Availableonlineatwww.sciencedirect.com ScienceDirect materials letters ELSEVIER Materials Letters 61(2007)312-315 www.elsevier.com/locate/matlet Preparation and mechanical properties of c/Sic composites with carbon fiber woven preform Junqiang Ma", Yongdong Xu, Litong Zhang, Laifei Cheng, Jingjiang Nie, Hong L National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China Received 19 December 2006; accepted 9 April 2006 Available online 17 May 2006 Abstract o C/SiC composites reinforced with multilayer carbon fiber woven preforms were fabricated by isothermal chemical vapor infiltration (ICVD) rocess. To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tension, bending and shear loads. The results indicated that the composites, with superior intrinsic through-the-thickness properties exhibited high in-plane mechanical properties. Therefore, the composites developed can well meet the demands of the reusable nose cap, i.e. the easiness of near-net shaping and the capability of withstanding multidirectional mechanical and thermal stresses. o 2006 Elsevier B V. All rights reserved eywords: Composite materials; Chemical vapor infiltration: Multilayer preform; Mechanical properties 1. Introduction The objective of this paper is to assess the mechanical properties of the novel C/SiC composites under various loading Fiber reinforced composite materials are widely used in the conditions, including tension, bending and shear loads, which design of thermostructural components like nozzles, nose cap, etc. are expected to provide valuable information for effectively for its acceptable performance at elevated temperatures [1,2]. utilizing the composites to manufacture the very demanding Traditional C/C nose cap is fabricated by stacking bi-directional nose cap carbon cloth one over the other on a horizontal plane. However, when the carbon cloth is shaped around the dome-shaped mold, 2. Experimental the carbon cloth will undergo in-plane shear deformation, which is accompanied by the changes of intemal microstructure. These 2. I. Fabrication of the composites variations in intemal geometry have much influence on local thermal conductivity [3 and make it difficult to develop the The carbon fiber utilized was T300 6 K carbon fiber from the models which predict the stiffness and strength properties of the Nippon Toray corporation. The weave pattern of the carbon fiber composites [4]. Further, the very tough reentry environment [5 preforms is illustrated in Fig. 1(a) and their weave parameters are can significantly degrade the C/C nose cap and lower its reliability presented in Table 1. Composite panels were processed by and reusability isothermal chemical vapor infiltration (ICVi)to deposit pyrolytic The solution to the above problems associated with the carbon interphase (0. 2 um thickness)and Sic matrix, which has luction of the nose cap using 2D C/C composites is the use been described previously in full detail [6]. The resultant com- SiC composites reinforced with near-net shaped multilayer posites had a nominal fiber volume fraction of 45%0, an open orms. Most recently, the authors have applied the unique porosity of 14-17% and a density of 1.9-2.1 g/em multilayer preforms to successfully produce the near-net shaped C/SiC nose cap by the Icvi process 2.2. Mechanical testing s Corresponding author. Tel. +86 29 88494616: fax: +86 29 88494620 Of a total of 15 composite panels processed in the same fur nace runs, five were cut into test specimens along the warp and 0167-577X/S-see front matter e 2006 Elsevier B V. All rights reserved. doi:10.1016 malet006.04.099
Preparation and mechanical properties of C/SiC composites with carbon fiber woven preform Junqiang Ma ⁎, Yongdong Xu, Litong Zhang, Laifei Cheng, Jingjiang Nie, Hong Li National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China Received 19 December 2006; accepted 9 April 2006 Available online 17 May 2006 Abstract C/SiC composites reinforced with multilayer carbon fiber woven preforms were fabricated by isothermal chemical vapor infiltration (ICVI) process. To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tension, bending and shear loads. The results indicated that the composites, with superior intrinsic through-the-thickness properties, exhibited high in-plane mechanical properties. Therefore, the composites developed can well meet the demands of the reusable nose cap, i.e. the easiness of near-net shaping and the capability of withstanding multidirectional mechanical and thermal