Availableonlineatwww.sciencedirect.com Scripta materialia ELSEVIER Scripta Materialia 54(2006)1967-1971 www.actamat-journals.com Microstructure characterization and tensile behavior of 2. 5D C/SiC composites fabricated by chemical vapor infiltration Junqiang Ma", Yongdong Xu, Litong Zhang, Aifei Cheng, Jingjiang Nie, Ning Dong National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, 547 Mailbox, Xian Shaanxi 710072, Peoples Republic of china Received 23 November 2005: received in revised form 18 January 2006: accepted 30 January 2006 Available online 9 March 2006 Abstract a The microstructure of 2. 5D C/SiC composite was characterized in terms of fiber architecture and processing-induced cracks. a geo- petric model of the weave architecture which can be used for visualization and further tive analysis, was generated. Tension tests along the warp and weft directions were conducted at room temperature. The results that the stress-strain curves exhibit mostly nonlinear behavior. Microstructural observations reveal that the differences in tensile be between two loading directions are attrib- uted to the characteristics of the weave architecture o 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved Keywords: Ceramic matrix composites; Weave architecture; Microstructure; Tension tests; Fracture 1. Introduction Although three-dimensional (3D) four-step braided C/SiC omposites have improved delamination resistance and Carbon fiber reinforced silicon carbide ceramic matrix can be used to fabricate complex net or near-net shaped composites(C/SiC CMCs)are promising candidates for components [8-10], they are still not applicable for manu- many applications, particularly as aerospace and aircraft facturing components with one closed end (e.g,a nose thermostructural components, since they retain the advan- cap). Most recently, the authors have develop a unique tages of silicon carbide ceramics while providing an kind of multilayer C/SiC composites with through-the- enhanced degree of damage tolerance [1, 2]. Up to now, thickness reinforcements. The composites developed have chemical vapor infiltration (CvI) has proved the most two-and-a-half-dimensional (2.5D)architecture in the real promising process for fabricate me abilities to manip- architecture reported in literature [11-15]. The characteris- ulate and modify the microstructure of the matrix, to tailor tics of the weave technique make the fabric preform partic- the fiber/matrix interface, and to fabricate complex net or ularly suitable for conforming to the mould surface of near-net shaped components at relatively low temperatures dome-shaped components(e.g, nose cap), and allow net or near-net shaping Two-dimensional C/Sic composites have been investi In order to utilize these novel composites most efi ated extensively [5-7]. However, their widespread applica- ciently, thorough understanding of their mechanical prop- ions in many structural components have been limited by erties is essential. It is well known that the fiber architecture fabrication problems and poor delamination resistance. determines the composite microstructures and microstress distribution, thereby determining the composite properties ing author. Tel. +86 29 88494616 to characterize the composite microstructures with an E-mail address: junqianqma@ kcnh com(J. Ma) emphasis on the fiber architecture and processing-induced 1359-6462/S- see front matter C 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi: 10. 1016/j-scriptamat. 2006.01.047
Microstructure characterization and tensile behavior of 2.5D C/SiC composites fabricated by chemical vapor infiltration Junqiang Ma *, Yongdong Xu, Litong Zhang, Laifei Cheng, Jingjiang Nie, Ning Dong National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, 547 Mailbox, Xi’an Shaanxi 710072, People’s Republic of China Received 23 November 2005; received in revised form 18 January 2006; accepted 30 January 2006 Available online 9 March 2006 Abstract The microstructure of 2.5D C/SiC composite was characterized in terms of fiber architecture and processing-induced cracks. A geometric model of the weave architecture, which can be used for visualization and further qualitative analysis, was generated. Tension tests along the warp and weft directions were conducted at room temperature. The results show that the stress–strain curves exhibit mostly nonlinear behavior. Microstructural observations reveal that the differences in tensile behavior between two loading directions are attributed to the characteristics of the weave architecture. