- Email: [email protected]
C composites
Carbon 41 (2003) 1819–1826
Interfacial shear strength of C / C composites Yuko Furukawa a , Hiroshi Hatta b , *, Yasuo Kogo a a
Tokyo University of Science, Department of Material Science and Technology, Faculty of Industrial Science and Technology, 2641 Yamazaki, Noda, Chiba 278 -8510, Japan b Research Division of Space Propulsion, Institute of Space and Astronautical Science, 3 -1 -1, Yoshinodai, Sagamihara, Kanagawa 229 -8510, Japan Received 26 December 2002; received in revised form 24 March 2003; accepted 26 March 2003
Abstract Fiber-bundle push-out, single-fiber push-in, and single-fiber push-out tests were conducted in order to examine the applicability of these methods for determining the interfacial shear strength of carbon–carbon composites. The fiber-bundle push-out test resulted mostly in fractures along the fiber / matrix interface but created a small amount of fractures in the matrix. Hence, the evaluated strength was regarded as an approximate value. In order to precisely evaluate the interfacial strength, push-in and push-out tests for a single fiber were performed using a micro-Vickers indentation tester. In these tests, the load has to be placed within a target fiber, and the indentation should not extend to the matrix. This condition restricted the load that could be applied to a carbon fiber. Within this limit, a single carbon fiber could not be pushed-in. For the sake of load reduction, single-fiber push-out tests were conducted using thin specimens. The thickness appropriate for a single-fiber push-out specimen was estimated based on the interfacial shear strength obtained by the bundle push-out tests. Below this thickness, single-fiber push-out tests could be successfully performed. 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Carbon / carbon composites; C. Scanning electron microscopy (SEM); D. Interfacial properties
1. Introduction Carbon / carbon composites (C / Cs) maintain excellent strength and toughness at temperatures exceeding 2273 K in a non-oxidizing atmosphere. Due to this advantage, C / Cs are expected to be applied to structures that experience ultra-high temperatures in non-oxidizing atmospheres [1–4]. However, a general understanding of C / Cs is still in the primitive stage. For example, even the mechanisms governing the behavior of tensile fractures in C / Cs have not been clarified [5]. Recently, it was suggested that the tensile strength of C / Cs is improved when the interfacial shear strength between the fiber and the matrix is lowered [6]. Hence, the precise evaluation of the interfacial strength has become important. Several attempts have been made to measure the interfacial shear strength of C / Cs using, for example, fiber push-in, push-out, and
pull-out tests [7–11]. However, to the authors’ knowledge, the interfacial shear strength of C / Cs has not been successfully determined, probably due to problems associated with specimen preparation, testing conditions, and the calculation procedures necessary for interfacial shear strength determination. In the present study, three methods for the determination of interfacial strength often adopted for other composites, polymer matrix and ceramic matrix composites [12–24], were applied to C / Cs. The purpose of this study is to identify problems associated with the measurement of the fiber / matrix interfacial shear strength of C / Cs, and then to find a measuring technique suitable for C / Cs.
2. Experimental procedure
2.1. Materials *Corresponding author. Tel.: 181-427-598-293; fax; 181427-598-461. E-mail address: [email protected] (H. Hatta).
The C / Cs used in the present study were of a symmetric cross-ply lamination type, (08 / 908) 8s , with various den-
0008-6223 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00144-1
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Fig. 1. Schematic drawings of diamond indenters used for the single fiber push-out tests. (a) Pyramidal indenter and (b) spherical indenter.
sities from 1.31 to 1.81 g / cm 3 . These C / Cs were produced by Toho-Rayon, Japan, via the resin-char method. In this fabrication process, a carbon fiber-reinforced phenolic– resin matrix composite, CFRP, was first produced. The CFRP was then carbonized at 1273 K, and the resultant material was applied several cycles of phenolic resin infiltration, carbonization of the resin at 1273 K, and HTT (high temperature treatment) of the matrix at 2273 K. The C / C density was varied by performing this cycle 1–6 times. The reinforcing fiber of these C / Cs was PAN-based high strength type IM-600 (Toho-Rayon, diameter of fibers: 5.3 mm). The fiber volume fraction was 60% at the CFRP stage.
2.2. Single fiber tests Single-fiber push-in and push-out tests were conducted using a micro-Vickers indentation tester (maximum capacity of 5 N, Shimadzu MCTE-501). In these tests, the load
was applied to a fiber in a C / C specimen at a constant loading rate of 3.87 mN / s. Two types of diamond indenters were used for the loading as shown in Fig. 1, i.e., a pyramidal indenter with a facing angle of 1368 and a spherical indenter with a diameter of 5.3 mm. The latter indenter was formed by grinding a 908 cone indenter. After the tests, the fracture surfaces were observed using scanning electronic microscopy (SEM).
2.2.1. Push-in test In the single-fiber push-in test, a compressive load was applied to an 8-mm thick specimen, as shown in Fig. 2a. The interfacial debonding and sliding stresses can in principle be determined based on the relations between force and indenter displacement during repetitions of loading and unloading [12]. 2.2.2. Push-out test For single-fiber push-out tests, the specimens have to be
Fig. 2. Schematic drawings illustrating the method of measurement of the interfacial fracture stress of C / Cs. (a) Single fiber push-in test, (b) single fiber push-out test and (c) fiber bundle push-out test.