stresses. © 2006 Elsevier B.V. All rights reserved. Keywords: Composite materials; Chemical vapor infiltration; Multilayer preform; Mechanical properties 1. Introduction Fiber reinforced composite materials are widely used in the design of thermostructural components like nozzles, nose cap, etc. for its acceptable performance at elevated temperatures [1,2]. Traditional C/C nose cap is fabricated by stacking bi-directional carbon cloth one over the other on a horizontal plane. However, when the carbon cloth is shaped around the dome-shaped mold, the carbon cloth will undergo in-plane shear deformation, which is accompanied by the changes of internal microstructure. These variations in internal geometry have much influence on local thermal conductivity [3] and make it difficult to develop the models which predict the stiffness and strength properties of the composites [4]. Further, the very tough reentry environment [5] can significantly degrade the C/C nose cap and lower its reliability and reusability. The solution to the above problems associated with the production of the nose cap using 2D C/C composites is the use of C/SiC composites reinforced with near-net shaped multilayer preforms. Most recently, the authors have applied the unique multilayer preforms to successfully produce the near-net shaped C/SiC nose cap by the ICVI process. The objective of this paper is to assess the mechanical properties of the novel C/SiC composites under various loading conditions, including tension, bending and shear loads, which are expected to provide valuable information for effectively utilizing the composites to manufacture the very demanding nose cap. 2. Experimental 2.1. Fabrication of the composites The carbon fiber utilized was T300 6 K carbon fiber from the Nippon Toray corporation. The weave pattern of the carbon fiber preforms is illustrated in Fig. 1(a) and their weave parameters are presented in Table 1. Composite panels were processed by isothermal chemical vapor infiltration (ICVI) to deposit pyrolytic carbon interphase (0.2 μm thickness) and SiC matrix, which has been described previously in full detail [6]. The resultant composites had a nominal fiber volume fraction of 45%, an open porosity of 14–17% and a density of 1.9–2.1 g/cm3 . 2.2. Mechanical testing Of a total of 15 composite panels processed in the same furnace runs, five were cut into test specimens along the warp and Materials Letters 61 (2007) 312–315 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 29 88494616; fax: +86 29 88494620. E-mail address: junqiangma@kcnh.com (J. Ma). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.099
J. Ma et al. Materials Letters 61(2007)312-315 (a) Warp yarn Warp yarn Weft yarn Fig. 1. Weave patten of the carbon fiber preform and the three-dimensional view of the composites (a)Weave pattem of the carbon fiber preform (b)Three- imensional view of the composite weft directions, and then the specimens were sealed with CVd further qualitative analysis. It is seen that the warp yarns presented the Sic to cover all exposed fiber ends. All the mechanical testing shape of a quasi rectangle and contacted tightly with each other, while the was performed on Instron 1196 machine at room temperature. weft yams presented the shape of a lentil and parallelogram alternately Tension tests were performed according to the ASTM C1275 along the warp direction and distributed separately with each other. All test method. Three-point bending tests with a span-to-depth these phenomena are attributed to the combination of compaction in the ratio of 16 were conducted according to the ASTM C1341 test the carbon fiber preforms method. In-plane shear tests were carried out based on the guidelines of the ASTM C1292 test method. Short-beam shear 3.2. Tension tests tests were conducted to measure interlaminar shear strength according to the ASTM D2344 test method; the span-to-depth The typical tensile stress-strain curves of the composites are shown ratio was approximately 5 in Fig. 2. It is seen that all the stress-strain curves exhibited largely nonlinear behavior, which can be qualitatively understood by the 3. Results and discussion processing-induced damage and damage accumulation occurring in the composites under increasing tensile stresses. The damage accumulation 3. 