- Email: [email protected]
Interface-like fracture mechanism in pyrolytic carbon matrix-based carbon–carbon composites
March 2001
Materials Letters 48 Ž2001. 117–120 www.elsevier.comrlocatermatlet
Interface-like fracture mechanism in pyrolytic carbon matrix-based carbon–carbon composites Xianghui Hou a,) , Hejun Li b, Shouyang Zhang b, Jian Shen a a
b
College of Chemistry and Chemical Engineering, Nanjing UniÕersity, Nanjing, People’s Republic of China College of Materials Science and Engineering, Northwestern Polytechnical UniÕersity, Xi’an, People’s Republic of China Received 24 March 2000; received in revised form 16 August 2000; accepted 18 August 2000
Abstract Interface fracture mechanism is specially important for the properties of composites. In the present research of carbon–carbon composites, an interface-like fracture mechanism in the pyrolytic carbon matrix is first reported. The interface-like fracture can provide carbon–carbon composites with significantly improved strength and toughness, which has the effects similar to the appropriate interface fracture mechanism between fibers and matrix. This special behavior can be considered as a useful supplement for the traditional conception of interface fracture of composites. q 2001 Elsevier Science B.V. All rights reserved. Keywords: A. composites; Interfaces; D. fracture; Mechanical properties
1. Introduction The interface fracture mechanism influences the mechanical properties of composites greatly w1–3x. Under external loading, the interfacial behavior, which may significantly affect the properties of composites, includes interface debonding, pull-out of fibers, rupture of fibers, and the effects of weakening or terminating microcracks at interfacial regions. Appropriate interface damage modes will result in significant improvement for fracture properties of composites, especially for brittle matrix composites Žceramic matrix, carbon matrix, etc... Depending upon the relative interfacial shear strength and the cohesive strength of matrix, the )
Corresponding author. Fax: q86-25-359-4933. E-mail address: hou – [email protected] ŽX. Hou..
damage mechanisms in composites are usually divided into three categories: interface fracture, cohesive fracture of matrix, and mixed fracture w4x. Among these mechanisms, appropriate interface fracture can provide composites with high strength, good toughness and improved elongation. In this paper, we first find a interface-like fracture mechanism in pyrolytic carbon matrix of carbon–carbon composites, which plays a similar role to an appropriate interface fracture between fibers and matrix, and results in high fracture properties of composites.
2. Experimental Preforms used in this experiment were prepared by lamination technique with 1K high-strength PAN carbon woven cloth. These preforms were disc ge-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 9 0 - 1
X. Hou et al.r Materials Letters 48 (2001) 117–120
118
Table 1 Deposition conditions and flexural properties of different samples Type
I
II
III
Deposition temperature Ž8C. Precursor flow rate Žsccm. Flexural strength ŽMPa. Flexural modulus ŽGPa. Fracture displacement Žmm.
800–900
1200–1300
1000–1100
4800
3000
4000
213.3"15.8
189.4"7.7
241.6"12.0
28.4"0.7
25.3"0.6
27.6"0.9
0.49"0.03
0.58"0.02
0.64"0.02
ometry and their size were all f 100 = 5mm. The fiber volume fraction of preforms was controlled to about 40%. Then preforms were put into furnace and densified by chemical vapor infiltration ŽCVI. process. The infiltration process was operated under negative gas pressure with propylene as precursor, and deposition temperature in 800–13008C. After several hundred hours of infiltration, the 2D carbon–carbon components were achieved with bulk density about 1.70–1.73 grcm3 Žthe theoretical density was around 1.83 grcm3 .. With different deposition temperature and precursor flow rate, three types of composites were obtained. The deposition condition of every CVI process was given in Table 1. Samples for mechanical properties test were cut from the composites, with rectangular cross-section 10 mm in width, 4 mm in thickness, and 55 mm in length. Three-point bend testing was performed using a CSS-1100 Electric Mechanical Tester. The loading–displacement curves were obtained during loading. Between five and seven samples were used for each type of composites. The average flexural strength and displacement were calculated. Toughness in the material was evaluated from the relative shape of the loading–displacement curves.