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramic matrix composites; Weave architecture; Microstructure; Tension tests; Fracture 1. Introduction Carbon fiber reinforced silicon carbide ceramic matrix composites (C/SiC CMCs) are promising candidates for many applications, particularly as aerospace and aircraft thermostructural components, since they retain the advantages of silicon carbide ceramics while providing an enhanced degree of damage tolerance [1,2]. Up to now, chemical vapor infiltration (CVI) has proved the most promising process for fabricating the composites. The primary advantages of this process are the abilities to manipulate and modify the microstructure of the matrix, to tailor the fiber/matrix interface, and to fabricate complex net or near-net shaped components at relatively low temperatures [3,4]. Two-dimensional C/SiC composites have been investigated extensively [5–7]. However, their widespread applications in many structural components have been limited by fabrication problems and poor delamination resistance. Although three-dimensional (3D) four-step braided C/SiC composites have improved delamination resistance and can be used to fabricate complex net or near-net shaped components [8–10], they are still not applicable for manufacturing components with one closed end (e.g., a nose cap). Most recently, the authors have developed a unique kind of multilayer C/SiC composites with through-thethickness reinforcements. The composites developed have two-and-a-half-dimensional (2.5D) architecture in the real sense, which differ significantly from the so-called 2.5D architecture reported in literature [11–15]. The characteristics of the weave technique make the fabric preform particularly suitable for conforming to the mould surface of dome-shaped components (e.g., nose cap), and allow net or near-net shaping. In order to utilize these novel composites most effi- ciently, thorough understanding of their mechanical properties is essential. It is well known that the fiber architecture determines the composite microstructures and microstress distribution, thereby determining the composite properties. Therefore, the present work has two objectives. The first is to characterize the composite microstructures with an emphasis on the fiber architecture and processing-induced 1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.01.047 * Corresponding author. Tel.: +86 29 88494616. E-mail address: junqianqma@kcnh.com (J. Ma). www.actamat-journals.com Scripta Materialia 54 (2006) 1967–1971
J. Ma et al. Scripta Materialia 54(2006)1967-1971 defects. The second is to investigate the in-plane tensile behavior of the composites and its relation to the micro structures. Also, the differences in tensile behavior between 30L3 warp and weft directions were compared, and suggestions were offered for tailoring the mechanical properties to achieve the desired performance of the components or Fig. 2. Geometry and dimensions of tension specimen(dimensions in structures millimeters) 2. Experimental a chemical vapor deposited Sic coating. The geometry and dimensions of tension specimens are shown in Fig. 2. 2.1. Composite fabrication The tensile stress-strain curves were measured using an Instron 1196 test machine at a crosshead speed of The carbon fiber utilized was T300( Nippon Toray cor- 0.05 mm/min Strains were recorded with an extensometer poration), and each yarn contained 6000 fibers. The fiber with a gauge length of 25 mm. Five specimens were tested preforms fabricated by the 2.5D woven method were sup- for each loading direction in this manner. The fracture sur plied by Nanjing Institute of Glass Fiber, People's Repub- faces of tested specimens were examined by SEM lic of China. A schematic of the fiber preform is shown in ig. 1. Composite panels were processed by isothermal 3 Results and discussion CVI to deposit the pyrolytic carbon(PyC) interphase and the silicon carbide matrix, which has been described previ- 3. 