition.
Fig. 1 – Actual and SEM image of the natural sponge used in this study.
Fig. 2 – Diagram of the wear-testing device.
Coefficient of friction was obtained from friction torque, which was recorded by a registering instrument in the wear tester, and then divided by the load and radius of the ring. The wear rates were determined by dividing the wear volume loss of the specimens by the sliding distance. The morphologies of the worn surfaces were analyzed using an OM.
3.
Results
3.1.
Microstructures
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4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
C
Intensity (u.a.) Intensity (u.a.) Intensity (u.a.)
Fig. 3 shows the actual and OM image of the carbon/epoxy resin composite. The black netlike phase inside the composite is carbon derived from sponge, which is interpenetrated by the gray phase of epoxy resin. Both phases are continuous, forming an interpenetrating network structure. During the course of sintering in vacuum, phenolic resin on the sponge fibers was decomposed into carbon in situ [12]. This gives rise to a netlike carbon with the morphology derived from sponge. The three-dimensional net structure of the sponge was not damaged. It was retained in the carbon-network perfectly, as seen in Figs. 1 and 3. After
injecting epoxy resin into the carbon and curing, this natural net structure was brought once more into the composite. Therefore, the distribution of every phase in this polymer matrix composite is controlled by the natural structure of sponge. This is different from the traditional polymer matrix composites, the structures of which are obtained artificially. Fig. 4 presents the XRD patterns of the epoxy resin, carbon/epoxy composite, and netlike materials after the phenolic resin covering, solidifying and by sintering at 800 and 1400 C. Seen from their XRD patterns, the netlike materials are composed of nongraphitizable carbon, i.e. glassy carbon from phonelic resin [12]. It can also be seen that the peak intensity of carbon in the netlike carbon sintered at 1400 C is higher than that sintered at 800 C. This indicates that crystallization extent of carbon increased while the sintering temperature rises. Different state of carbon could influence the properties of the composite further more. Table 2 lists group data of the netlike carbon and carbon/ epoxy resin composites obtained in the present investigation. The densities of the composites are all lower than that of epoxy resin (1.17 g/cm3). Volume content of epoxy matrix in the composites can been calculated according to the following law: qPorC þ qEpoxy VEpoxy ¼ qComposite , where qComposite , qPorC , and qEpoxy represent the density of the composite, the netlike carbon, and the epoxy matrix, respectively. VEpoxy represents the volume content of the epoxy matrix in the composites. It should be noticed that qPorC is the bulk density of the netlike carbon, so it need not multiplied the volume content of the carbon. The volume content of carbon in composite can been calculated through the law: VCarbon ¼ 1 VEpoxy . The results are shown in the last row of the table. It is obvious that the content of carbon in C2/Epoxy is much higher than the other two because that the sponge piece used as its template absorbed more volume of the phenolic resin.
Intensity (u.a.)
CARBON
C3 sintered at 1400°C
1200 800 C 400
C
0
C
800
C1 sintered at 800°C C
400
C
0
Epoxy resin
400 0
C C2/Epoxy
400 C 0 20
40
60
80
2θ ( degr ee)
Fig. 3 – Actual and OM image of the carbon/epoxy resin composite.
Fig. 4 – XRD pattern of the netlike materials, epoxy resin and carbon/epoxy composite.
9
Table 2 – Densities and content of some phases of the materials.
0.108
0.153
0.113
1.006
0.945
0.998
23.25
32.31
24.35
3
C3
-2
Bulk density of netlike carbon (g/cm3) Density of carbon/epoxy resin composite (g/cm3) Volume content of carbon in composite (%)
C2
8
Wear rate (10 mm /m)
C1
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
7 6 5 4 3 2
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
CARBON
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
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1 0
3.2.
Thermal diffusivity
49N
Fig. 5 shows the thermal diffusivities of the epoxy resin and composites, where the thermal diffusivities of the composites are all higher than that of epoxy resin. Usually, the capacity of heat transmission of carbon is better than that of epoxy resin [13,14]; therefore, introducing carbon could improve the thermal diffusivity of epoxy resin remarkably. Moreover, the thermal diffusivity of the composites may rise by increasing the content of carbon, which can be seen from the data of C1/ Epoxy and C2/Epoxy in Fig. 5. Compared with the C1/Epoxy, the C3/Epoxy exhibited a little higher thermal diffusivity. This may be due to the higher crystallization extent of carbon in C3/Epoxy than that in C1/Epoxy. The carbon with higher crystallization extent possesses higher capacity of heat transmission [15].
3.3.
Wear properties
Fig. 6 shows the variation of wear rates as a function of the applied load at room temperature for the composites and epoxy resin. The wear rates for all materials increase with increasing load. Apparently, the wear rate of the epoxy resin is relatively larger than that of the composites. It indicates that the wear resistance of the composites is significantly improved, as compared to that of the matrix. This improvement is attributed to the following reasons: (1) in the composites, the glassy carbon derived from phenolic resin usually possesses higher hardness [16]. It may provide resistance to abrasion during the composites wearing, and decrease the wear rate. (2) The composites have higher thermal diffusivity than epoxy resin, as shown in Fig. 5. It can help the composites
2
Thermal Diffusivity (mm /s)
0.40
C2/Epoxy
0.35 0.30
Epoxy
C1/Epoxy
C3/Epoxy
0.25 0.20 0.15 0.10 0.05 0.00
Fig. 5 – Thermal diffusivities of carbon/epoxy resin composites and epoxy resin.
