SEI TERIALS ENGE& ENGIEERN ELSEVIER Materials Science and Engineering A300(2001)196-202 www.elsevier.com/locate/msea Mechanical properties of 3D fiber reinforced C/SiC composites Yongdong Xu, Aifei Cheng, Litong Zhang, Hongfeng Yin, Xiaowei Yin State Key Laboratory of solidifi n Polytechnical Unicersity, Xian, Shaanxi 710072, People's Republic of China Received 9 May 2000: received in revised form 17 August 2000 abstract High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0-2. 1 g cm after the three dimensional carbon preform was infiltrated for 30 h The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300C, and 447 MPa at 1600oC. At elevated temperatures(1300 and 1600.C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m"2 and 12.0 kJ m-2, respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23. 3. The value of dynamic fracture toughness was 62- measured by Charpy impact tests. C 2001 Elsevier Science B.v. All rights reserved Keywords: 3D textile C/SiC composites; Toughness; Reliability; Chemical vapor infiltration 1. Introduction rently being applied to the extent that they could be Even when they have been employed, relatively low Continuous fiber reinforced ceramic matrix com stress applications and large safety factors were usually posites(CFCCs)are very interesting structural materi considered. The main reason is the difficulty and uncer- als because of their higher performance compared with tainty that exist in determining their failure strength, super-alloy at elevated temperatures, and higher frac fracture toughness, operating lifetime in severe condi ture toughness compared with monolithic ceramics [I tions because the nature of the deformation and failure potential to be used in advanced aero-engines. Among 13,14 of the composites were very complicated 4]. For this reason, CFCCs are considered as the most these CFCCs. both carbon fiber and silicon carbide This paper examined the mechanical properties over fiber reinforced silicon carbide composites(C/SiC and a large temperature range of 3D textile C/SiC com- posite materials produced by chemical vapor infiltra- SiC/SiC)are most promising and have been received tion. The aims of current contribution are (1)to considerable interest [1, 5-8. Many investigations hav develop the understanding of the effects of architecture been conducted on two dimensional woven C/SiC and on the mechanical properties and the damage behavior SiC/SiC composite materials. Recently, attention has of the composites, (2) to expand the experimental been focused on three dimensional woven or braided knowledge for the design of the three dimensional ceramic matrix composite materials in order to meet textile composite materials mechanical and thermal properties requirements along he thickness of the composites [9-12] espite the attractiveness of fiber-reinforced com- 2. Materials and experimental procedures tes as engineering components, they are not cu 2. 1. Fabrication of the composites orresponding author. Tel. +86-29-8491427: fax: +86-29. 8491000 PAN-based carbon fiber was employed and each yarn contained 3000 filaments. The three-dimensional 0921-5093/01/s- see front matter o 2001 Elsevier Science B.V. All rights reserved PI:S0921-509300)01533-1
Materials Science and Engineering A300 (2001) 196–202 Mechanical properties of 3D fiber reinforced C/SiC composites Yongdong Xu *, Laifei Cheng, Litong Zhang, Hongfeng Yin, Xiaowei Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical Uni6ersity, Xian, Shaanxi 710072, People’s Republic of China Received 9 May 2000; received in revised form 17 August 2000 Abstract High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm−3 after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m1/2 and 12.0 kJ·m−2 , respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m−2 measured by Charpy impact tests. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 3D textile C/SiC composites; Toughness; Reliability; Chemical vapor infiltration www.elsevier.com/locate/msea 1. Introduction Continuous fiber reinforced ceramic matrix composites (CFCCs) are very interesting structural materials because of their higher performance compared with super-alloy at elevated temperatures, and higher fracture toughness compared with monolithic ceramics [1– 4]. For this reason, CFCCs are considered as the most potential to be used in advanced aero-engines. Among these CFCCs, both carbon fiber and silicon carbide fiber reinforced silicon carbide composites(C/SiC and SiC/SiC) are most promising and have been received considerable interest [1,5–8]. Many investigations have been conducted on two dimensional woven C/SiC and SiC/SiC composite materials. Recently, attention has been focused on three dimensional woven or braided ceramic matrix composite materials in order to meet mechanical and thermal properties requirements along the thickness of the composites [9–12]. Despite the attractiveness of fiber-reinforced composites as engineering components, they are not currently being applied to the extent that they could be. Even when they have been employed, relatively low stress applications and large safety factors were usually considered. The main reason is the difficulty and uncertainty that exist in determining their failure strength, fracture toughness, operating lifetime in severe conditions because the nature of the deformation and failure behavior of the composites were very complicated [13,14]. This paper examined the mechanical properties over a large temperature range of 3D textile C/SiC composite materials produced by chemical vapor infiltration. The aims of current contribution are (1) to develop the understanding of the effects of architecture on the mechanical properties and the damage behavior of the composites, (2) to expand the experimental knowledge for the design of the three dimensional textile composite materials. 