1. Preform characterization ously in detail [1o As shown in Fig. l, the 2.5D preform is a unique kind of 2. Weave architecture and microstructure multilayer fabric. It is composed of layers of straight weft characterization yarns and a set of sinusoidal warp yarns, with the adjacent The as-fabricated composites were cut parallel to the From left to right, the odd column has seven rows of weft warp and weft directions, and then the cross-sections were yarns, while the even column has five rows of weft yarns ground and polished. The microstructural observations For the odd column, the uppermost and lowest weft yarns and the determination of geometrical parameters of the differ significantly from the other weft yarns which are weave architecture were conducted using scanning electron interlocked by the sinusoidal warp yarns. In contrast, in microscopy(SEM). A computer model which was an the case of the even column, all the weft yarns are in the approximate description of the weave architecture was gen- same situation, i.e., the weft yarns are enclosed in the qua- erated using the AutoCAD software drangular outer tubes which are formed by the warp yarns. According to group theory, the preform is a symmetrical 2.3 Tension tests and microstructural observation object. Firstly, the preform is symmetrical in the warp and weft directions with respect to their respective mid Of a total of 15 composite panels processed in the same planes, and secondly, two adjacent layers of warp yarns furnace runs, three composite panels were cut into the are symmetric with respect to the symmetry center dumbbell-shaped tension specimens along the warp and weft directions, and then the specimens were sealed with 3. 2. Weave architecture and microstructure The macro-mechanical performance of the composites is primarily dependent on their final weave architecture or microstructure, so adequate control over their final weave architecture or microstructure is of prime importance. A 3D view of the as-fabricated composites is shown in Fig. 3. It is seen that the warp yarns take an approximately sinusoidal path, and the shapes of lentils and parallelograms alternately. 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 or Z element which contributes particularly to improved delamination resistance. It is also evident from the trans- Weft yarn verse cross-section(plane y-z)that the warp yarns present Fig. I. Schematic of fiber preform. thought that the different cross-sectional shapes of weft
defects. The second is to investigate the in-plane tensile behavior of the composites and its relation to the microstructures. Also, the differences in tensile behavior between warp and weft directions were compared, and suggestions were offered for tailoring the mechanical properties to achieve the desired performance of the components or structures. 2. Experimental 2.1. Composite fabrication The carbon fiber utilized was T300 (Nippon Toray corporation), and each yarn contained 6000 fibers. The fiber preforms fabricated by the 2.5D woven method were supplied by Nanjing Institute of Glass Fiber, People’s Republic of China. A schematic of the fiber preform is shown in Fig. 1. Composite panels were processed by isothermal CVI to deposit the pyrolytic carbon (PyC) interphase and the silicon carbide matrix, which has been described previously in detail [10]. 2.2. Weave architecture and microstructure characterization The as-fabricated composites were cut parallel to the warp and weft directions, and then the cross-sections were ground and polished. The microstructural observations and the determination of geometrical parameters of the weave architecture were conducted using scanning electron microscopy (SEM). A computer model which was an approximate description of the weave architecture was generated using the AutoCAD software. 2.3. Tension tests and microstructural observation Of a total of 15 composite panels processed in the same furnace runs, three composite panels were cut into the dumbbell-shaped tension specimens along the warp and weft directions, and then the specimens were sealed with a chemical vapor deposited SiC coating. The geometry and dimensions of tension specimens are shown in Fig. 2. The tensile stress–strain curves were measured using an Instron 1196 test machine at a crosshead speed of 0.05 mm/min. Strains were recorded with an extensometer with a gauge length of 25 mm. Five specimens were tested for each loading direction in this manner. The fracture surfaces of tested specimens were examined by SEM. 3. Results and discussion 3.1. Preform characterization As shown in Fig. 1, the 2.5D preform is a unique kind of multilayer fabric. It is composed of layers of straight weft yarns and a set of sinusoidal warp yarns, with the adjacent layers of weft yarns interlaced together by warp yarns. From left to right, the odd column has seven rows of weft yarns, while the even column has five rows of weft yarns. For the odd column, the uppermost and lowest weft yarns differ significantly from the other weft yarns which are interlocked by the sinusoidal warp yarns. In contrast, in the case of the even column, all the weft yarns are in the same situation, i.e., the weft yarns are enclosed in the quadrangular outer tubes which are formed by the warp yarns. According to group theory, the preform is a symmetrical object. Firstly, the preform is symmetrical in the warp and weft directions with respect to their respective midplanes, and secondly, two adjacent layers of warp yarns are symmetric with respect to the symmetry center. 