1. Carbon fiber preform architecture includes the matrix cracking and interfacial debonding/sliding which had been discussed in full detail in literature [7]. The tension test data It is a well-known fact that the macro-mechanical properties of fiber are summarized in Table 2, and it is seen that the composites had much reinforced composite materials are primarily dependent on their fiber higher tensile properties in the direction than those in the weft preform architecture, so a thorough understanding of their fiber preform direction, which are attributed to the large differences in yarn density architecture can facilitate the qualitative analysis of their mechanical and yarn path between warp and weft yarns. properties. Closer observations of fracture surfaces revealed that the fracture It can be seen from Fig. 1(a) that the carbon fiber preforms are some warp yarns occurred mostly in the yarn crossover areas, while the variations of layer-to-layer angle interlock fabric and consist of two sets of yams. The weft yams, which take a nominally straight path, create the Y axis portion; while the warp yarns, which take a nominally sinusoidal path, travel from one weft layer to the adjacent weft layer, and back, thus holding together all the weft layers. Obviously, the warp yams undertake dual roles. Firstly, they create the X axis portion which contributes mainly warp direction to in-plane strength, and secondly, they form through-the-thickness reinforcements which contribute to improved delamination resistance, thereby the superior in-service impact resistance. Considering that the mechanical properties of the composites depend upon their final fiber preform architecture, the three-dimensional view of weft direction the real composites was illustrated in Fig. 1(b) for visualization and parameters of the fiber preforms 000.1020.30.4050.60.70.8 layers Ends/cm/layer Picks/cm/layer fraction Warp( K) 6 10 27.7 Weft(6 K) Fig. 2. Tensile stress-strain curves for two loading directions showing mostly
weft directions, and then the specimens were sealed with CVD SiC to cover all exposed fiber ends. All the mechanical testing was performed on Instron 1196 machine at room temperature. Tension tests were performed according to the ASTM C1275 test method. Three-point bending tests with a span-to-depth ratio of 16 were conducted according to the ASTM C1341 test method. In-plane shear tests were carried out based on the guidelines of the ASTM C1292 test method. Short-beam shear tests were conducted to measure interlaminar shear strength according to the ASTM D2344 test method; the span-to-depth ratio was approximately 5. 3. Results and discussion 3.1. Carbon fiber preform architecture It is a well-known fact that the macro-mechanical properties of fiber reinforced composite materials are primarily dependent on their fiber preform architecture, so a thorough understanding of their fiber preform architecture can facilitate the qualitative analysis of their mechanical properties. It can be seen from Fig. 1(a) that the carbon fiber preforms are some variations of layer-to-layer angle interlock fabric and consist of two sets of yarns. The weft yarns, which take a nominally straight path, create the Y axis portion; while the warp yarns, which take a nominally sinusoidal path, travel from one weft layer to the adjacent weft layer, and back, thus holding together all the weft layers. Obviously, the warp yarns undertake dual roles. Firstly, they create the X axis portion which contributes mainly to in-plane strength, and secondly, they form through-the-thickness reinforcements which contribute to improved delamination resistance, thereby the superior in-service impact resistance. Considering that the mechanical properties of the composites depend upon their final fiber preform architecture, the three-dimensional view of the real composites was illustrated in Fig. 1(b) for visualization and further qualitative analysis. It is seen that the warp yarns presented the shape of a quasi rectangle and contacted tightly with each other, while the weft yarns presented the shape of a lentil and parallelogram alternately along the warp direction and distributed separately with each other. All these phenomena are attributed to the combination of compaction in the graphite fixture during the CVI process and symmetrical characteristics of the carbon fiber preforms. 