Y. Furukawa et al. / Carbon 41 (2003) 1819–1826
thin enough to allow fractures over the whole interface of the loaded fiber and to push it out completely, as shown in Fig. 2b. Using results of fiber-bundle push-out tests, the specimen thickness was determined to be less than 105 mm. The specimens were prepared by first polishing a piece of C / C embedded in epoxy resin. The embedded specimens were, then, sectioned into approximately 1-mm thickness using a diamond cutter, and polished to thicknesses of 39 to 105 mm using diamond paste. A 70-mm groove was engraved on the surface of an aluminum base plate in order to make a room for the pushed-out fiber. Because of the fiber’s small diameter, 5.3 mm, precise loading points were confirmed by SEM observations after the tests, and if the load was applied near a fiber boundary, such test results were rejected. In the present study, the interfacial strength in the push-out tests was approximated by the average shear strength given by the following Eq. (1) [12,13]:
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able, because the openings of these cracks may alter the interfacial condition, i.e., excess compressive stress might be induced on the fiber / matrix interface due to radial displacement by the cracks. In order to clearly observe damage given to the fiber end by indentation loading, thin (sub-micron) gold coating was sputtered on the surface of part of specimens before the indentation tests. It is known that indentation lengths and occasionally appearing oval median / radial cracks obey Eqs. (2) and (3), respectively [25,26]: F~a 2
(2)
F~c 3 / 2
(3)
where F is the load applied to a fiber when the interfacial debonding or sliding occurred, R f is the fiber radius, and L is the specimen thickness.
where compressive load is denoted by F, a half length of an indentation and oval median crack by a and c, respectively. Following indentation tests using the pyramidal indenter, cross-shaped damage as shown in Fig. 3 was observed. Fig. 4 shows the relation between force and a half of the damage size as a function of indentation load. The solid line in the figure represents the crack length predicted by Eq. (4), in which the proportional constant was determined under the assumption that the fracture toughness is that of graphite (1.1 MPa m 1 / 2 ) [27]:
2.3. Fiber-bundle push-out test
F 5 7.70c 3 / 2
In the fiber-bundle push-out test, a fiber bundle was pushed out using a tungsten-carbide needle having a diameter of 50 mm, as shown in Fig. 2c. The load in this test was applied at a constant displacement rate of 0.1 mm / min by a screw-driven tensile testing machine (maximum capacity of 245 N, Orientec RTM-25). The specimens for this test were prepared by means of the same procedure as those for the single fiber push-out tests, and finished into thicknesses of 110–230 mm. A 70-mm diameter hole was drilled through the base plate so as to enable a fiber bundle to be pushed-out. Most of fracture area in this test was assumed to occur along the interface. In order to confirm this, specimen surfaces after the push-out tests were observed using SEM. The interfacial debonding and sliding stresses were expressed by the average shear stress, the load divided by the debonding and sliding areas, respectively.
The coincidence of the predicted and observed damage sizes indicates that the damage consisted of median / radial cracks induced during the indentation tests. In order for the test to be meaningful, cracks should at least be within the fiber. Thus from Fig. 4, the indentation load should be restricted to below 35 mN. Under this load, only partial debonding of a fiber was observed; thus interfacial strength cannot be determined by this test. Note that the force–
F ts 5 ]] 2p R f L
(1)
(4)
3. Experimental results
3.1. Single fiber tests 3.1.1. Push-in test 3.1.1.1. Pyramidal indenter. In single-fiber push-in tests using the pyramidal indenter, oval median / radial cracks occasionally appear [25,26]. These cracks are not desir-
Fig. 3. Scanning electron micrograph of the surface of a goldcoated C / C ( r 51.57 g / cm 3 ) after push-in test using the pyramidal indenter at a maximum force of 40 mN.
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Fig. 4. Relation between indentation load and damage size observed on the C / C cross-section ( r 51.57 g / cm 3 ) in the single fiber push-in test using a pyramidal indenter.
displacement curves obtained by this test were smooth as shown in Fig. 5 by a solid line.
3.1.1.2. Spherical indenter. Using the spherical indenter as shown Fig. 1b, push-in tests were also conducted. In this test, though no median cracks were observed, a crack suddenly appeared and extended across the fiber at an applied load exceeding 60 mN as shown in Fig. 6. Thus, the maximum load applicable to this indenter was determined to be 60 mN. As shown in Fig. 6, observations using SEM following the tests revealed that even at a load limit of 60 mN, the interfacial debonding occurred only partially just as in the case of using the pyramidal indenter. Hence, determination of interfacial debonding stress by
Fig. 5. Relation between force and indenter displacement in C / C ( r 51.57 g / cm 3 ) obtained by single fiber push-in test using pyramidal and spherical indenters without gold coating.
Fig. 6. Scanning electron micrograph of the surface of goldcoated C / C ( r 51.57 g / cm 3 ) after push-in test using a spherical indenter at a maximum force of 60 mN.
means of the single fiber push-in test using the spherical indenter was also concluded to be impossible. A typical load–displacement curve obtained by the push-in test using the spherical indenter is shown in Fig. 5 by dashed line.