73.5N Load (N)
98N
Fig. 6 – Wear rates as a function of the applied load for carbon/epoxy resin composites and epoxy resin.
eliminate the frictional heat faster than epoxy resin. Thus, the temperature on the worn surface of the composites during wearing may be lower, and the degree of softening on the worn surface was also lower, as compared to that of epoxy resin. Therefore, the composites exhibit higher wear resistance. It can also be seen that C2/Epoxy and C3/Epoxy show the better wear resistance than C1/Epoxy. This is also due to the increased content of carbon in the former two composites and their higher thermal diffusivity, as seen in Table 2 and Fig. 5. Fig. 7 shows the worn surfaces of specimens examined by OM. The worn surfaces of epoxy resin at 49 and 73.5 N are shown in Fig. 7a and b, respectively. Cracks (indicated by the arrows in figure), which are perpendicular to the siding direction, are evident on the worn surfaces of epoxy resin, and the density of cracks increases remarkably with the increasing of the load. The relative slip between the sliding ring and specimen can raise a stress of tearing on the worn surface. As the specimen of epoxy resin is soft, this stress could distort its worn surface leading to crack formation. The worn surfaces of C1/Epoxy at 49 and 73.5 N are shown in Fig. 7c and d, respectively. Only a few cracks perpendicular to the sliding direction can be found on their worn surface, which is different from the specimen of epoxy resin. The sponge-like network made up of the hard glassy carbon in the composite may play an important role. In the composite, the carbon and the matrix forms the tri-dimensionally interpenetrating structure. When the epoxy matrix was torn by the sliding ring, the hard carbon-network could nearly root the soft matrix at the original position without serious distortion. Therefore, much fewer perpendicular cracks emerged on the worn surface of composite as compared to the epoxy resin. This can also be another reason why composites exhibit better wear resistance than epoxy resin. Fig. 8 shows the average coefficients of friction as a function of applied load for the composites and epoxy resin. The coefficients of friction of both the epoxy resin and the composites increase with increasing load. Compared with epoxy resin, the composites exhibit lower coefficients of friction. The friction reduction is attributed to the following reasons:
CARBON
4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
2439
Fig. 7 – Worn surfaces of carbon/epoxy resin composite and epoxy resin (epoxy resin at 49 N (a) and 73.5 N (b), C1/Epoxy at 49 N (c) and 73.5 N (d)).
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
0.8
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
Coefficient of friction
1.0
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
1.2
49N
73.5N Load (N)
98N
0.6 0.4 0.2 0.0
Fig. 8 – Average coefficient of friction versus applied load.
(1) the carbon in the composite improves the hardness of material so that the adhesivity between the worn surface and the sliding ring was reduced during wear test. However, the simplex epoxy resin is softer than the composites, leading to stronger adhesivity and higher coefficient of friction. (2) The carbon that was peeled from the netlike material during wearing could also play lubrication action to reduce the friction. (3) The composites have higher thermal diffusivity; thus, the temperature or degree of softening of the worn surface was lower, as compared to the simplex epoxy resin. The adhesivity between the friction couple could also decrease.
Fig. 9 – Real-time coefficients of friction as a function of the sliding distance for (a) epoxy resin and (b) C1/Epoxy at 73.5 N.
Fig. 9a and b shows the real-time coefficients of friction of epoxy resin and C1/Epoxy as a function of the sliding distance, which were recorded by the register instrument. The load is 73.5 N. Compared with the curve of epoxy resin, there is less rising and falling in the curve of the composite in the whole wear process. Therefore, the curve of the composite looks straighter than that of epoxy resin. Moreover, the
2440
CARBON
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
coefficient of friction of the composite changes in a more narrow range than that of epoxy resin, which makes its curve thinner than the curve of epoxy resin. The results seen from Fig. 9 indicate that the friction behaviors of the composite are more stable than those of epoxy resin. The reason may be related to the carbon-network, which will be analyzed in Section 4.1.
4.
Discussions
4.1.
Role of the carbon-network
during wear test could lead to the obvious strain of the specimen. The deformation resilience could lead to the intense flutter of the specimen. Thus, the coefficient of friction of the epoxy resin may have a wider range (Fig. 9a). However, for the composite, the hard carbon-network inside could root the epoxy matrix in the original position to avoid obvious strain as the friction force changed. Moreover, the carbonnetwork could restrain the rebound of the epoxy matrix. Therefore, intense flutter was avoided and the coefficient of friction had a more narrow range than epoxy resin (Fig. 9b).