respectively. It can be easily determined that Type III has better flexural properties, compared with others. The flexural strength of Type III is 241.6 " 12.0 MPa, which is about 10–30% higher than those of Types I and II. Moreover, Type III also exhibits a high fracture displacement and the shape of the loading–displacement curve indicates an improvement in the toughness of these composites, compared to Types I and II. To explain the above results, the matrix and interface in different composites were observed using SEM, as well as the fracture surfaces ŽFig. 2a, b, c.. It should be noted that in all three types of composites mixed fracture occurs. Interfacial damage and matrix cohesive damage phenomena can be both observed in these carbon–carbon composites. The predominant fracture mechanisms are quite different in each composite. In Type I, the fracture mainly takes place at the interface and can be described as interfacial debonding ŽFig. 2a.. After damage, it can be seen that there is little pyrolytic carbon adhered on the carbon fibers. In Type II, apparently circular microcracks exist in the pyrolytic carbon matrix after the densification process. Under external loading, these microcracks may lead to large-scale delamination in the matrix ŽFig. 2b., and cause the failure of composites. For Type III ŽFig. 2c., the dominant damage also occurs inside the matrix. But unlike
3. Results and discussion Carbon–carbon samples densified by different CVI process have different microstructure and mechanical behavior. The flexural properties of different composites are listed in Table 1, and the typical loading–displacement curves are given in Fig. 1,
Fig. 1. Typical loading–displacement curves of different samples under flexural loading.
X. Hou et al.r Materials Letters 48 (2001) 117–120
119
Fig. 2. Predominant damage morphology in different carbon–carbon composites. Ža. Type I, Žb. Type II, Žc. Type III.
damage in Type II, the propagation paths of crack are no longer straight, but a step-like mode. It appears from the fracture behavior of the Type III composites that their high mechanical properties may directly be attributed to the step-delamination mechanism inside the matrix. During the CVI process, the deposition of pyrolytic carbon tends to form ring-shape laminar carbon layers around carbon fibers Žas in Fig. 3.. The strength and modulus of pyrolytic carbon is very large in the lamina plane, but small in the out-of-plane direction w5x. Thus, the cohesive fracture of carbon matrix is generally through the delamination of these carbon layers. When the Type III composites are loaded, since the interfacial strength between fibers and matrix is strong, cracks are mainly initiated at a carbon layer boundary in the matrix. As loading increases, these cracks begin to spread between the layers. However, the cohesive
strength between the carbon layers in Type III is much higher than that in Type II, and it is strong
Fig. 3. Laminar pyrolytic carbon layers around carbon fibers in carbon–carbon composites.
120
X. Hou et al.r Materials Letters 48 (2001) 117–120
enough to prevent the cracks from propagating linearly along their initial directions. The cracks turn to the adjacent layer boundary and continue to develop. Thus, only partial delamination of the layers occurs. As this course goes on, the special step-like fracture surfaces are formed finally. Compared with the general cohesive damage in pyrolytic carbon matrix, this special damage mode can sustain further loading and absorb more energy, which provides those carbon– carbon composites with a significant improvement in strength and toughness. It is interesting that the predominant damage in the matrix of Type III composites has some characteristics of the fracture of a moderately good interface. Firstly, it restrains the linear propagation of the cracks. Secondly, it can absorb more energy by partial delamination of the carbon layers and the step-like cracks. Thirdly, it makes composites more strong and tough. Thus, the fracture mechanism in the matrix involved in the Type III composites can be regarded as a special interface-like fracture. The step-like damage surfaces play a role similar to the interfacial fracture between fibers and matrix, and can be defined as ‘second interface’. The appearance of interface-like fracture has some prerequisite conditions. Firstly, the interfacial shear is high enough to restrain debonding between fibers and matrix. Secondly, the cohesive strength of the matrix must be suitable. If the cohesive strength is too high, the propagation of cracks in the matrix is limited, and the delamination of matrix does not occur. If the cohesive strength is too low, cracks will extend linearly along the pyrolytic carbon matrix Žlike in Fig. 2b., so that the step-like fracture surface cannot appear. The interface-like fracture mechanism in pyrolytic carbon matrix can be considered as a useful method for increasing the fracture properties of composites.