3.1.2. Push-out test Fig. 7 shows the results of the single fiber push-out tests using the spherical indenter for C / C having a thickness of 69 mm and a density of 1.57 g / cm 3 . Note that the force– displacement curves produced in the fiber push-out tests include a flat region. Since these tests were conducted under constant loading rate, the flat regions indicate abrupt displacements, in other words, complete debonding of the fiber interfaces. In order to confirm this debonding, the specimen surfaces were observed via SEM. The photo-
Fig. 7. Relation between force and indenter displacement in C / C (t569 mm, r 51.57 g / cm 3 ) obtained by single fiber push-out test using a spherical indenter.
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Fig. 8. Scanning electron micrographs of a C / C surface (t569 mm, r 51.57 g / cm 3 ) after push-out test using a spherical indenter. (a) Maximum force; 70 mN, (b) maximum force; 44 mN.
graphs shown in Fig. 8a and b were taken after the flat regions appearing in the force–displacement curves, respectively. When the load surpassed the flat load, a compressive fracture around the loaded fiber was clearly observed as shown in Fig. 8a. On the other hand, only a pushed-out fiber without damage to the surrounding matrix was observed for specimens loaded up to the flat load as shown in Fig. 8b. Based on these observations, the interfacial debonding shear stress was determined based on the load of the flat region using Eq. (1) only in the case of the flat load being less than 60 mN. The resulting interfacial shear debonding stress was about 37.3 MPa. As well, similar flat regions in the force–displacement curves were found in the case of the pyramidal indenter. The average interfacial strength in this case was 24.6 MPa.
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This method is easy to perform and specimen preparation is also easy because rather thick specimens can be used. The other merit of the bundle push-out test is that sliding load can be easily determined in addition to debonding load. The present testing apparatus for the single fiber push-out test applies force at a constant loading rate. Thus, the machine does not permit load drop. On the other hand, in the bundle push-out test, a load can be applied using a general purpose mechanical testing machine. Thus the sliding load can be easily detected. Fig. 9 shows a typical result of the fiber bundle push-out tests. A fiber bundle in this case was pushed out at an applied (maximum) load of 1 N. As can be seen in Fig. 10a and b, fracture was confirmed to occur almost along the interface between the fiber and matrix, but with a small amount of matrix failure. Thus, this method was judged to be useful for the rough evaluation of interfacial shear strength. Substitution of the maximum load into Eq. (1) yielded an interfacial shear debonding stress of 17.2 MPa. After the application of the maximum load, sliding along the interface occurred as indicated in Fig. 9. Using this load, the interfacial sliding stress (sliding load divided by sliding area) was determined to be 6.0 MPa. Fig. 11 shows the relation between specimen thickness and the interfacial debonding and sliding stresses. As shown in this figure, the interfacial debonding and sliding stresses were confirmed to only slightly depend on specimen thickness. The interfacial debonding stresses obtained by the single fiber push-out test and the fiber bundle push-out tests were compared as a function of bulk density of the C / C in Fig. 12. As this figure shows, the interfacial debonding stresses obtained by the bundle push-out tests were much lower than those obtained by the single fiber tests. However, both sets of results showed a similar tendency. This indicates that the bundle push-out test is useful at least for compara-
3.2. Fiber-bundle push-out tests The single fiber push-out test was rather difficult to perform. First of all, the preparation of a thin specimen required for the single fiber push-out test is difficult, especially when the interfacial strength is low. In addition, the single fiber push-out test requires SEM observations to confirm that the indenter is pushing at near the center of a fiber. Hence, another method was explored to more easily evaluate interfacial strength. The present study proposes the bundle push-out test for performing this evaluation.
Fig. 9. Typical relation between force and indenter displacement of C / C (t5155 mm, r 51.81 g / cm 3 ) obtained by fiber bundle push-out test.
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Fig. 12. Comparison of interfacial debonding stresses obtained by single fiber push-out and fiber bundle push-out tests.
4. Discussion Fig. 10. Scanning electron micrographs of the surface of an indented specimen (t5155 mm, r 51.81 g / cm 3 ) after the bundle push-out test. (a) Fractured surface of the specimen after the test and (b) magnification of the fracture surface.
tive study. The difference shown by the two measuring techniques—bundle and single fiber push-out tests—is probably due to the difference in specimen geometry. The true characterization of the interface should include, for example, consideration of stress concentrations appearing around the free surfaces. This analysis is now underway.
Interfacial strengths have been successfully determined for ceramic matrix composites (CMCs) [12–24], but not for C / Cs. In the case of CMCs, the importance of interface characterization for the optimization of mechanical responses was understood in their early development stages. These efforts were motivated by analytical models, which include the interfacial strength or toughness as a ruling parameter of macroscopic mechanical responses. Hence, much effort was devoted to characterization of the interfaces. In contrast, in the case of C / Cs, analysis of their mechanical properties is still in a primitive stage [28]. Thus, the effects of interfacial strength on various mechanical properties, for example, strength and toughness, are not clearly understood, and only a few studies [7–11] have been performed regarding the characterization of these interfaces. In addition to this background, the following difficulties should be noted for C / C experiments.