4.2. In the carbon/epoxy resin composites, the carbon-network plays an important role in stabilizing the friction process and coefficient of friction for the composites. For the epoxy specimen, the matters were peeled off from the specimen unevenly during the wear test. Some parts showed serious abrasions, while only slight abrasions were found in other parts of the worn surface. Fig. 10a shows the worn surface of the epoxy specimen at 73.5 N. The severely worn parts are found in the fuscous and white regions, where most surfaces have been peeled off and could not touch the sliding ring. Hence, the contact area between the specimen here and the sliding ring is smaller. The slightly worn parts can be found in the yellow regions where there is a larger contact area. During the wear test, the severe wear region could transform into a mild region, or vice versa. Therefore, the total contact area between the specimen and the sliding ring also changed frequently in the wear process, which leads to the frequent altering of the total friction force. Thus the coefficient of friction of the epoxy resin fluctuated seriously in the whole wear process (Fig. 9a). However, for the composites, the phenomenon is not the same with that of the epoxy resin. The carbon-network divided the epoxy on the worn surface into innumerable units so that the epoxy matrix could be peeled off evenly from the surface. Therefore, its worn surface is always kept smooth as seen in Fig. 10b. There are few changes in the total contact area and friction force during the wear test, thus only a slight rise and fall in its curve can be observed (Fig. 9b). The curve of the composite exhibits more straight than that of the epoxy resin. The thinner friction curve of the carbon/epoxy resin composite is attributed to the carbon-network. Similar to some soft polymers, the epoxy resin has low elastic modulus and high rebound ability. The slight altering of friction force
Abrasion mechanism
Wear and friction between the matrix epoxy resin and sliding ring is believed to initiate by deformation and cutting of the bulges on the epoxy resin by the sliding ring, resulting in an increased contact area. Therefore, its coefficient of friction would increase at the beginning, as shown in Fig. 9a. For the carbon/epoxy composites, the wear and friction behavior would become more complex. Cutting and deformation of the matrix resin bulges by the harder sliding ring could increase the contact area, which may increase the coefficient of friction. However, carbon bleed out onto the sliding surface, forming a carbon film on the exposed resin surfaces, that decreases the likelihood of adhesion, and results in a reduced coefficient of friction at the beginning, as shown in Fig. 9b. The wear behavior of a material is a function of many factors, such as applied load, sliding velocity, counterface, lubrication and atmosphere. Generally, wear studies on engineering materials are concerned with the effects of applied load. Sliding wear takes place due to the relative sliding of the two surfaces in contact with each other under a certain applied load. In the case of the matrix epoxy resin, with increasing applied load bulges on the surface of the sliding ring penetrate deeper into the surface of sample and more heat can be generated on the sliding surfaces due to friction. This raises the sliding surface temperature and softens the sample, which then exhibits a higher degree of adhesion and an increase in coefficient of friction (as shown in Fig. 8). Moreover, with increasing applied load, the effective stress on the bulges on the surface of the sample can by far exceed the strength and consequently they break off, which results in a higher wear rate, as shown in Fig. 6. In the case of carbon/epoxy composites, bulges on the sliding ring penetrate deeper into the surface of sample with increasing applied
Fig. 10 – Worn surfaces of (a) epoxy resin and (b) C1/Epoxy at 73.5 N.
CARBON
4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
load, which results in an increase of the friction-induced hot surface temperature and a gradual flaking off of the carbon films. These lead to an increase in metal-to-resin contact surface area and a mild adhesive wear, resulting in the simultaneous increase of wear rate and coefficient of friction, Figs. 6 and 8.
5.
[2]
[3]
Conclusion
Based on the three-dimensional net structure of natural sponge, the carbon/epoxy resin composites with interpenetrating network were produced by injecting the epoxy resin into the netlike carbon derived from sponge. This carbon-network remained perfectly in the composite, and the natural sponge controlled the interpenetrating structure of these composites. The thermal diffusivities of these composites are higher than that of epoxy resin, and increased with the increasing content of carbon and crystallization extent of carbon. The wear rates of the composites are shown to be lower than that of the epoxy resin. The lower wear rates are attributed to the abrasion of carbon in the composites, higher thermal diffusivity, and carbon-network of the composites. The composites exhibit a lower average coefficient of friction compared with epoxy resin. The reason may be that the composites possessed higher hardness during wearing so that it was more difficult for the composites to adhere to the sliding ring than for epoxy resin. As the carbon-network in the composites divided the epoxy on the worn surface into innumerable units, the friction curve of the composite appeared straighter than that of the epoxy resin. Furthermore, the carbon-network could restrict the strain and rebound of the epoxy matrix, which made the friction curves of the composites thinner than that of the epoxy resin. These results demonstrate a more stable friction state for this type of composite.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Acknowledgement [13]
The authors wish to express thanks to Natural Science Foundation of Jiangsu (Nos. BK2007596 and BK2008407), and Science and Technology Development Program of Nanjing University of Science and Technology (Nos. XKF07017 and XKF09068).