3.2. Weave architecture and microstructure The macro-mechanical performance of the composites is primarily dependent on their final weave architecture or microstructure, so adequate control over their final weave architecture or microstructure is of prime importance. A 3D view of the as-fabricated composites is shown in Fig. 3. It is seen that the warp yarns take an approximately sinusoidal path, and the weft yarns present cross-sectional shapes of lentils and parallelograms alternately. 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 or Z element which contributes particularly to improved delamination resistance. It is also evident from the transverse cross-section (plane y–z) that the warp yarns present the cross-sectional shape of quasi-rectangles. It was Fig. 1. Schematic of fiber preform. thought that the different cross-sectional shapes of weft Fig. 2. Geometry and dimensions of tension specimen (dimensions in millimeters). 1968 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971
J. Ma et al Scripta Materialia 54(2006)1967-1971 The yarns are the fundamental structural elements of the Warp yarn composites. The geometrical structures of the composites are mainly determined by the cross-section and the center- line configurations of their constituent yarns In the present study, the basic idea of the computer modeling is to create each yarn by sweeping a closed curve along a centerline path. The closed curve and centerline path represent the ctional shape and Weft yarn the constituent yarns, respectively. Assuming the cross-sec- tional shapes of the warp and weft yarns to be rectangular and elliptical/parallelogrammic, and the spatial path of Fig 3. Three-dimensional view of as-fabricated composites warp and weft yarns to be sinusoidal and straight, a geo. metric model(shown in Fig. 5) was generated. Although yarns are attributable to the combination of compaction in it is only an approximate description of the spatial path the graphite fixture and the symmetrical characteristics of and geometry of the yarns in the composites, it does conve- fiber preform. niently visualize the weave architecture and quantify Shown in Fig. 4 are five kinds of cracks in the compos- geometric characteristics that can be correlated with the ites: transverse cracks in the warp and weft yarns, longitu- global composite properties dinal cracks in the warp and weft yarns and splitting cracks between two adjacent warp yarns. Transverse cracks corre- 3.4. Monotonic tension test results spond to the pre-existing cracks due to processing-related thermal residual stresses, while longitudinal cracks are 3.4.1. Tension tests formed by the propagation of transverse cracks into adja- The tensile stress-strain curves for monotonic loading cent yarns. Since all these cracks are observed after com- parallel to the warp and weft directions are shown in posite processing and before mechanical loading, they are Fig. 6. All the curves exhibited mostly nonlinear behavior referred to as"processing-induced cracks The curves can be generally divided into three regions: a very small initial linear region followed by a large nonlin- 3.3. Geometric model ear region and finally a quasi-linear region. The large non linear region was accompanied by a significant decrease in The cross-sections parallel to the warp and weft yarns the elastic modulus. In the quasi-linear region, the elastic were examined by SEM, and some of the major geometri- modulus recovered a little and gradually tended to be sta cal parameters (listed in Table 1)were evaluated approxi- ble up to the failure of the composites. For the convenience mately to generate the geometric model. of comparison, the tension test data are summarized in Fig 4. SEM images showing five kinds of processing-induced cracks. The geometrical parameters of weave architecture in the final 2. 5D C/SiC composites Rectangular warp Crimp angle of Weft density Elliptical weft yarn Parallelogrammic warp yarns(deg) (Picks/cm) Width Height Length of side 0.39 0.26 0.71