The interface strength is usually the key factor in composites. Strong interfaces restrain the debonding of interface and leads to brittle failure of composites. Poor interfacial bonding results in large-scale interfacial debonding under low loading. Only moderate interface strengths lead to brittle matrix composites with good fracture properties, such as high strength and high toughness. In this case, cracks in the composites turn at the interface between fibers and matrix, and induce the partial debonding phenomenon. However, in the Type III composites, because of the high interface strength, debonding of the interface does not occur. Partial delamination of the pyrolytic carbon layers comes into play and leads to the improved strength and toughness in carbon–carbon composites, which has the effects similar to the moderate interface fracture mechanism. 4. Conclusion An interface-like fracture mechanism in pyrolytic carbon matrix based carbon–carbon composites is reported. The interface-like fracture can provide carbon–carbon composites with significant improved strength and toughness, which has the effects similar to the moderate interface fracture mechanism between fibers and matrix. References w1x M. Sakai, T. Miyajima, M. Inagaki, Compos. Sci. Technol. 40 Ž1991. 231. w2x C. Ahearn, B. Rand, Carbon 34 Ž1996. 239. w3x S.M. Oh, J.Y. Lee, Carbon 27 Ž1989. 423. w4x X.J. Xian, R.Y. Li, Fracture Analysis and Microstructural Photograph of Composites, Science Press, Beijing, 1993, p. 46. w5x J.D. Buckley, D.D. Edie, Carbon–Carbon Materials and Composites, Park Ridge, New Jersey, 1993, p. 127.
Materials Letters 48 Ž2001. 117–120 www.elsevier.comrlocatermatlet
Interface-like fracture mechanism in pyrolytic carbon matrix-based carbon–carbon composites Xianghui Hou a,) , Hejun Li b, Shouyang Zhang b, Jian Shen a a
b
College of Chemistry and Chemical Engineering, Nanjing UniÕersity, Nanjing, People’s Republic of China College of Materials Science and Engineering, Northwestern Polytechnical UniÕersity, Xi’an, People’s Republic of China Received 24 March 2000; received in revised form 16 August 2000; accepted 18 August 2000
Abstract Interface fracture mechanism is specially important for the properties of composites. In the present research of carbon–carbon composites, an interface-like fracture mechanism in the pyrolytic carbon matrix is first reported. The interface-like fracture can provide carbon–carbon composites with significantly improved strength and toughness, which has the effects similar to the appropriate interface fracture mechanism between fibers and matrix. This special behavior can be considered as a useful supplement for the traditional conception of interface fracture of composites. q 2001 Elsevier Science B.V. All rights reserved. Keywords: A. composites; Interfaces; D. fracture; Mechanical properties
1. Introduction The interface fracture mechanism influences the mechanical properties of composites greatly w1–3x. Under external loading, the interfacial behavior, which may significantly affect the properties of composites, includes interface debonding, pull-out of fibers, rupture of fibers, and the effects of weakening or terminating microcracks at interfacial regions. Appropriate interface damage modes will result in significant improvement for fracture properties of composites, especially for brittle matrix composites Žceramic matrix, carbon matrix, etc... Depending upon the relative interfacial shear strength and the cohesive strength of matrix, the )
Corresponding author. Fax: q86-25-359-4933. E-mail address: hou – [email protected] ŽX. Hou..
damage mechanisms in composites are usually divided into three categories: interface fracture, cohesive fracture of matrix, and mixed fracture w4x. Among these mechanisms, appropriate interface fracture can provide composites with high strength, good toughness and improved elongation. In this paper, we first find a interface-like fracture mechanism in pyrolytic carbon matrix of carbon–carbon composites, which plays a similar role to an appropriate interface fracture between fibers and matrix, and results in high fracture properties of composites.