[14]
[15] R E F E R E N C E S
[16] [1] Greil P, Vogli E, Fey T, Bezold A, Popovska N, Gerhard H, et al. Effect of microstructure on the fracture behavior of
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biomorphous silicon carbide ceramics. J Eur Ceram Soc 2002;22(14–15):2697–707. Zampieri A, Kullmann S, Selvam T, Bauer J, Schwieger W, Sieber H, et al. Bioinspired rattan-derived SiSiC/Zeolite monoliths preparation and characterisation. Microporous Mesoporous Mater 2006;90(1–3):162–74. Pancholi V, Mallick D, AppaRao C, Samajdar I, Chakrabarti OP, Maiti HS, et al. Microstructural characterization using orientation imaging microscopy of cellular Si/SiC ceramics synthesized by replication of Indian dicotyledonous plants. J Eur Ceram Soc 2007;27(1):367–76. Kaul VS, Faber KT, Sepulveda R, Lopez AR, Martinez J. Precursor selection and its role in the mechanical properties of porous SiC derived from wood. Mater Sci Eng A 2006;428(1– 2):225–32. Macedo JS, Otubo L, Ferreira OP, Gimenez I, Mazali IO, Barreto LS. Biomorphic activated porous carbons with complex microstructures from lignocellulosic residues. Microporous Mesoporous Mater 2008;107(3):276–85. Singh M, Yee B. Reactive process of environmentally conscious, biomorphic ceramics from natural wood precursors. J Eur Ceram Soc 2004;24(2):209–17. Qiao JM, Wang JP, Qiao GJ, Jin ZH. Preparation of porous SiC ceramic with a woodlike microstructure by sol–gel and carbothermal reduction processing. J Eur Ceram Soc 2004;24(2):3251–9. Munoz A, Fernandez JM, Singh M. High temperature compressive mechanical behavior of joined biomorphic silicon carbide ceramics. J Eur Ceram Soc 2002;22(2):2727–33. Jhaver R, Tippur H. Processing, compression response and finite element modeling of syntactic foam based interpenetrating phase composite (IPC). Mater Sci Eng A 2009;499(1–2):507–17. Tilbrook MT, Moon RJ, Hoffman M. On the mechanical properties of alumina-epoxy composites with an interpenetrating network structure. Mater Sci Eng A 2005;393(1–2):170–8. Wegner LD, Gibson LJ. The mechanical behaviour of interpenetrating phase composites-III: resin-impregnated porous stainless steel. Mech Sci 2001;43(4):1061–72. Xie XQ, Zhang D, Fan TX, Sakata T, Mori H, Okabe T, et al. The fabrication of composites with interpenetrating networks based on woodceramics. Mater Lett 2002;56(1–2):102–7. Ganguli S, Roy AK, Anderson DP. Improved thermal conductivity for chemically functionalized exfoliated graphite/epoxy composites. Carbon 2008;46(5):806–17. Tantawy FE, Kamada K, Ohnabe H. In situ network structure, electrical and thermal properties of conductive epoxy resincarbon black composites for electrical heater applications. Mater Lett 2002;56(1–2):112–26. Michalowski J, Mikociak D, Konsztowicz KJ, Blazewicz S. Thermal conductivity of 2D C–C composites with pyrolytic and glass-like carbon matrices. J Nucl Mater 2009;393(1):47–53. Kawamura K, Jenkins GM. Mechanical properties of glassy carbon fibres derived from phenolic resin. J Mater Sci 1972;7(10):1099–112.
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation and wear behavior of carbon/epoxy resin composites with an interpenetrating network structure derived from natural sponge Wang Tianchi *, Xiong Dangsheng, Zhou Tianle Department of Material Science and English, Nanjing University of Science and Technology, Nanjing 210094, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Natural sponge was used as the template to produce carbon/epoxy resin composites with
Received 12 November 2009
interpenetrating network structure. Carbon with a network structure was first obtained by
Accepted 9 March 2010
the pyrolysis of natural sponge. The composites were then obtained by injecting epoxy
Available online 12 March 2010
resin into the carbon. Their microstructures and wear properties were analyzed. The results show that the natural structure of sponge controlled the interpenetrating network structures of the carbon/epoxy resin composites. The carbon-network in the composites was shown to stabilize the coefficient of friction of epoxy resin. The carbon also reduces the wear rate of the epoxy resin remarkably. 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Life-forms, after hundreds of millions of years of evolution, possess rational and graceful microstructures that cannot be obtained artificially. In recent years, researchers have used plants as a template to prepare new materials with plant structure. The method has been used by several workers who used plants such as wood, bamboo, and medium-density board to prepare ceramics with plant structure such as SiC, Si/ SiC/C, SiOC/C, Al2O3, TiO2, ZrO2, SnO2 [1–8]. Results show that the natural structure of plants can play an important role in obtaining excellent properties for some new ceramics. These ceramics may be extensively used for molecular screen, filters, catalyst support, thermal insulation materials, gas-sensing materials, electromagnetic shielding materials, far-infrared radiation materials, etc. The method of using life-form to prepare new materials has broken through the traditional concept of material design, reflecting the human desire to learn from nature, and also provides a new research idea for material development. Recently, new developments of polymer matrix composite fabrication have drawn attention in that both phases are