4.1. Fiber compressive failure
Fig. 11. Relation between interfacial debonding and sliding stresses and specimen thickness obtained by fiber bundle push-out test of C / C (t5155 mm, r 51.81 g / cm 3 ).
The average compressive stress over the fiber cross section under the permitted load, 60 mN, for the single fiber push-in tests is about 3 GPa. Taking stress concentration into account, real compressive stress at the contact area of the pyramidal or spherical indenter easily surpasses the compressive strength of the carbon fiber, approximately 4–6 GPa [29]. On the other hand, ceramic fibers have much higher compressive strength than do carbon fibers, the compressive strength of ceramic fiber being more than one order higher than its tensile strength. Thus, a greater load can be applied to CMCs than that to
Y. Furukawa et al. / Carbon 41 (2003) 1819–1826
C / Cs. This indicates that a load applicable to a ceramic fiber should be much higher than that to a carbon fiber.
4.2. Scale of fiber diameter The diameter of the present carbon fiber is 5.3 mm, while that of ceramic fiber is usually several times larger than carbon fiber; for example, SiC fiber (Nicaron) is about 15 mm. Thus, in the case of CMCs, it is much easier to find the indentation and to observe the propagation of the interfacial debonding during the test as compared with C / Cs.
4.3. Specimen preparation The lower compressive strength and smaller diameter of carbon fiber require thinner specimens than CMCs. This requirement leads to difficulty in specimen preparation, especially when interfacial strength is low; specimens often fracture during preparation. The fiber bundle pushout test permits the use of a thicker specimen than does the single fiber push-out test. Even in the fiber bundle push-out test, the applicable load is restricted to (cross sectional area)3(compressive strength of C / C; about 600 MPa). When a 50 mm needle is used for pushing out, the load should be less than 1.2 N, and this load requires a thickness of less than 250 mm.
5. Conclusions In order to establish a technique for measuring the interfacial strength of C / Cs, fiber bundle push-out and single fiber push-in and -out tests were performed. The following conclusions were drawn from this study: 1. The interfacial debonding strength and the interfacial sliding strength of C / Cs could be approximately determined by means of a fiber bundle push-out test. 2. Interfacial strength was successfully determined by single fiber push-out tests but not by push-in tests because of the high load required by the latter method. 3. Single fiber push-out and fiber bundle push-out tests resulted in a similar tendency in regard to interfacial strength, but expressed different absolute values.
Acknowledgements This research was partly supported by a grant-in-aid for basic science (No. 11305047) from the Ministry of Education, Sports, Culture, Science and Technology of Japan.
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References [1] Buckley JD, Edie DD. Carbon–carbon materials and composites. NASA Ref Publ 1992;1254:267–81. [2] Fitzer E, Gkogkidis A, Heine M. Carbon fibers and their composites. High Temp–High Press 1984;16:363–92. [3] Fitzer E, Geigl K-H, Huttner W. The influence of carbon fibre surface treatment on the mechanical properties of carbon / carbon composites. Carbon 1979;18:265–70. [4] Fitzer E, Huttner W. Structure and strength of carbon / carbon composites. J Phys D: Appl Phys 1981;14(3):347–71. [5] Kagawa Y, Hatta H. Tailoring ceramics composite. Aguneshofusha, 1990;164–193, in Japanese. [6] Fitzer E, Terwiesch B. Carbon–carbon composites unidirectionally reinforced with carbon and graphite fibers. Carbon 1972;10:383–90. [7] Serizawa H. The study about the physical properties of fiber-reinforced ceramics composites, Ph.D. thesis, Tokyo University, 1997, in Japanese. [8] Kohyama A, Serizawa H, Sato S, Hinoki T, Wen C. Interfacial microstructure and properties of C / C composites. In: Proceedings of the 8th symposium on high performance materials for severe environments. Tokyo International Forum (Tokyo, Japan): International symposium on advanced materials, 1997, pp. 447–57, In Japanese. [9] Fujita K, Sakai H, Iwashita N, Sawada Y. Influence of heat treatment temperature on interfacial shear strength of C / C. In: Proceedings of the 5th Japan international SAMPE symposium, Tokyo Big Sight (Tokyo, Japan): Society for the advancement of material and process engineering, 1997, pp. 1557–62. [10] Sakai M, Matsuyama R, Miyajima T. The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite. Carbon 2000;38:2123–31. [11] Komori K, Yasuda K, Matsuo Y, Kimura S. Pore structure and fracture behavior of carbon fiber / pitch-based carbon matrix composites. In: Proceedings of the 27th carbon material conference. The Kitakyushu International Conference Center, Kyushu, Carbon material society, 2000, pp. 98–9, In Japanese. [12] Marshall DB, Oliver WC. measurement of interfacial mechanical properties in fiber-reinforced ceramic composites. J Am Ceram Soc 1987;70(8):542–8. [13] Marshall DB. An indentation method for measuring matrixfiber frictional stresses in ceramic composites. J Am Ceram Soc 1984;67:C259–60. ´ [14] Bianchi V, Goursat P, Sinkler W, Monthioux M, Menessier E. Carbon fiber-reinforced (YMAS) glass–ceramic matrix composites. III. Interfacial aspects. J Eur Ceram Soc 1999;19:317–27. [15] Marshall DB, Oliver WC. An indentation method for measuring residual stresses in fiber-reinforced ceramics. Mater Sci Eng A 1990;126:95–103. [16] Shetty DK. Shear-lag analysis of fiber push-out (indentation) tests for estimating interfacial friction stress in ceramic– matrix composites. J Am Ceram Soc 1988;71(2):C107–9. [17] Honda K, Kagawa Y. Debonding criterion in the pushout process of fiber-reinforced ceramics. Acta Mater 1996;44(8):3267–77. [18] Honda K, Kagawa Y. Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics. Acta Metall Mater 1995;43:1477–87.