3.2. Tension tests The typical tensile stress–strain curves of the composites are shown in Fig. 2. It is seen that all the stress–strain curves exhibited largely nonlinear behavior, which can be qualitatively understood by the processing-induced damage and damage accumulation occurring in the composites under increasing tensile stresses. The damage accumulation includes the matrix cracking and interfacial debonding/sliding, which had been discussed in full detail in literature [7]. The tension test data are summarized in Table 2, and it is seen that the composites had much higher tensile properties in the warp direction than those in the weft direction, which are attributed to the large differences in yarn density and yarn path between warp and weft yarns. Closer observations of fracture surfaces revealed that the fracture of warp yarns occurred mostly in the yarn crossover areas, while the Fig. 1. Weave pattern of the carbon fiber preform and the three-dimensional view of the composites. (a) Weave pattern of the carbon fiber preform. (b) Threedimensional view of the composites. Table 1 Weave parameters of the fiber preforms Weave direction Number of yarn layers Yarn density Fiber volume fraction Ends/cm/layer Picks/cm/layer Warp (6 K) 6 10 27.7 Weft (6 K) 7/5 3 8.3 Fig. 2. Tensile stress–strain curves for two loading directions showing mostly nonlinear behavior. J. Ma et al. / Materials Letters 61 (2007) 312–315 313
314 J. Ma et al./ Materials Letters 61(2007)312-315 Table 2 Table 3 Tension test data of the composites Bending test data of the composites Specimen Tensile Strength(MPa) Failure strain(%)Initial m Loading Flexural Strain at flexural Strain at fracture direction strength modulus ngth (%) direction 326(35) 06920.07) 153(16) Weft direction 145(16) 0.70500.04 522(48) (11) Standard deviations are given in parentheses. 313 1(4.6)0.780.052)0.790.054 deviations are fracture of weft yams occurred at random positions. When the loads s are subjected to tension along the warp direction, the tensile strength, it was just the reverse. The relatively lower strain at the flexural loads are primarily carried by warp yams, thus the straightening strength of the warp-direction specimen further confirms large stress tendency of warp yarns will cause high values of stresses in yam concentrations in the yarn crossover areas crossover areas. Such stress concentrations can trigger localized damage in warp yarn even when the overall applied tensile stress is 3. 4. Shear tests much lower than the ultimate tensile strength. This can well explain why the fracture of warp yarns mostly occurred in yam crossover areas. In Considering that the thermostructural components will undergo ontrast, when the specimens are subjected to tension weft-wise, the complex loads during service, it is valuable to carry out shear tests, be- tensile loads are primarily carried by weft yams. Obviously, almost no cause shear tests can provide information on the strength and stress concentrations exist in yarn crossover areas, therefore the weft deformation of materials under shear stresses yarns fractured at random positions. Typical in-plane shear stress-strain curves are shown in Fig. 4. It can be seen that the specimens cut along the warp and weft directions 3.3. Bending tests had comparable shear strength (listed in Table 4)and exhibited similar stress-strain trends. All the curves displayed nonlinearity from o The typical flexural strength-displacement curves are shown in Fig. 3, the very beginning due to the processing-induced matrix cracks With me of the test data are summarized in Table 3. It is seen that all the increasing the applied loads the degree of nonlinearity was increased curves exhibited similar trends except for the stress history after peak due to the damage accumulation, including the multiplication of stresses For the specimen cut along the warp direction, the curve showed matrix cracks, the propagation of cracks along the fiber directions and nonlinearity to a displacement value of 1.01 mm to reach the peak stress the failure of some fibers by tension in the transverse-loading (522 MPa). After the peak stress, the stress dropped a little and then directions. At the last stage, the applied loads were maintained due to remained in a large range of displacement until the primary load drop the friction of fiber and fiber bundle pullout until the final failure of occurs at a displacement of about 1.37 mm. The crucial phenomenon is the specimens believed to be attributed to a lockup mechanism involving yam waviness To further investigate the shear properties of the composites, short- and pinching feature of the adjacent warp yams. In the case of the beam shear tests were conducted to determine the interlaminar shear specimen cut along the weft direction, the curve displayed similar