2. Materials and experimental procedures 2.1. Fabrication of the composites PAN-based carbon fiber was employed and each yarn contained 3000 filaments. The three-dimensional * Corresponding author. Tel.: +86-29-8491427; fax: +86-29- 8491000. E-mail address: ydxu@nwpu.edu.cn (Y. Xu). 0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)01533-1
Y. Xu et al. Materials Science and Engineering 4300 (2001)196-202 500 400 200 1.0 3.0 Displacement, mm Fig. I. Stress-displacement curve for 3D C/SiC textile composites at 1600C (3-D) fabric preform was braided by four-step process- thinned to 75 um, dimpled to a center using Ar+ions ing. The fiber volume fraction was 40%. In the present at an incident angle of 15. The specimens were exam- experiment, isothermal /forced flow chemical vapor ined with an operated voltage of 200 kV. infiltration was employed to deposit pyrolytic carbon layer and silicon carbide which has been described previously in detail [Il, 12]. A carbon layer of 200 nm 3. Results and discussion thickness was deposited on the surface of carbon fiber as interfacial layer with butane prior to densification. 3. 1. Flexural loading Methytrichosilane(MTS, CH3,)was used for depo- sition of SiC and carried by bubbling hydrogen. Typical The mechanical properties of fiber reinforced com- onditions used for the densification of silicon carbide posite materials are largely governed by the stress matrix are 1100C, a hydrogen to MTS mol ratio of 10, transfer capability of fiber /matrix interface. The and a pressure of a few kPa. Argon(Ar) was employed interfacial compatibility is related to the interfacial as diluent gas to slow the rate of deposition shear stress. which characterizes the combination of stress necessary to de-bond the interface and the fric- 2. 2. Mechanical properties measurement tional forces developed at the interface. The density of Mechanical properties of the composite materials vere characterized under bending, shear, and impact loading. Flexural strength was measured using the three-point-bending method at temperatures from room temperature up to the elevated temperatures(1300 and 1600C)in vacuum. Shear strength was measured using the short beam bending method with a span of 15 mm edged-notched beam method. The impact tests gle fracture toughness was determined with the sir performed with instrumented Charpy equipment. The sample size was3.0×20×70mm, and the impact velocity of 3 ms-1 2.3. Microstructure observation and surface analysi The density of the samples was determined by the water displacement method. The microstructure of frac- ture surface was observed by a scanning electron micro- scope. Transmission electron microscope samples were prepared by cutting 300 um thickness using a low speed diamond saw. The composites were mechanically Fig. 2. Fiber fracture in the SiC matrix at 1600C
Y. Xu et al. / Materials Science and Engineering A300 (2001) 196–202 197 Fig. 1. Stress–displacement curve for 3D C/SiC textile composites at 1600°C. (3-D) fabric preform was braided by four-step processing. The fiber volume fraction was 40%. In the present experiment, isothermal/forced flow chemical vapor infiltration was employed to deposit pyrolytic carbon layer and silicon carbide, which has been described previously in detail [11,12]. A carbon layer of 200 nm thickness was deposited on the surface of carbon fiber as interfacial layer with butane prior to densification. Methytrichosilane (MTS, CH3SiCl3) was used for deposition of SiC and carried by bubbling hydrogen. Typical conditions used for the densification of silicon carbide matrix are 1100°C, a hydrogen to MTS mol ratio of 10, and a pressure of a few kPa. Argon (Ar) was employed as diluent gas to slow the rate of deposition. 2.2. Mechanical properties measurement Mechanical properties of the composite materials were characterized under bending, shear, and impact loading. Flexural strength was measured using the three-point-bending method at temperatures from room temperature up to the elevated temperatures (1300 and 1600°C) in vacuum. Shear strength was measured using the short beam bending method with a span of 15 mm. Fracture toughness was determined with the single edged-notched beam method. The impact tests were performed with instrumented Charpy equipment. The sample size was 3.0×20×70 mm, and the impact velocity of 3 ms−1 . 2.3. Microstructure obser6ation and surface analysis The density of the samples was determined by the water displacement method. The microstructure of fracture surface was observed by a scanning electron microscope. Transmission electron microscope samples were prepared by cutting 300 mm thickness using a low speed diamond saw. The composites were mechanically thinned to 75 mm, dimpled to a center using Ar+ ions at an incident angle of 15°. The specimens were examined with an operated voltage of 200 kV. 3. Results and discussion 3.1. Flexural loading The mechanical properties of fiber reinforced composite materials are largely governed by the stresstransfer capability of fiber/matrix interface. The interfacial compatibility is related to the interfacial shear stress, which characterizes the combination of stress necessary to de-bond the interface and the frictional forces developed at the interface. The density of Fig. 2. Fiber fracture in the SiC matrix at 1600°C