yarns are attributable to the combination of compaction in the graphite fixture and the symmetrical characteristics of fiber preform. Shown in Fig. 4 are five kinds of cracks in the composites: transverse cracks in the warp and weft yarns, longitudinal cracks in the warp and weft yarns and splitting cracks between two adjacent warp yarns. Transverse cracks correspond to the pre-existing cracks due to processing-related thermal residual stresses, while longitudinal cracks are formed by the propagation of transverse cracks into adjacent yarns. Since all these cracks are observed after composite processing and before mechanical loading, they are referred to as ‘‘processing-induced cracks’’. 3.3. Geometric model The cross-sections parallel to the warp and weft yarns were examined by SEM, and some of the major geometrical parameters (listed in Table 1) were evaluated approximately to generate the geometric model. The yarns are the fundamental structural elements of the composites. The geometrical structures of the composites are mainly determined by the cross-section and the centerline configurations of their constituent yarns. In the present study, the basic idea of the computer modeling is to create each yarn by sweeping a closed curve along a centerline path. The closed curve and centerline path represent the cross-sectional shape and the centerline configurations of the constituent yarns, respectively. Assuming the cross-sectional shapes of the warp and weft yarns to be rectangular and elliptical/parallelogrammic, and the spatial path of warp and weft yarns to be sinusoidal and straight, a geometric model (shown in Fig. 5) was generated. Although it is only an approximate description of the spatial path and geometry of the yarns in the composites, it does conveniently visualize the weave architecture and quantify geometric characteristics that can be correlated with the global composite properties. 3.4. Monotonic tension test results 3.4.1. Tension tests The tensile stress–strain curves for monotonic loading parallel to the warp and weft directions are shown in Fig. 6. All the curves exhibited mostly nonlinear behavior. The curves can be generally divided into three regions: a very small initial linear region followed by a large nonlinear region and finally a quasi-linear region. The large nonlinear region was accompanied by a significant decrease in the elastic modulus. In the quasi-linear region, the elastic modulus recovered a little and gradually tended to be stable up to the failure of the composites. For the convenience of comparison, the tension test data are summarized in Fig. 3. Three-dimensional view of as-fabricated composites. Fig. 4. SEM images showing five kinds of processing-induced cracks. Table 1 The geometrical parameters of weave architecture in the final 2.5D C/SiC composites Warp density (Ends/cm) Rectangular warp yarn Crimp angle of warp yarns (deg) Weft density (Picks/cm) Elliptical weft yarn Parallelogrammic weft yarn Width (mm) Height (mm) Major axis (mm) Minor axis (mm) Length of side (mm) 10 1 0.39 14 3 1.8 0.26 0.71 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971 1969
J. Ma et al. Scripta Materialia 54(2006)1967-1971 Warp yarn tion occurring in the composites under increasing tensile stresses. The damage accumulation includes matrix crack- ing and interfacial debonding/sliding, which has been dis- cussed fully in the literature [16] The great tensile anisotropy of the composites is closely related to the differences in yarn density and yarn path between warp and weft yarns As shown in Table 2 the com- posites have 3.33 times as many yarns in the warp direction as compared to the weft direction. However, the tensile Weft varn strength in the warp direction is only 2.2 times that for the weft direction. Such a decrease in strength is expected Fig. 5. Geometric model of 2. 5D weave architecture because the undulation of warp yarns can significantly reduce the in-plane mechanical properties. Obviously, the eduction in the in-plane mechanical properties is influ- enced to a large extent by the waviness of the warp yarns, i.e., the wavier the warp yarns are, the weaker the in-plane mechanical properties of the composites. For the given yarns, the undulation degree of warp yarns can be altered by adjusting the weft density. A lower weft density indicates a lower undulation degree of warp yarns and vice versa. However, a lower weft density will significantly reduce the mechanical properties in the weft direction. That is to say, Weft direction the mechanical properties of the composites can be tailore for specific applications by varying the weave parameter 000.10203040506070809 Strain 3. 