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[19] Kallas MN, Koss DA, Hahn HT, Hellmann JR. Interfacial stress state present in a ‘‘thin-slice’’ fiber push-out test. J Mater Sci 1992;27:3821–6. [20] Zhou X-F, Wagner HD, Nutt SR. Interfacial properties of polymer composites measured by push-out and fragmentation tests. Composites 2001;A32:1543–51. [21] Kerans RJ, Jero PD, Parthasarathy TA. Issues in the control of fiber / matrix interfaces in ceramic composites. Comp Sci Technol 1994;51:291–6. [22] Kerans RJ, Parthasarathy TA. Theoretical analysis of the fiber pullout and pushout tests. J Am Ceram Soc 1991;74(7):1585–96. [23] Kerans RJ, Rebillat F, Lamon J. Fiber–matrix interface properties of single-fiber microcomposites as measured by fiber push in tests. J Am Ceram Soc 1997;80(2):506–8. [24] Kerans RJ, Parthasarathy TA, Rebillat F, Lamon J. Interface properties in high-strength Nicalon / C / SiC composites, as determined by rough surface analysis of fiber push-out tests. J Am Ceram Soc 1998;81(7):1881–7.
[25] Lawn B. In: Fracture of brittle solids, 2nd ed., Cambridge University Press, 1975, pp. 249–307. [26] Lawn BR, Evans AG, Marshall DB. Elastic / plastic indentation damage in ceramics: the median / radial crack system. J Am Ceram Soc 1980;63(9–10):574–81. [27] Sakai M. Fracture toughness and R-curve behavior of graphite. Tanso 1988;134:211–23, in Japanese. [28] Hatta H, Suzuki K, Shigei T, Somiya S, Sawada Y. Strength improvement by densification of C / C composites. Carbon 2001;39:83–90. [29] Sumida A, Ono K, Kawazu Y. PAN based high modulus graphitized carbon fiber TORAYCA M60J. In: Proceedings of the 34th international SAMPE symposium. Reno Convention Center, Reno, NV, Society for the Advancement of Material and Process Engineering, 1989, pp. 2579–89, Vol. 34(2).
Interfacial shear strength of C / C composites Yuko Furukawa a , Hiroshi Hatta b , *, Yasuo Kogo a a
Tokyo University of Science, Department of Material Science and Technology, Faculty of Industrial Science and Technology, 2641 Yamazaki, Noda, Chiba 278 -8510, Japan b Research Division of Space Propulsion, Institute of Space and Astronautical Science, 3 -1 -1, Yoshinodai, Sagamihara, Kanagawa 229 -8510, Japan Received 26 December 2002; received in revised form 24 March 2003; accepted 26 March 2003
Abstract Fiber-bundle push-out, single-fiber push-in, and single-fiber push-out tests were conducted in order to examine the applicability of these methods for determining the interfacial shear strength of carbon–carbon composites. The fiber-bundle push-out test resulted mostly in fractures along the fiber / matrix interface but created a small amount of fractures in the matrix. Hence, the evaluated strength was regarded as an approximate value. In order to precisely evaluate the interfacial strength, push-in and push-out tests for a single fiber were performed using a micro-Vickers indentation tester. In these tests, the load has to be placed within a target fiber, and the indentation should not extend to the matrix. This condition restricted the load that could be applied to a carbon fiber. Within this limit, a single carbon fiber could not be pushed-in. For the sake of load reduction, single-fiber push-out tests were conducted using thin specimens. The thickness appropriate for a single-fiber push-out specimen was estimated based on the interfacial shear strength obtained by the bundle push-out tests. Below this thickness, single-fiber push-out tests could be successfully performed. 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Carbon / carbon composites; C. Scanning electron microscopy (SEM); D. Interfacial properties
1. Introduction Carbon / carbon composites (C / Cs) maintain excellent strength and toughness at temperatures exceeding 2273 K in a non-oxidizing atmosphere. Due to this advantage, C / Cs are expected to be applied to structures that experience ultra-high temperatures in non-oxidizing atmospheres [1–4]. However, a general understanding of C / Cs is still in the primitive stage. For example, even the mechanisms governing the behavior of tensile fractures in C / Cs have not been clarified [5]. Recently, it was suggested that the tensile strength of C / Cs is improved when the interfacial shear strength between the fiber and the matrix is lowered [6]. Hence, the precise evaluation of the interfacial strength has become important. Several attempts have been made to measure the interfacial shear strength of C / Cs using, for example, fiber push-in, push-out, and
pull-out tests [7–11]. However, to the authors’ knowledge, the interfacial shear strength of C / Cs has not been successfully determined, probably due to problems associated with specimen preparation, testing conditions, and the calculation procedures necessary for interfacial shear strength determination. In the present study, three methods for the determination of interfacial strength often adopted for other composites, polymer matrix and ceramic matrix composites [12–24], were applied to C / Cs. The purpose of this study is to identify problems associated with the measurement of the fiber / matrix interfacial shear strength of C / Cs, and then to find a measuring technique suitable for C / Cs.