2. Experimental Preforms used in this experiment were prepared by lamination technique with 1K high-strength PAN carbon woven cloth. These preforms were disc ge-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 9 0 - 1
X. Hou et al.r Materials Letters 48 (2001) 117–120
118
Table 1 Deposition conditions and flexural properties of different samples Type
I
II
III
Deposition temperature Ž8C. Precursor flow rate Žsccm. Flexural strength ŽMPa. Flexural modulus ŽGPa. Fracture displacement Žmm.
800–900
1200–1300
1000–1100
4800
3000
4000
213.3"15.8
189.4"7.7
241.6"12.0
28.4"0.7
25.3"0.6
27.6"0.9
0.49"0.03
0.58"0.02
0.64"0.02
ometry and their size were all f 100 = 5mm. The fiber volume fraction of preforms was controlled to about 40%. Then preforms were put into furnace and densified by chemical vapor infiltration ŽCVI. process. The infiltration process was operated under negative gas pressure with propylene as precursor, and deposition temperature in 800–13008C. After several hundred hours of infiltration, the 2D carbon–carbon components were achieved with bulk density about 1.70–1.73 grcm3 Žthe theoretical density was around 1.83 grcm3 .. With different deposition temperature and precursor flow rate, three types of composites were obtained. The deposition condition of every CVI process was given in Table 1. Samples for mechanical properties test were cut from the composites, with rectangular cross-section 10 mm in width, 4 mm in thickness, and 55 mm in length. Three-point bend testing was performed using a CSS-1100 Electric Mechanical Tester. The loading–displacement curves were obtained during loading. Between five and seven samples were used for each type of composites. The average flexural strength and displacement were calculated. Toughness in the material was evaluated from the relative shape of the loading–displacement curves.
respectively. It can be easily determined that Type III has better flexural properties, compared with others. The flexural strength of Type III is 241.6 " 12.0 MPa, which is about 10–30% higher than those of Types I and II. Moreover, Type III also exhibits a high fracture displacement and the shape of the loading–displacement curve indicates an improvement in the toughness of these composites, compared to Types I and II. To explain the above results, the matrix and interface in different composites were observed using SEM, as well as the fracture surfaces ŽFig. 2a, b, c.. It should be noted that in all three types of composites mixed fracture occurs. Interfacial damage and matrix cohesive damage phenomena can be both observed in these carbon–carbon composites. The predominant fracture mechanisms are quite different in each composite. In Type I, the fracture mainly takes place at the interface and can be described as interfacial debonding ŽFig. 2a.. After damage, it can be seen that there is little pyrolytic carbon adhered on the carbon fibers. In Type II, apparently circular microcracks exist in the pyrolytic carbon matrix after the densification process. Under external loading, these microcracks may lead to large-scale delamination in the matrix ŽFig. 2b., and cause the failure of composites. For Type III ŽFig. 2c., the dominant damage also occurs inside the matrix. But unlike
3. Results and discussion Carbon–carbon samples densified by different CVI process have different microstructure and mechanical behavior. The flexural properties of different composites are listed in Table 1, and the typical loading–displacement curves are given in Fig. 1,
Fig. 1. Typical loading–displacement curves of different samples under flexural loading.