Y. Xu et al. Materials Science and Engineering 4300 (2001)196-202 262228KV 23∠1mmD3 Fig 3. Fracture surface of 3D C/Sic composites under fiexural loading. 261728KV X251mD38 Fig 4. Fracture surface of 3D C/SiC composites under shear loading 200 8120 Displacement, mm Fig. 5. Failure behavior of notched 3D C/Sic composites with a notch
198 Y. Xu et al. / Materials Science and Engineering A300 (2001) 196–202 Fig. 3. Fracture surface of 3D C/SiC composites under flexural loading. Fig. 4. Fracture surface of 3D C/SiC composites under shear loading. Fig. 5. Failure behavior of notched 3D C/SiC composites with a notch
Y. Xu et al. Materials Science and Engineering 4300 (2001)196-202 Table I glass phase at the grain boundary. For C/Sic com Flexural strength data for 3D C/SiC composites posites, there is no glass phase in materials. Hence, the Number dr MPa flexural strength of the composites was nearly constant when the temperature ranged from room temperature up to 1600C. The average values flexural strength were 441 MPa at room temperature, 450 MPa at 1300oC and 447 MPa at 1600C. From Fig. l. it was also observed that the failure behavior of the composites was varied with the increase of the temperature. At room temperature, the stress drop was very gradual 435 after the maximum stress point. This observation sug gested that the fracture energy of the materials was very high. However, the failure behavior became brittle and the composites exhibited steep stress drops after the maximum stress point at high temperatures(Fig. Ib and c) The variation of failure behavior of composites was caused by alteration of the interfacial bonding between fiber and matrix. The t300 carbon fiber is anan- 475 sotropic material and usually characterized by two thermal expansion coefficients (TECs), a radial TEC (7.0 x 10-6oC-)and a longitudinal TEC(-01 to A prefor cm-3 after the three infiltrated SiC matrIx Is 4.8 10-6oC-[15, 16]. Hence, the composites was 2.0-2.1 1.1 x 10-6C-). The TEc of chemical vapor dimensional carbon m was chemical vapor the tensile stress within the interfacial phase along the infiltrated for 30 h. Fig. la shows the typical failure fiber radial direction was generated after the composites behavior of 3D C/SiC textile composites at room tem- were cooled down from the infiltration temperature perature,which is different from that of monolithic(1100 C)to room temperature. It was easy for the ceramIcs The present composite materials exhibited the carbon fiber to debond and be pulled out from the substantial non-linear failure behavior. From the silicon carbide matrix. Above the infiltration tempera stress-displacement curve, it could be observed that the ture, however, the interfacial stress became compressive failure of 3D C/SiC composites occurred in a controlled which led to the tight bond between the fiber and manner. In addition, the materials exhibited significant matrix. In addition, the tensile stress was generated in ailure deflection (1.2 mm) the carbon fiber. In such case. the carbon fiber was In general, the strength of monolithic ceramics(such easily damaged in the silicon carbide matrix as illus- as silicon nitride, mullite) decreases significantly at ele- trated in Fig. 2. As a result, the composites exhibited vated temperature due to the softening and sliding of brittle failure behavior because it was difficulty for the 234 95 Fig. 6. Weibull plot for flexural strength of 3 C/Sic composites
Y. Xu et al. / Materials Science and Engineering A300 (2001) 196–202 199 Table 1 Flexural strength data for 3D C/SiC composites sf , MPaNumber 3941 4062 4183 4 425 4265 4276 7 430 4358 4359 10 438 44011 44312 44413 44914 45915 45916 47017 18 473 47519 47820 glass phase at the grain boundary. For C/SiC composites, there is no glass phase in materials. Hence, the flexural strength of the composites was nearly constant when the temperature ranged from room temperature up to 1600°C. The average values flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. From Fig. 1, it was also observed that the failure behavior of the composites was varied with the increase of the temperature. At room temperature, the stress drop was very gradual after the maximum stress point. This observation suggested that the fracture energy of the materials was very high. However, the failure behavior became brittle and the composites exhibited steep stress drops after the maximum stress point at high temperatures (Fig. 1b and c). The variation of failure behavior of composites was caused by alteration of the interfacial bonding between fiber and matrix. The T300 carbon fiber is an anisotropic material and usually characterized by two thermal expansion coefficients (TECs), a radial TEC (7.0×10−6 °C−1 ) and a longitudinal TEC (−0.1 to 1.1×10−6 °C−1 ). The TEC of chemical vapor infiltrated SiC matrix is 4.8×10−6 °C−1 [15,16]. Hence, the tensile stress within the interfacial phase along the fiber