2. Experimental procedure
2.1. Materials *Corresponding author. Tel.: 181-427-598-293; fax; 181427-598-461. E-mail address: [email protected] (H. Hatta).
The C / Cs used in the present study were of a symmetric cross-ply lamination type, (08 / 908) 8s , with various den-
0008-6223 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00144-1
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Fig. 1. Schematic drawings of diamond indenters used for the single fiber push-out tests. (a) Pyramidal indenter and (b) spherical indenter.
sities from 1.31 to 1.81 g / cm 3 . These C / Cs were produced by Toho-Rayon, Japan, via the resin-char method. In this fabrication process, a carbon fiber-reinforced phenolic– resin matrix composite, CFRP, was first produced. The CFRP was then carbonized at 1273 K, and the resultant material was applied several cycles of phenolic resin infiltration, carbonization of the resin at 1273 K, and HTT (high temperature treatment) of the matrix at 2273 K. The C / C density was varied by performing this cycle 1–6 times. The reinforcing fiber of these C / Cs was PAN-based high strength type IM-600 (Toho-Rayon, diameter of fibers: 5.3 mm). The fiber volume fraction was 60% at the CFRP stage.
2.2. Single fiber tests Single-fiber push-in and push-out tests were conducted using a micro-Vickers indentation tester (maximum capacity of 5 N, Shimadzu MCTE-501). In these tests, the load
was applied to a fiber in a C / C specimen at a constant loading rate of 3.87 mN / s. Two types of diamond indenters were used for the loading as shown in Fig. 1, i.e., a pyramidal indenter with a facing angle of 1368 and a spherical indenter with a diameter of 5.3 mm. The latter indenter was formed by grinding a 908 cone indenter. After the tests, the fracture surfaces were observed using scanning electronic microscopy (SEM).
2.2.1. Push-in test In the single-fiber push-in test, a compressive load was applied to an 8-mm thick specimen, as shown in Fig. 2a. The interfacial debonding and sliding stresses can in principle be determined based on the relations between force and indenter displacement during repetitions of loading and unloading [12]. 2.2.2. Push-out test For single-fiber push-out tests, the specimens have to be
Fig. 2. Schematic drawings illustrating the method of measurement of the interfacial fracture stress of C / Cs. (a) Single fiber push-in test, (b) single fiber push-out test and (c) fiber bundle push-out test.
Y. Furukawa et al. / Carbon 41 (2003) 1819–1826
thin enough to allow fractures over the whole interface of the loaded fiber and to push it out completely, as shown in Fig. 2b. Using results of fiber-bundle push-out tests, the specimen thickness was determined to be less than 105 mm. The specimens were prepared by first polishing a piece of C / C embedded in epoxy resin. The embedded specimens were, then, sectioned into approximately 1-mm thickness using a diamond cutter, and polished to thicknesses of 39 to 105 mm using diamond paste. A 70-mm groove was engraved on the surface of an aluminum base plate in order to make a room for the pushed-out fiber. Because of the fiber’s small diameter, 5.3 mm, precise loading points were confirmed by SEM observations after the tests, and if the load was applied near a fiber boundary, such test results were rejected. In the present study, the interfacial strength in the push-out tests was approximated by the average shear strength given by the following Eq. (1) [12,13]:
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able, because the openings of these cracks may alter the interfacial condition, i.e., excess compressive stress might be induced on the fiber / matrix interface due to radial displacement by the cracks. In order to clearly observe damage given to the fiber end by indentation loading, thin (sub-micron) gold coating was sputtered on the surface of part of specimens before the indentation tests. It is known that indentation lengths and occasionally appearing oval median / radial cracks obey Eqs. (2) and (3), respectively [25,26]: F~a 2
(2)
F~c 3 / 2
(3)
where F is the load applied to a fiber when the interfacial debonding or sliding occurred, R f is the fiber radius, and L is the specimen thickness.
where compressive load is denoted by F, a half length of an indentation and oval median crack by a and c, respectively. Following indentation tests using the pyramidal indenter, cross-shaped damage as shown in Fig. 3 was observed. Fig. 4 shows the relation between force and a half of the damage size as a function of indentation load. The solid line in the figure represents the crack length predicted by Eq. (4), in which the proportional constant was determined under the assumption that the fracture toughness is that of graphite (1.1 MPa m 1 / 2 ) [27]:
2.3. Fiber-bundle push-out test
F 5 7.70c 3 / 2
In the fiber-bundle push-out test, a fiber bundle was pushed out using a tungsten-carbide needle having a diameter of 50 mm, as shown in Fig. 2c. The load in this test was applied at a constant displacement rate of 0.1 mm / min by a screw-driven tensile testing machine (maximum capacity of 245 N, Orientec RTM-25). The specimens for this test were prepared by means of the same procedure as those for the single fiber push-out tests, and finished into thicknesses of 110–230 mm. A 70-mm diameter hole was drilled through the base plate so as to enable a fiber bundle to be pushed-out. Most of fracture area in this test was assumed to occur along the interface. In order to confirm this, specimen surfaces after the push-out tests were observed using SEM. The interfacial debonding and sliding stresses were expressed by the average shear stress, the load divided by the debonding and sliding areas, respectively.