4.2. Fracture observation Fig. 6. Tensile stress-strain curves showing mostly nonlinear behavior The fracture surface morphology can well reflect the fracture characteristics of the composites, so extensive frac- Table 2. It is seen that the composites exhibited very differ- tographic studies were conducted by SEM in this work. ent tensile properties between warp and weft directions The top view of the fracture surfaces for tension specimens The mostly nonlinear tensile stress-strain behavior of in both loading directions is shown in Fig. 7. Detailed 2.5D C/SiC composites can be qualitatively understood observations revealed that the yarns fractured at various by the processing-induced damage and damage accumula- elevations. The warp yarns fractured in the yarn crossover areas, while the weft yarns fractured at random positions. The side view of the fracture surfaces indicated that the Table 2 fracture surfaces of the yarns were very Tension tests data for 2.5D C/SiC composites fibers exhibiting multi-step fracture and extensive pullout. Failur nitial The fracture characteristics mentioned above are closel strength(MPa) strain(%) modulus(GPa related to the different yarn path and suitable thickness of Warp direction 326(35) 0682(0.08)153(16) Weft direction 145(16) the Pyc interphase. In the present composites, the warp 0.705(0.04)62(28) and weft yarns take nominally sinusoidal and straight paths Standard deviations are given in brackets. respectively. When the specimens are subjected to tension Fig. 7. Typical SEM images showing the top view of fracture surfaces of tension specimens: (a) specimen cut along warp direction and(b) specimen cut
Table 2. It is seen that the composites exhibited very different tensile properties between warp and weft directions. The mostly nonlinear tensile stress–strain behavior of 2.5D C/SiC composites can be qualitatively understood by the processing-induced damage and damage accumulation occurring in the composites under increasing tensile stresses. The damage accumulation includes matrix cracking and interfacial debonding/sliding, which has been discussed fully in the literature [16]. The great tensile anisotropy of the composites is closely related to the differences in yarn density and yarn path between warp and weft yarns. As shown in Table 2 the composites have 3.33 times as many yarns in the warp direction as compared to the weft direction. However, the tensile strength in the warp direction is only 2.2 times that for the weft direction. Such a decrease in strength is expected because the undulation of warp yarns can significantly reduce the in-plane mechanical properties. Obviously, the reduction in the in-plane mechanical properties is influenced to a large extent by the waviness of the warp yarns, i.e., the wavier the warp yarns are, the weaker the in-plane mechanical properties of the composites. For the given yarns, the undulation degree of warp yarns can be altered by adjusting the weft density. A lower weft density indicates a lower undulation degree of warp yarns and vice versa. However, a lower weft density will significantly reduce the mechanical properties in the weft direction. That is to say, the mechanical properties of the composites can be tailored for specific applications by varying the weave parameters. 3.4.2. Fracture observation The fracture surface morphology can well reflect the fracture characteristics of the composites, so extensive fractographic studies were conducted by SEM in this work. The top view of the fracture surfaces for tension specimens in both loading directions is shown in Fig. 7. Detailed observations revealed that the yarns fractured at various elevations. The warp yarns fractured in the yarn crossover areas, while the weft yarns fractured at random positions. The side view of the fracture surfaces indicated that the fracture surfaces of the yarns were very ragged, with the fibers exhibiting multi-step fracture and extensive pullout. The fracture characteristics mentioned above are closely related to the different yarn path and suitable thickness of the PyC interphase. In the present composites, the warp and weft yarns take nominally sinusoidal and straight paths, respectively. When the specimens are subjected to tension Fig. 5. Geometric model of 2.5D weave architecture. Fig. 6. Tensile stress–strain curves showing mostly nonlinear behavior. Table 2 Tension tests data for 2.5D C/SiC composites Specimen Tensile strength (MPa) Failure strain (%) Initial modulus (GPa) Warp direction 326 (35) 0.682 (0.08) 153 (16) Weft direction 145 (16) 0.705 (0.04) 62 (2.8) Standard deviations are given in brackets. Fig. 7. Typical SEM images showing the top view of fracture surfaces of tension specimens: (a) specimen cut along warp direction and (b) specimen cut along weft direction. 1970 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971