radial direction was generated after the composites were cooled down from the infiltration temperature (1100°C) to room temperature. It was easy for the carbon fiber to debond and be pulled out from the silicon carbide matrix. Above the infiltration temperature, however, the interfacial stress became compressive which led to the tight bond between the fiber and matrix. In addition, the tensile stress was generated in the carbon fiber. In such case, the carbon fiber was easily damaged in the silicon carbide matrix as illustrated in Fig. 2. As a result, the composites exhibited brittle failure behavior because it was difficulty for the the composites was 2.0–2.1 g cm−3 after the three dimensional carbon preform was chemical vapor infiltrated for 30 h. Fig. 1a shows the typical failure behavior of 3D C/SiC textile composites at room temperature, which is different from that of monolithic ceramics. The present composite materials exhibited the substantial non-linear failure behavior. From the stress–displacement curve, it could be observed that the failure of 3D C/SiC composites occurred in a controlled manner. In addition, the materials exhibited significant failure deflection (1.2 mm). In general, the strength of monolithic ceramics (such as silicon nitride, mullite) decreases significantly at elevated temperature due to the softening and sliding of a Fig. 6. Weibull plot for flexural strength of 3D C/SiC composites
Y. Xu et al. Materials Science and Engineering 4300 (2001)196-202 SiC matrix 100nm 0999415KV84”忘 Fig. 7. Insufficient uniformity of interfacial layer. fiber to be pulled out from the matrix. Moreover, the bonding is usually considered as a kind of weak interfa composites showed significant non-linear failure behav- cial bonding because of the residual pores in the com- ior at 1600 C, which was attributed to creep of the posites caused by the bottom neck effectduring the manometer grain size of silicon carbide matrix. The chemical vapor infiltration process. Accordingly, both mis-match along the fiber axis of TECs between the fiber pull-out and bundle pull-out were observed at silicon carbide matrix and the fiber resulted in many room temperature(Fig. 3). At elevated temperatures micro-cracks in the matrix. It is believed that these 1300 and 1600 C), the fiber /matrix interfacial bonding micro-cracks have some contribution to the non-linear became strong but the bundle/bundle interfacial bond ailure behavior of the materials by deflection of th ing was still weak enough. In this case, the bundle pull-out was dominated, which resulted in the brittle Microstructural observations revealed that the failure failure behavior at high temperatures behavior of 3D textile C/SiC composites was domi- nated by the damage mode. As discussed above, the interfacial bonding between the carbon fiber and silicon 3. 2. Failure behavior of notched specimen under shear carbide matrix was dependent on the properties of loadin interfacial phase and temperature. However, the inter- facial bonding between fiber bundle and bundle was Shear strength of 3D C/SiC composites was mea- only influenced by the density of the composites but sured by the short shear beam method of three-point- independent of the temperature. For the three dimen- bending. The shear strength was calculated by the sional textile CFCCs, the bundle/bundle interfacial following equation [17]
200 Y. Xu et al. / Materials Science and Engineering A300 (2001) 196–202 Fig. 7. Insufficient uniformity of interfacial layer. Fig. 8. Impact fracture surface of C/SiC composites. fiber to be pulled out from the matrix. Moreover, the composites showed significant non-linear failure behavior at 1600°C, which was attributed to creep of the manometer grain size of silicon carbide matrix. The mis-match along the fiber axis of TECs between the silicon carbide matrix and the fiber resulted in many micro-cracks in the matrix. It is believed that these micro-cracks have some contribution to the non-linear failure behavior of the materials by deflection of the main crack. Microstructural observations revealed that the failure behavior of 3D textile C/SiC composites was dominated by the damage mode. As discussed above, the interfacial bonding between the carbon fiber and silicon carbide matrix was dependent on the properties of interfacial phase and temperature. However, the interfacial bonding between fiber bundle and bundle was only influenced by the density of the composites but independent of the temperature. For the three dimensional textile CFCCs, the bundle/bundle interfacial bonding is usually considered as a kind of weak interfacial bonding because of the residual pores in the composites caused by the ‘bottom neck effect’ during the chemical vapor infiltration process. Accordingly, both fiber pull-out and bundle pull-out were observed at room temperature (Fig. 3). At elevated temperatures (1300 and 1600°C), the fiber/matrix interfacial bonding became strong but the bundle/bundle interfacial bonding was still weak enough. In this case, the bundle pull-out was dominated, which resulted in the brittle failure behavior at high temperatures. 3.2. Failure beha6ior of notched specimen under shear loading Shear strength of 3D C/SiC composites was measured by the short shear beam method of three-pointbending. The shear strength was calculated by the following equation [17]