X. Hou et al.r Materials Letters 48 (2001) 117–120
119
Fig. 2. Predominant damage morphology in different carbon–carbon composites. Ža. Type I, Žb. Type II, Žc. Type III.
damage in Type II, the propagation paths of crack are no longer straight, but a step-like mode. It appears from the fracture behavior of the Type III composites that their high mechanical properties may directly be attributed to the step-delamination mechanism inside the matrix. During the CVI process, the deposition of pyrolytic carbon tends to form ring-shape laminar carbon layers around carbon fibers Žas in Fig. 3.. The strength and modulus of pyrolytic carbon is very large in the lamina plane, but small in the out-of-plane direction w5x. Thus, the cohesive fracture of carbon matrix is generally through the delamination of these carbon layers. When the Type III composites are loaded, since the interfacial strength between fibers and matrix is strong, cracks are mainly initiated at a carbon layer boundary in the matrix. As loading increases, these cracks begin to spread between the layers. However, the cohesive
strength between the carbon layers in Type III is much higher than that in Type II, and it is strong
Fig. 3. Laminar pyrolytic carbon layers around carbon fibers in carbon–carbon composites.
120
X. Hou et al.r Materials Letters 48 (2001) 117–120
enough to prevent the cracks from propagating linearly along their initial directions. The cracks turn to the adjacent layer boundary and continue to develop. Thus, only partial delamination of the layers occurs. As this course goes on, the special step-like fracture surfaces are formed finally. Compared with the general cohesive damage in pyrolytic carbon matrix, this special damage mode can sustain further loading and absorb more energy, which provides those carbon– carbon composites with a significant improvement in strength and toughness. It is interesting that the predominant damage in the matrix of Type III composites has some characteristics of the fracture of a moderately good interface. Firstly, it restrains the linear propagation of the cracks. Secondly, it can absorb more energy by partial delamination of the carbon layers and the step-like cracks. Thirdly, it makes composites more strong and tough. Thus, the fracture mechanism in the matrix involved in the Type III composites can be regarded as a special interface-like fracture. The step-like damage surfaces play a role similar to the interfacial fracture between fibers and matrix, and can be defined as ‘second interface’. The appearance of interface-like fracture has some prerequisite conditions. Firstly, the interfacial shear is high enough to restrain debonding between fibers and matrix. Secondly, the cohesive strength of the matrix must be suitable. If the cohesive strength is too high, the propagation of cracks in the matrix is limited, and the delamination of matrix does not occur. If the cohesive strength is too low, cracks will extend linearly along the pyrolytic carbon matrix Žlike in Fig. 2b., so that the step-like fracture surface cannot appear. The interface-like fracture mechanism in pyrolytic carbon matrix can be considered as a useful method for increasing the fracture properties of composites.
The interface strength is usually the key factor in composites. Strong interfaces restrain the debonding of interface and leads to brittle failure of composites. Poor interfacial bonding results in large-scale interfacial debonding under low loading. Only moderate interface strengths lead to brittle matrix composites with good fracture properties, such as high strength and high toughness. In this case, cracks in the composites turn at the interface between fibers and matrix, and induce the partial debonding phenomenon. However, in the Type III composites, because of the high interface strength, debonding of the interface does not occur. Partial delamination of the pyrolytic carbon layers comes into play and leads to the improved strength and toughness in carbon–carbon composites, which has the effects similar to the moderate interface fracture mechanism. 4. Conclusion An interface-like fracture mechanism in pyrolytic carbon matrix based carbon–carbon composites is reported. The interface-like fracture can provide carbon–carbon composites with significant improved strength and toughness, which has the effects similar to the moderate interface fracture mechanism between fibers and matrix. References w1x M. Sakai, T. Miyajima, M. Inagaki, Compos. Sci. Technol. 40 Ž1991. 231. w2x C. Ahearn, B. Rand, Carbon 34 Ž1996. 239. w3x S.M. Oh, J.Y. Lee, Carbon 27 Ž1989. 423. w4x X.J. Xian, R.Y. Li, Fracture Analysis and Microstructural Photograph of Composites, Science Press, Beijing, 1993, p. 46. w5x J.D. Buckley, D.D. Edie, Carbon–Carbon Materials and Composites, Park Ridge, New Jersey, 1993, p. 127.