J. Ma et al Scripta Materialia 54(2006)1967-1971 the final densification, the weft yarns presented the cross-sectional shapes of a lentil and a parallelogram alternately, while the warp yarns presented the cross-sec- tional shape of a quasi-rectangle. It was found that the different cross-sectional shapes of the yarns are the cou pled results of symmetrical characteristics of the pre- form, the spatial path of yarns and the compaction in the graphite fixture. Five kinds of processing-induced Carbon fiber cracks were identified in the as-fabricated composit The geometric model of the final weave architecture gen erated by the AutoCAD software can be used for conve- nient visualization and further qualitative analysis 2. The tensile stress-strain curves of 2. 5D C/Sic co Fig 8. Typical SEM micrograph showing interfacial debonding in the be generally divided into three regions: a very smale. can ites exhibit mostly nonlinear behavior. The curves tial linear region followed by a large nonlinear region along warp direction, the tensile loads are primarily carried and finally a quasi-linear region. The differences in by warp yarns. Thus the straightening tendency of warp tensile properties between warp and weft directions are ause high values of stresses in yarn crossover attributed to the characteristics of weave architecture areas. Such stress concentrations can trigger localized dam The failures of the composites exhibit multiple fractures age in warp yarns even when the overall applied tensile stress of the yarns, with the fibers showing multi-step fracture is much lower than the ultimate tensile strength. This can and extensive pullout explain well why the fracture of warp yarns always occurred subjected to tension weftwise, the tensile loads are primarily Acknowledgements carried by the weft yarns. Obviously, almost no stress con centrations exist in yarn crossover areas, therefore the weft The authors acknowledge the financial support of the yarns fractured at random positions Natural Science Foundation of China ( Contract No 90405015)and the National Young Elitists Foundation The composite properties are markedly influenced Dy (Contract No. 50425208) the interfacial properties. The main functions of the inter- phase are: (1) to act as a mechanical fuse, i.e., to deflect the matrix microcracks, and(2)to maintain a good load References transfer between the fibers and the matrix [17]. It has been postulated that the best interphase materials might be those [1] Lamouroux F, Bourrat X R. Carbon1993:31:1273. H. Acta Astronautica with a layered crystal structure or a layered microstructure 2004:55:409 and the interphase being strongly bonded to the fiber sur- [3] Lin W, Yang JM. J Mater Sci 1991: 26: 1le face [18]. The interphase of the tested specimen is shown Chiang YM, Haggerty JS, Messner RP, et al. Am Ceram Soc Bull in Fig. 8. The multilayer PyC interphase was strongly I C, Laurent G, Stephane B. Compos Sci Technol 1996: 56: 1363 bonded to the fiber surface while being significantly sepa-[6]Wang M, Laird C Mater Sci Eng A1997: 230:171 rated from the SiC matrix, indicating that the interphase [7 Staehler JM, Mall S, Zawada LP Compos Sci Technol 2003: 63: 2121 an better deflect the matrix microcracks along the sic [8] Ko FK. Am Ceram Soc Bull 1989 68: 401 matrix/ PyC interphase interface. Therefore, 2. 5D C/SiC 9 Xu YD, Zhang LT. J Am Ceram Soc 1997: 80: 1897. composites exhibited non-brittle fracture characteristics. [10] Xu YD, Zhang LT, Cheng LF, Yan D.Carbon 1998: 36: 1051 with the matrix cracks displaying a step-like path and th [11] Almaz A, Reynaud P, Rouby D, et al. Compos Sci Technol 1998 failure of the yarns showing multi-step fiber fracture as well [12] Almaz A, Ducret D, Gurerjouma RE, et al. Compos Sci Technol as extensive fiber pullo [13] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2000: 279: 73 4. Conclusions [14] Boitier G, Chermant JL, Vicens J. Mater Sci Eng A 2000: 289: 265 [15] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2001: 313: 53. [16] Stephane B. Compos Sci Technol 2001: 61: 2285 1. The 2.5D fiber preform is a symmetrical object which [17]Naslain R Compos Part A 1998:29A: 114: has two symmetry planes (i.e, two mid-planes). After [18] Naslain R Compos Sci Technol 2004: 64: 155