strength. The test results are presented in Table 4. The specimen that nonlinearity to a displacement value of 1.25 mm to reach the peak stress was cut along the warp direction had much higher interlaminar shear (313 MPa). However, after the peak stress, the stress descended rapidly in strength than the specimen cut along the weft direction, which a steep manner. It is also noted that the strain at the flexural strength of the attributed to the difference in density between warp and weft yarns. It is men cut along the warp direction was much lower than that of the evident that the composites displayed higher interlaminar shear men cut along the weft direction, while for the strain at fracture properties relative to their 2D counterparts reported in literature [8], warp direction 雪20 weft direction 0.00.20.40.60.81012141.61.8 002040.60.81.012141.61.8 Displacement(mm) Strain (%) and, weft direct isnrengthi-displacement curves tor specimens cut along the warp wilt d inctiane shear stressestrain curves for specimens cut aiong the warp an
fracture of weft yarns occurred at random positions. When the specimens are subjected to tension along the warp direction, the tensile loads are primarily carried by warp yarns, thus the straightening tendency of warp yarns will cause high values of stresses in yarn crossover areas. Such stress concentrations can trigger localized damage in warp yarn even when the overall applied tensile stress is much lower than the ultimate tensile strength. This can well explain why the fracture of warp yarns mostly occurred in yarn crossover areas. In contrast, when the specimens are subjected to tension weft-wise, the tensile loads are primarily carried by weft yarns. Obviously, almost no stress concentrations exist in yarn crossover areas, therefore the weft yarns fractured at random positions. 3.3. Bending tests The typical flexural strength–displacement curves are shown in Fig. 3, and some of the test data are summarized in Table 3. It is seen that all the curves exhibited similar trends except for the stress history after peak stresses. For the specimen cut along the warp direction, the curve showed nonlinearity to a displacement value of 1.01 mm to reach the peak stress (522 MPa). After the peak stress, the stress dropped a little and then remained in a large range of displacement until the primary load drop occurs at a displacement of about 1.37 mm. The crucial phenomenon is believed to be attributed to a lockup mechanism involving yarn waviness and pinching feature of the adjacent warp yarns. In the case of the specimen cut along the weft direction, the curve displayed similar nonlinearity to a displacement value of 1.25 mm to reach the peak stress (313 MPa). However, after the peak stress, the stress descended rapidly in a steep manner. It is also noted that the strain at the flexural strength of the specimen cut along the warp direction was much lower than that of the specimen cut along the weft direction, while for the strain at fracture strength, it was just the reverse. The relatively lower strain at the flexural strength of the warp-direction specimen further confirms large stress concentrations in the yarn crossover areas. 3.4. Shear tests Considering that the thermostructural components will undergo complex loads during service, it is valuable to carry out shear tests, because shear tests can provide information on the strength and deformation of materials under shear stresses. Typical in-plane shear stress–strain curves are shown in Fig. 4. It can be seen that the specimens cut along the warp and weft directions had comparable shear strength (listed in Table 4) and exhibited similar stress–strain trends. All the curves displayed nonlinearity from the very beginning due to the processing-induced matrix cracks. With increasing the applied loads the degree of nonlinearity was increased due to the damage accumulation, including the multiplication of matrix cracks, the propagation of cracks along the fiber directions and the failure of some fibers by tension in the transverse-loading directions. At the last stage, the applied loads were maintained due to the friction of fiber and fiber bundle pullout until the final failure of the specimens. To further investigate the shear properties of the composites, shortbeam shear tests were conducted to determine the interlaminar shear strength. The test results are presented in Table 4. The specimen that was cut along the warp direction had much higher interlaminar shear strength