continuous and tri-dimensionally interpenetrating through the microstructure [9–11]. This kind of composite, which mimics many natural structural materials such as bone, bamboo and so on, presents multifunctional characteristics. Each phase contributes its own properties. The most general method of fabricating the composites with an interpenetrating network is to impregnate a desired second phase into a preexisting open-cell porous ceramic or porous metal. This then can produce the requisite connectivity and spatial distribution of the two or more component phases. Usually, the structures of porous preform depend on the artificial fabrication processes, such as foaming. Through these processes, we can determine the diameter of pores, distribution of pores, porosity of porous preform, etc. However, the process of fabricating materials with desired and delicate microstructures remains to be a challenge. Natural sponge is an old cellulous halobios, which is adapted to warm seawater. Sponges are an abundant resource and at present, 5000 species have been identified microscopic investigation reveals a skeleton of interlaced sponge that exhibits a three-dimensional net structure. It possesses a high capacity of absorbing water or grease, and it is often
* Corresponding author: Fax: +86 2584315776. E-mail address: [email protected] (W. Tianchi). 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.03.011
2436
CARBON
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
harvested from the sea for the manufacture of bathing and cleaning products. Its structure can also give us illumination for manufacturing polymer matrix composites with interpenetrating network. In this work, we developed a method to manufacture the interpenetrating polymer matrix composite with microstructures controlled by the natural structures of sponge. We manufactured the carbon/epoxy resin composites through infiltrating the epoxy resin into the netlike carbon preform derived from natural sponge templates. The microstructures, thermal diffusivity, and wear behavior of the resin composites were analyzed.
Table 1 – The codes of the three netlike carbon and their processing parameters.
Volume ration of phenolic resin and sponge (ml/cm3) Sintering temperature (C)
C1
C2
C3
0.4
0.6
0.4
800
800
1400
2.
Experimental
100 g, supplied by Wuxi Resin Co. Then at a pressure of 1 MPa, polymer matrix was injected into the three carbons. After heating at 160 C for 2 h, the matrix cured and the carbon/ epoxy resin composites were obtained.
2.1.
Sample preparation
2.2.
The natural sponge (family of origin: filaceous sponge, producing area: Mediterranean Sea) was selected as the template. Fig. 1 shows its actual photo and the image captured by the scanning electron microscopy (SEM, JSM-6380, JEOL, Japan). It can be seen that skeleton fibers in the sponge link up, interlace and form a complex net structure. Three shares of thermoset phenolic resin (solid content: 45–55%, solvent: ethanol, Tu-4 cup viscosity at 25 C: 640 s) with different volumes were poured into three pieces of this kind of sponge. The volume ratio of the phenolic resin poured and the sponge for one piece was 0.6 ml resin/1 cm3 sponge. The ratios for the other two pieces of sponge were 0.4 ml/cm3. After the phenolic resin was adequately absorbed in the sponges, the sponges were squeezed gently to spread the resin coat evenly on the sponge fibers. The three resin-coated sponges were dried at 80 C for 6 h and heated at 160 C for 2 h for resin cure. Then, one cured piece coated by phenolic resin in 0.4 ml/cm3 ration was sintered at 1400 C for 2 h in the vacuum condition and the other two pieces were sintered at 800 C for 2 h. During sintering, the phenolic resin was pyrolyzed to carbon in situ, whereby the sponge pieces turned to netlike carbon that maintained the structure of template. The codes of C1, C2, and C3 are used to represent these three carbons, and their processing parameters are presented in Table 1. The liquid of polymer matrix can be compounded through mixing the epoxy resin (WSR618), curing agent, and absolute alcohol in a 4:1:1 weight ratio. Here, the epoxy resin is diglycidyl ether of bisphenol A (DGEBA) with the epoxy value of 0.48–0.54 mol/
Microstructures and thermal diffusivity
The microstructures of the materials were observed using optical microscopy (OM). The phases of netlike materials were identified using a X-ray diffractometer (XRD, Cu-Ka, RigakuX 2038, Japan). The bulk density (geometrical density) was determined by measuring the weight and the volume of the cuboid specimens. The thermal diffusivity of the composites and epoxy resin were measured using a laser conductometer (Nanuflashi LFA447, Netzsch, Germany). The shape of thermal diffusivity specimen was a column with size of u12.6 · 1 mm3.
2.3.
Wear tests
Friction and wear tests were conducted with a MM-200 wear tester by a block-on-ring arrangement in air at room temperature without lubricant (see Fig. 2). The sliding ring with a diameter of uouter40–uinner16 · 10 mm was made of GR15 bearing steel (oil hardened to HRC 62). The wide rectangular faces of the block specimens with dimension of 10 · 10 · 30 mm3 were put in line contact with the sliding ring. Prior to testing, the test surfaces of the specimens and ring were ground to a smooth finish on the 1000 grit SiC emery paper. The wear tests were performed at the following conditions: three load levels of 49, 73.5 and 98 N, and one rotational speed of 200 rpm (0.419 m/s). A standard test runs 50 min, which corresponds to a sliding distance of 1256 m. Three specimens of every material were tested for every wearing condition.