along 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 yarns even when the overall applied tensile stress is much lower than the ultimate tensile strength. This can explain well why the fracture of warp yarns always occurred in yarn crossover areas. In contrast, when the specimens are subjected to tension weftwise, the tensile loads are primarily carried by the weft yarns. Obviously, almost no stress concentrations exist in yarn crossover areas, therefore the weft yarns fractured at random positions. The composite properties are markedly influenced by the interfacial properties. The main functions of the interphase are: (1) to act as a mechanical fuse, i.e., to deflect the matrix microcracks, and (2) to maintain a good load transfer between the fibers and the matrix [17]. It has been postulated that the best interphase materials might be those with a layered crystal structure or a layered microstructure and the interphase being strongly bonded to the fiber surface [18]. The interphase of the tested specimen is shown in Fig. 8. The multilayer PyC interphase was strongly bonded to the fiber surface while being significantly separated from the SiC matrix, indicating that the interphase can better deflect the matrix microcracks along the SiC matrix/PyC interphase interface. Therefore, 2.5D C/SiC composites exhibited non-brittle fracture characteristics, with the matrix cracks displaying a step-like path and the failure of the yarns showing multi-step fiber fracture as well as extensive fiber pullout. 4. Conclusions 1. The 2.5D fiber preform is a symmetrical object which has two symmetry planes (i.e., two mid-planes). After the final densification, the weft yarns presented the cross-sectional shapes of a lentil and a parallelogram alternately, while the warp yarns presented the cross-sectional shape of a quasi-rectangle. It was found that the different cross-sectional shapes of the yarns are the coupled results of symmetrical characteristics of the preform, the spatial path of yarns and the compaction in the graphite fixture. Five kinds of processing-induced cracks were identified in the as-fabricated composites. The geometric model of the final weave architecture generated by the AutoCAD software can be used for convenient visualization and further qualitative analysis. 2. The tensile stress–strain curves of 2.5D C/SiC composites exhibit mostly nonlinear behavior. The curves can be generally divided into three regions: a very small initial linear region followed by a large nonlinear region and finally a quasi-linear region. The differences in tensile properties between warp and weft directions are attributed to the characteristics of weave architecture. The failures of the composites exhibit multiple fractures of the yarns, with the fibers showing multi-step fracture and extensive pullout. 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] Lamouroux F, Bourrat X, Sevely J, Naslain R. Carbon 1993;31:1273. [2] Schmidta S, Beyera S, Knabeb H, Immicha H. Acta Astronautica 2004;55:409. [3] Lin W, Yang JM. J Mater Sci 1991;26:116. [4] Chiang YM, Haggerty JS, Messner RP, et al. Am Ceram Soc Bull 1989;68:420. [5] Gerald C, Laurent G, Stephane B. Compos Sci Technol 1996;56:1363. [6] Wang M, Laird C. Mater Sci Eng A 1997;230:171. [7] Staehler JM, Mall S, Zawada LP. Compos Sci Technol 2003;63:2121. [8] Ko FK. Am Ceram Soc Bull 1989;68:401. [9] Xu YD, Zhang LT. J Am Ceram Soc 1997;80:1897. [10] Xu YD, Zhang LT, Cheng LF, Yan D. Carbon 1998;36:1051. [11] Dalmaz A, Reynaud P, Rouby D, et al. Compos Sci Technol 1998; 58:693. [12] Dalmaz A, Ducret D, Gurerjouma RE, et al. Compos Sci Technol 2000;60:913. [13] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2000;279:73. [14] Boitier G, Chermant JL, Vicens J. Mater Sci Eng A 2000;289:265. [15] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2001;313:53. [16] Stephane B. Compos Sci Technol 2001;61:2285. [17] Naslain R. Compos Part A 1998;29A:1145. [18] Naslain R. Compos Sci Technol 2004;64:155. Fig. 8. Typical SEM micrograph showing interfacial debonding in the specimens tested. J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971 1971