The coincidence of the predicted and observed damage sizes indicates that the damage consisted of median / radial cracks induced during the indentation tests. In order for the test to be meaningful, cracks should at least be within the fiber. Thus from Fig. 4, the indentation load should be restricted to below 35 mN. Under this load, only partial debonding of a fiber was observed; thus interfacial strength cannot be determined by this test. Note that the force–
F ts 5 ]] 2p R f L
(1)
(4)
3. Experimental results
3.1. Single fiber tests 3.1.1. Push-in test 3.1.1.1. Pyramidal indenter. In single-fiber push-in tests using the pyramidal indenter, oval median / radial cracks occasionally appear [25,26]. These cracks are not desir-
Fig. 3. Scanning electron micrograph of the surface of a goldcoated C / C ( r 51.57 g / cm 3 ) after push-in test using the pyramidal indenter at a maximum force of 40 mN.
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Fig. 4. Relation between indentation load and damage size observed on the C / C cross-section ( r 51.57 g / cm 3 ) in the single fiber push-in test using a pyramidal indenter.
displacement curves obtained by this test were smooth as shown in Fig. 5 by a solid line.
3.1.1.2. Spherical indenter. Using the spherical indenter as shown Fig. 1b, push-in tests were also conducted. In this test, though no median cracks were observed, a crack suddenly appeared and extended across the fiber at an applied load exceeding 60 mN as shown in Fig. 6. Thus, the maximum load applicable to this indenter was determined to be 60 mN. As shown in Fig. 6, observations using SEM following the tests revealed that even at a load limit of 60 mN, the interfacial debonding occurred only partially just as in the case of using the pyramidal indenter. Hence, determination of interfacial debonding stress by
Fig. 5. Relation between force and indenter displacement in C / C ( r 51.57 g / cm 3 ) obtained by single fiber push-in test using pyramidal and spherical indenters without gold coating.
Fig. 6. Scanning electron micrograph of the surface of goldcoated C / C ( r 51.57 g / cm 3 ) after push-in test using a spherical indenter at a maximum force of 60 mN.
means of the single fiber push-in test using the spherical indenter was also concluded to be impossible. A typical load–displacement curve obtained by the push-in test using the spherical indenter is shown in Fig. 5 by dashed line.
3.1.2. Push-out test Fig. 7 shows the results of the single fiber push-out tests using the spherical indenter for C / C having a thickness of 69 mm and a density of 1.57 g / cm 3 . Note that the force– displacement curves produced in the fiber push-out tests include a flat region. Since these tests were conducted under constant loading rate, the flat regions indicate abrupt displacements, in other words, complete debonding of the fiber interfaces. In order to confirm this debonding, the specimen surfaces were observed via SEM. The photo-
Fig. 7. Relation between force and indenter displacement in C / C (t569 mm, r 51.57 g / cm 3 ) obtained by single fiber push-out test using a spherical indenter.
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Fig. 8. Scanning electron micrographs of a C / C surface (t569 mm, r 51.57 g / cm 3 ) after push-out test using a spherical indenter. (a) Maximum force; 70 mN, (b) maximum force; 44 mN.
graphs shown in Fig. 8a and b were taken after the flat regions appearing in the force–displacement curves, respectively. When the load surpassed the flat load, a compressive fracture around the loaded fiber was clearly observed as shown in Fig. 8a. On the other hand, only a pushed-out fiber without damage to the surrounding matrix was observed for specimens loaded up to the flat load as shown in Fig. 8b. Based on these observations, the interfacial debonding shear stress was determined based on the load of the flat region using Eq. (1) only in the case of the flat load being less than 60 mN. The resulting interfacial shear debonding stress was about 37.3 MPa. As well, similar flat regions in the force–displacement curves were found in the case of the pyramidal indenter. The average interfacial strength in this case was 24.6 MPa.