Fig. 1 – Actual and SEM image of the natural sponge used in this study.
Fig. 2 – Diagram of the wear-testing device.
Coefficient of friction was obtained from friction torque, which was recorded by a registering instrument in the wear tester, and then divided by the load and radius of the ring. The wear rates were determined by dividing the wear volume loss of the specimens by the sliding distance. The morphologies of the worn surfaces were analyzed using an OM.
3.
Results
3.1.
Microstructures
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4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
C
Intensity (u.a.) Intensity (u.a.) Intensity (u.a.)
Fig. 3 shows the actual and OM image of the carbon/epoxy resin composite. The black netlike phase inside the composite is carbon derived from sponge, which is interpenetrated by the gray phase of epoxy resin. Both phases are continuous, forming an interpenetrating network structure. During the course of sintering in vacuum, phenolic resin on the sponge fibers was decomposed into carbon in situ [12]. This gives rise to a netlike carbon with the morphology derived from sponge. The three-dimensional net structure of the sponge was not damaged. It was retained in the carbon-network perfectly, as seen in Figs. 1 and 3. After
injecting epoxy resin into the carbon and curing, this natural net structure was brought once more into the composite. Therefore, the distribution of every phase in this polymer matrix composite is controlled by the natural structure of sponge. This is different from the traditional polymer matrix composites, the structures of which are obtained artificially. Fig. 4 presents the XRD patterns of the epoxy resin, carbon/epoxy composite, and netlike materials after the phenolic resin covering, solidifying and by sintering at 800 and 1400 C. Seen from their XRD patterns, the netlike materials are composed of nongraphitizable carbon, i.e. glassy carbon from phonelic resin [12]. It can also be seen that the peak intensity of carbon in the netlike carbon sintered at 1400 C is higher than that sintered at 800 C. This indicates that crystallization extent of carbon increased while the sintering temperature rises. Different state of carbon could influence the properties of the composite further more. Table 2 lists group data of the netlike carbon and carbon/ epoxy resin composites obtained in the present investigation. The densities of the composites are all lower than that of epoxy resin (1.17 g/cm3). Volume content of epoxy matrix in the composites can been calculated according to the following law: qPorC þ qEpoxy VEpoxy ¼ qComposite , where qComposite , qPorC , and qEpoxy represent the density of the composite, the netlike carbon, and the epoxy matrix, respectively. VEpoxy represents the volume content of the epoxy matrix in the composites. It should be noticed that qPorC is the bulk density of the netlike carbon, so it need not multiplied the volume content of the carbon. The volume content of carbon in composite can been calculated through the law: VCarbon ¼ 1 VEpoxy . The results are shown in the last row of the table. It is obvious that the content of carbon in C2/Epoxy is much higher than the other two because that the sponge piece used as its template absorbed more volume of the phenolic resin.
Intensity (u.a.)
CARBON
C3 sintered at 1400°C
1200 800 C 400
C
0
C
800
C1 sintered at 800°C C
400
C
0
Epoxy resin
400 0
C C2/Epoxy
400 C 0 20
40
60
80
2θ ( degr ee)
Fig. 3 – Actual and OM image of the carbon/epoxy resin composite.
Fig. 4 – XRD pattern of the netlike materials, epoxy resin and carbon/epoxy composite.
9
Table 2 – Densities and content of some phases of the materials.
0.108
0.153
0.113
1.006
0.945
0.998
23.25
32.31
24.35
3
C3
-2
Bulk density of netlike carbon (g/cm3) Density of carbon/epoxy resin composite (g/cm3) Volume content of carbon in composite (%)
C2
8
Wear rate (10 mm /m)
C1
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
7 6 5 4 3 2
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
CARBON
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
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1 0
3.2.
Thermal diffusivity
49N
Fig. 5 shows the thermal diffusivities of the epoxy resin and composites, where the thermal diffusivities of the composites are all higher than that of epoxy resin. Usually, the capacity of heat transmission of carbon is better than that of epoxy resin [13,14]; therefore, introducing carbon could improve the thermal diffusivity of epoxy resin remarkably. Moreover, the thermal diffusivity of the composites may rise by increasing the content of carbon, which can be seen from the data of C1/ Epoxy and C2/Epoxy in Fig. 5. Compared with the C1/Epoxy, the C3/Epoxy exhibited a little higher thermal diffusivity. This may be due to the higher crystallization extent of carbon in C3/Epoxy than that in C1/Epoxy. The carbon with higher crystallization extent possesses higher capacity of heat transmission [15].
3.3.
Wear properties
Fig. 6 shows the variation of wear rates as a function of the applied load at room temperature for the composites and epoxy resin. The wear rates for all materials increase with increasing load. Apparently, the wear rate of the epoxy resin is relatively larger than that of the composites. It indicates that the wear resistance of the composites is significantly improved, as compared to that of the matrix. This improvement is attributed to the following reasons: (1) in the composites, the glassy carbon derived from phenolic resin usually possesses higher hardness [16]. It may provide resistance to abrasion during the composites wearing, and decrease the wear rate. (2) The composites have higher thermal diffusivity than epoxy resin, as shown in Fig. 5. It can help the composites
2
Thermal Diffusivity (mm /s)
0.40
C2/Epoxy
0.35 0.30
Epoxy
C1/Epoxy
C3/Epoxy
0.25 0.20 0.15 0.10 0.05 0.00
Fig. 5 – Thermal diffusivities of carbon/epoxy resin composites and epoxy resin.