than the specimen cut along the weft direction, which is attributed to the difference in density between warp and weft yarns. It is evident that the composites displayed higher interlaminar shear properties relative to their 2D counterparts reported in literature [8], Table 2 Tension test data of the composites Specimen Tensile Strength (MPa) Failure strain (%) Initial modulus (GPa) Warp direction 326 (35) 0.692 (0.07) 153 (16) Weft direction 145 (16) 0.705 (0.04) 62 (2.8) Standard deviations are given in parentheses. Fig. 3. Flexural strength–displacement curves for specimens cut along the warp and weft directions. Table 3 Bending test data of the composites Loading direction Flexural strength (MPa) Flexural modulus (GPa) Strain at flexural strength (%) Strain at fracture strength (%) Warp 522 (48) 109 (11) 0.62 (0.071) 0.86 (0.093) Weft 313 (22) 51 (4.6) 0.78 (0.052) 0.79 (0.054) Standard deviations are given in parentheses. Fig. 4. In-plane shear stress–strain curves for specimens cut along the warp and weft directions. 314 J. Ma et al. / Materials Letters 61 (2007) 312–315
J. Ma et al. Materials Letters 61(2007)312-315 Table 4 composites developed can well meet the demands of the nose cap Shear test data of the composites i.e. the easiness of near-net shaping and the capability of with- ane shear strength(MPa) Interlaminar shear steer standing multidirectional mechanical and thermal stresses tion77(82) Standard deviations are given in parentheses The authors acknowledge the financial support of the Natural Science Foundation of China(Contract No. 90405015)and the which is ascribed to the intrinsic through-the-thickness reinforcements National Young Elitists Foundation( Contract No 50425208) of the fiber preforms References 4. Conclusion [ w.C. Chang, N.H. Tai, CC M. Ma, J Mater. Sci. 30(1995)1225 C/SiC composites reinforced with multilayer preforms were 2)CCM Ma, N.H. Tai, W.C. Chang. Y P. Tsal, Carbon 34(1996)1175 fabricated by isothermal chemical vapor infiltration (CvD) [4].A Naik, NASA Contract. Rep. 194930(1994) process. Their mechanical behavior under tension, bending and [5] Dr.-Ing Hermann Hald, Dipl.- Ing Hendrik Weihs, Dipl-Ing Thomas Reime shear loads exhibited great anisotropy, which was attributed to the DipL.-Min Thomas Ullmann. AIAA 2003-2696. differences in path and density between warp and weft yams. Due [6] Y.D. Xu, L.T. Zhang, L.E. Cheng, D. Yan, Carbon 36(1998)1051 to the intrinsic through-the-thickness reinforcements of the fiber [7] B Step preforms, the composites displayed higher interlaminar she [8] w. Krenkel, Ceram. Eng. Sci. Proc. 24 (4)(2003)583 properties compared to their 2D counterparts. Therefore, the
which is ascribed to the intrinsic through-the-thickness reinforcements of the fiber preforms. 4. Conclusion C/SiC composites reinforced with multilayer preforms were fabricated by isothermal chemical vapor infiltration (ICVI) process. Their mechanical behavior under tension, bending and shear loads exhibited great anisotropy, which was attributed to the differences in path and density between warp and weft yarns. Due to the intrinsic through-the-thickness reinforcements of the fiber preforms, the composites displayed higher interlaminar shear properties compared to their 2D counterparts. Therefore, the composites developed can well meet the demands of the nose cap, i.e. the easiness of near-net shaping and the capability of withstanding multidirectional mechanical and thermal stresses. Acknowledgements The authors acknowledge the financial support of the Natural Science Foundation of China (Contract No. 90405015) and the National Young Elitists Foundation (Contract No. 50425208). References [1] W.C. Chang, N.H. Tai, C.C.M. Ma, J. Mater. Sci. 30 (1995) 1225. [2] C.C.M. Ma, N.H. Tai, W.C. Chang, Y.P. Tsai, Carbon 34 (1996) 1175. [3] A. Dasgupta, R.K. Agarwal, J. Compos. Mater. 26 (1992) 2736. [4] R.A. Naik, NASA Contract. Rep. 194930 (1994). [5] Dr.-Ing Hermann Hald, Dipl.-Ing Hendrik Weihs, Dipl.-Ing Thomas Reimer, Dipl.-Min Thomas Ullmann. AIAA 2003–2696. [6] Y.D. Xu, L.T. Zhang, L.F. Cheng, D. Yan, Carbon 36 (1998) 1051. [7] B. Stephane, Compos. Sci. Technol. 61 (2001) 2285. [8] W. Krenkel, Ceram. Eng. Sci. Proc. 24 (4) (2003) 583. Table 4 Shear test data of the composites Specimen In-plane shear strength (MPa) Interlaminar shear strength (MPa) Warp direction 77 (8.2) 62 (3.7) Weft direction 81 (7.1) 27 (3.1) Standard deviations are given in parentheses. J. Ma et al. / Materials Letters 61 (2007) 312–315 315