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This method is easy to perform and specimen preparation is also easy because rather thick specimens can be used. The other merit of the bundle push-out test is that sliding load can be easily determined in addition to debonding load. The present testing apparatus for the single fiber push-out test applies force at a constant loading rate. Thus, the machine does not permit load drop. On the other hand, in the bundle push-out test, a load can be applied using a general purpose mechanical testing machine. Thus the sliding load can be easily detected. Fig. 9 shows a typical result of the fiber bundle push-out tests. A fiber bundle in this case was pushed out at an applied (maximum) load of 1 N. As can be seen in Fig. 10a and b, fracture was confirmed to occur almost along the interface between the fiber and matrix, but with a small amount of matrix failure. Thus, this method was judged to be useful for the rough evaluation of interfacial shear strength. Substitution of the maximum load into Eq. (1) yielded an interfacial shear debonding stress of 17.2 MPa. After the application of the maximum load, sliding along the interface occurred as indicated in Fig. 9. Using this load, the interfacial sliding stress (sliding load divided by sliding area) was determined to be 6.0 MPa. Fig. 11 shows the relation between specimen thickness and the interfacial debonding and sliding stresses. As shown in this figure, the interfacial debonding and sliding stresses were confirmed to only slightly depend on specimen thickness. The interfacial debonding stresses obtained by the single fiber push-out test and the fiber bundle push-out tests were compared as a function of bulk density of the C / C in Fig. 12. As this figure shows, the interfacial debonding stresses obtained by the bundle push-out tests were much lower than those obtained by the single fiber tests. However, both sets of results showed a similar tendency. This indicates that the bundle push-out test is useful at least for compara-
3.2. Fiber-bundle push-out tests The single fiber push-out test was rather difficult to perform. First of all, the preparation of a thin specimen required for the single fiber push-out test is difficult, especially when the interfacial strength is low. In addition, the single fiber push-out test requires SEM observations to confirm that the indenter is pushing at near the center of a fiber. Hence, another method was explored to more easily evaluate interfacial strength. The present study proposes the bundle push-out test for performing this evaluation.
Fig. 9. Typical relation between force and indenter displacement of C / C (t5155 mm, r 51.81 g / cm 3 ) obtained by fiber bundle push-out test.
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Fig. 12. Comparison of interfacial debonding stresses obtained by single fiber push-out and fiber bundle push-out tests.
4. Discussion Fig. 10. Scanning electron micrographs of the surface of an indented specimen (t5155 mm, r 51.81 g / cm 3 ) after the bundle push-out test. (a) Fractured surface of the specimen after the test and (b) magnification of the fracture surface.
tive study. The difference shown by the two measuring techniques—bundle and single fiber push-out tests—is probably due to the difference in specimen geometry. The true characterization of the interface should include, for example, consideration of stress concentrations appearing around the free surfaces. This analysis is now underway.
Interfacial strengths have been successfully determined for ceramic matrix composites (CMCs) [12–24], but not for C / Cs. In the case of CMCs, the importance of interface characterization for the optimization of mechanical responses was understood in their early development stages. These efforts were motivated by analytical models, which include the interfacial strength or toughness as a ruling parameter of macroscopic mechanical responses. Hence, much effort was devoted to characterization of the interfaces. In contrast, in the case of C / Cs, analysis of their mechanical properties is still in a primitive stage [28]. Thus, the effects of interfacial strength on various mechanical properties, for example, strength and toughness, are not clearly understood, and only a few studies [7–11] have been performed regarding the characterization of these interfaces. In addition to this background, the following difficulties should be noted for C / C experiments.
4.1. Fiber compressive failure
Fig. 11. Relation between interfacial debonding and sliding stresses and specimen thickness obtained by fiber bundle push-out test of C / C (t5155 mm, r 51.81 g / cm 3 ).
The average compressive stress over the fiber cross section under the permitted load, 60 mN, for the single fiber push-in tests is about 3 GPa. Taking stress concentration into account, real compressive stress at the contact area of the pyramidal or spherical indenter easily surpasses the compressive strength of the carbon fiber, approximately 4–6 GPa [29]. On the other hand, ceramic fibers have much higher compressive strength than do carbon fibers, the compressive strength of ceramic fiber being more than one order higher than its tensile strength. Thus, a greater load can be applied to CMCs than that to
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C / Cs. This indicates that a load applicable to a ceramic fiber should be much higher than that to a carbon fiber.
4.2. Scale of fiber diameter The diameter of the present carbon fiber is 5.3 mm, while that of ceramic fiber is usually several times larger than carbon fiber; for example, SiC fiber (Nicaron) is about 15 mm. Thus, in the case of CMCs, it is much easier to find the indentation and to observe the propagation of the interfacial debonding during the test as compared with C / Cs.
4.3. Specimen preparation The lower compressive strength and smaller diameter of carbon fiber require thinner specimens than CMCs. This requirement leads to difficulty in specimen preparation, especially when interfacial strength is low; specimens often fracture during preparation. The fiber bundle pushout test permits the use of a thicker specimen than does the single fiber push-out test. Even in the fiber bundle push-out test, the applicable load is restricted to (cross sectional area)3(compressive strength of C / C; about 600 MPa). When a 50 mm needle is used for pushing out, the load should be less than 1.2 N, and this load requires a thickness of less than 250 mm.
5. Conclusions In order to establish a technique for measuring the interfacial strength of C / Cs, fiber bundle push-out and single fiber push-in and -out tests were performed. The following conclusions were drawn from this study: 1. The interfacial debonding strength and the interfacial sliding strength of C / Cs could be approximately determined by means of a fiber bundle push-out test. 2. Interfacial strength was successfully determined by single fiber push-out tests but not by push-in tests because of the high load required by the latter method. 3. Single fiber push-out and fiber bundle push-out tests resulted in a similar tendency in regard to interfacial strength, but expressed different absolute values.
Acknowledgements This research was partly supported by a grant-in-aid for basic science (No. 11305047) from the Ministry of Education, Sports, Culture, Science and Technology of Japan.
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