73.5N Load (N)
98N
Fig. 6 – Wear rates as a function of the applied load for carbon/epoxy resin composites and epoxy resin.
eliminate the frictional heat faster than epoxy resin. Thus, the temperature on the worn surface of the composites during wearing may be lower, and the degree of softening on the worn surface was also lower, as compared to that of epoxy resin. Therefore, the composites exhibit higher wear resistance. It can also be seen that C2/Epoxy and C3/Epoxy show the better wear resistance than C1/Epoxy. This is also due to the increased content of carbon in the former two composites and their higher thermal diffusivity, as seen in Table 2 and Fig. 5. Fig. 7 shows the worn surfaces of specimens examined by OM. The worn surfaces of epoxy resin at 49 and 73.5 N are shown in Fig. 7a and b, respectively. Cracks (indicated by the arrows in figure), which are perpendicular to the siding direction, are evident on the worn surfaces of epoxy resin, and the density of cracks increases remarkably with the increasing of the load. The relative slip between the sliding ring and specimen can raise a stress of tearing on the worn surface. As the specimen of epoxy resin is soft, this stress could distort its worn surface leading to crack formation. The worn surfaces of C1/Epoxy at 49 and 73.5 N are shown in Fig. 7c and d, respectively. Only a few cracks perpendicular to the sliding direction can be found on their worn surface, which is different from the specimen of epoxy resin. The sponge-like network made up of the hard glassy carbon in the composite may play an important role. In the composite, the carbon and the matrix forms the tri-dimensionally interpenetrating structure. When the epoxy matrix was torn by the sliding ring, the hard carbon-network could nearly root the soft matrix at the original position without serious distortion. Therefore, much fewer perpendicular cracks emerged on the worn surface of composite as compared to the epoxy resin. This can also be another reason why composites exhibit better wear resistance than epoxy resin. Fig. 8 shows the average coefficients of friction as a function of applied load for the composites and epoxy resin. The coefficients of friction of both the epoxy resin and the composites increase with increasing load. Compared with epoxy resin, the composites exhibit lower coefficients of friction. The friction reduction is attributed to the following reasons:
CARBON
4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
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Fig. 7 – Worn surfaces of carbon/epoxy resin composite and epoxy resin (epoxy resin at 49 N (a) and 73.5 N (b), C1/Epoxy at 49 N (c) and 73.5 N (d)).
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
0.8
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
Coefficient of friction
1.0
Epoxy C1/Epoxy C2/Epoxy C3/Epoxy
1.2
49N
73.5N Load (N)
98N
0.6 0.4 0.2 0.0
Fig. 8 – Average coefficient of friction versus applied load.
(1) the carbon in the composite improves the hardness of material so that the adhesivity between the worn surface and the sliding ring was reduced during wear test. However, the simplex epoxy resin is softer than the composites, leading to stronger adhesivity and higher coefficient of friction. (2) The carbon that was peeled from the netlike material during wearing could also play lubrication action to reduce the friction. (3) The composites have higher thermal diffusivity; thus, the temperature or degree of softening of the worn surface was lower, as compared to the simplex epoxy resin. The adhesivity between the friction couple could also decrease.
Fig. 9 – Real-time coefficients of friction as a function of the sliding distance for (a) epoxy resin and (b) C1/Epoxy at 73.5 N.
Fig. 9a and b shows the real-time coefficients of friction of epoxy resin and C1/Epoxy as a function of the sliding distance, which were recorded by the register instrument. The load is 73.5 N. Compared with the curve of epoxy resin, there is less rising and falling in the curve of the composite in the whole wear process. Therefore, the curve of the composite looks straighter than that of epoxy resin. Moreover, the
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CARBON
4 8 ( 2 0 1 0 ) 2 4 3 5 –2 4 4 1
coefficient of friction of the composite changes in a more narrow range than that of epoxy resin, which makes its curve thinner than the curve of epoxy resin. The results seen from Fig. 9 indicate that the friction behaviors of the composite are more stable than those of epoxy resin. The reason may be related to the carbon-network, which will be analyzed in Section 4.1.
4.
Discussions
4.1.
Role of the carbon-network
during wear test could lead to the obvious strain of the specimen. The deformation resilience could lead to the intense flutter of the specimen. Thus, the coefficient of friction of the epoxy resin may have a wider range (Fig. 9a). However, for the composite, the hard carbon-network inside could root the epoxy matrix in the original position to avoid obvious strain as the friction force changed. Moreover, the carbonnetwork could restrain the rebound of the epoxy matrix. Therefore, intense flutter was avoided and the coefficient of friction had a more narrow range than epoxy resin (Fig. 9b).
4.2. In the carbon/epoxy resin composites, the carbon-network plays an important role in stabilizing the friction process and coefficient of friction for the composites. For the epoxy specimen, the matters were peeled off from the specimen unevenly during the wear test. Some parts showed serious abrasions, while only slight abrasions were found in other parts of the worn surface. Fig. 10a shows the worn surface of the epoxy specimen at 73.5 N. The severely worn parts are found in the fuscous and white regions, where most surfaces have been peeled off and could not touch the sliding ring. Hence, the contact area between the specimen here and the sliding ring is smaller. The slightly worn parts can be found in the yellow regions where there is a larger contact area. During the wear test, the severe wear region could transform into a mild region, or vice versa. Therefore, the total contact area between the specimen and the sliding ring also changed frequently in the wear process, which leads to the frequent altering of the total friction force. Thus the coefficient of friction of the epoxy resin fluctuated seriously in the whole wear process (Fig. 9a). However, for the composites, the phenomenon is not the same with that of the epoxy resin. The carbon-network divided the epoxy on the worn surface into innumerable units so that the epoxy matrix could be peeled off evenly from the surface. Therefore, its worn surface is always kept smooth as seen in Fig. 10b. There are few changes in the total contact area and friction force during the wear test, thus only a slight rise and fall in its curve can be observed (Fig. 9b). The curve of the composite exhibits more straight than that of the epoxy resin. The thinner friction curve of the carbon/epoxy resin composite is attributed to the carbon-network. Similar to some soft polymers, the epoxy resin has low elastic modulus and high rebound ability. The slight altering of friction force
Abrasion mechanism
Wear and friction between the matrix epoxy resin and sliding ring is believed to initiate by deformation and cutting of the bulges on the epoxy resin by the sliding ring, resulting in an increased contact area. Therefore, its coefficient of friction would increase at the beginning, as shown in Fig. 9a. For the carbon/epoxy composites, the wear and friction behavior would become more complex. Cutting and deformation of the matrix resin bulges by the harder sliding ring could increase the contact area, which may increase the coefficient of friction. However, carbon bleed out onto the sliding surface, forming a carbon film on the exposed resin surfaces, that decreases the likelihood of adhesion, and results in a reduced coefficient of friction at the beginning, as shown in Fig. 9b. The wear behavior of a material is a function of many factors, such as applied load, sliding velocity, counterface, lubrication and atmosphere. Generally, wear studies on engineering materials are concerned with the effects of applied load. Sliding wear takes place due to the relative sliding of the two surfaces in contact with each other under a certain applied load. In the case of the matrix epoxy resin, with increasing applied load bulges on the surface of the sliding ring penetrate deeper into the surface of sample and more heat can be generated on the sliding surfaces due to friction. This raises the sliding surface temperature and softens the sample, which then exhibits a higher degree of adhesion and an increase in coefficient of friction (as shown in Fig. 8). Moreover, with increasing applied load, the effective stress on the bulges on the surface of the sample can by far exceed the strength and consequently they break off, which results in a higher wear rate, as shown in Fig. 6. In the case of carbon/epoxy composites, bulges on the sliding ring penetrate deeper into the surface of sample with increasing applied
Fig. 10 – Worn surfaces of (a) epoxy resin and (b) C1/Epoxy at 73.5 N.
CARBON
4 8 ( 20 1 0 ) 2 4 3 5–24 4 1
load, which results in an increase of the friction-induced hot surface temperature and a gradual flaking off of the carbon films. These lead to an increase in metal-to-resin contact surface area and a mild adhesive wear, resulting in the simultaneous increase of wear rate and coefficient of friction, Figs. 6 and 8.
5.
[2]
[3]
Conclusion
Based on the three-dimensional net structure of natural sponge, the carbon/epoxy resin composites with interpenetrating network were produced by injecting the epoxy resin into the netlike carbon derived from sponge. This carbon-network remained perfectly in the composite, and the natural sponge controlled the interpenetrating structure of these composites. The thermal diffusivities of these composites are higher than that of epoxy resin, and increased with the increasing content of carbon and crystallization extent of carbon. The wear rates of the composites are shown to be lower than that of the epoxy resin. The lower wear rates are attributed to the abrasion of carbon in the composites, higher thermal diffusivity, and carbon-network of the composites. The composites exhibit a lower average coefficient of friction compared with epoxy resin. The reason may be that the composites possessed higher hardness during wearing so that it was more difficult for the composites to adhere to the sliding ring than for epoxy resin. As the carbon-network in the composites divided the epoxy on the worn surface into innumerable units, the friction curve of the composite appeared straighter than that of the epoxy resin. Furthermore, the carbon-network could restrict the strain and rebound of the epoxy matrix, which made the friction curves of the composites thinner than that of the epoxy resin. These results demonstrate a more stable friction state for this type of composite.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Acknowledgement [13]
The authors wish to express thanks to Natural Science Foundation of Jiangsu (Nos. BK2007596 and BK2008407), and Science and Technology Development Program of Nanjing University of Science and Technology (Nos. XKF07017 and XKF09068).
[14]
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