CERAMICS INTERNATIONAL ELSEVIER Ceramics International 27(2001)565-570 www.elsevier.com/locate/ceramint High performance 3d textile Hi-Nicalon SiC/SiC composites by chemical vapor infiltration Yongdong Xu*, Aifei Cheng, Litong Zhang, Xiaowei Yin, Hongfeng Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xian, Shaanxi 710072, China Received Il September 2000: received in revised form 30 October 2000: accepted 18 December 2000 Abstract Three dimensional textile Hi-Nicalon silicon carbide fiber reinforced silicon carbide composites with high toughness and relia- bending, shear, and impact loading. bor infiltration. The mechanical properties of the composite materials were investigated under b lity were fabricated by chemical val ne density of the composites was 2.5 g cm-3 after the three dimension silicon carbide perform has been infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300C in vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mis-match of thermal expansion coefficients between fiber and matrix. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance and the value of dynamic fracture toughness is 36.0 kJ m-2 was measured with Charpy impact tests. C 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved Keywords: C. Mechanical properties; 3D SiC/SiC composites; Chemical vapor infiltration 1. Introduction tration and examined the mechanical properties over the temperature range from room temperature to 1300C. Continuous fiber reinforced ceramic matrix compo- The aims of the current contribution are to develop an sites(CFCCs) show superior performance when super- understanding of the architecture on the mechanical lloy at elevated temperatures and higher toughness properties and the damage behavior of the 3D Hi-Nica- wheen compared with monolithic ceramics [1-4]. For lon SiC/Sic composites and to expand the experimental these reasons, CFCCs have the most potential to be used knowledge for the three dimensional textile composite in advanced aero-engines, space, and fusion power reac materials tors [1-4]. Among these CFCCs, silicon carbide fiber reinforced silicon carbide composites(SiC/SiC)are pro- mising and have received considerable attention. Many 2. Materials and experimental procedures investigations have been conducted on one dimension (ID)and two-dimension(2D)woven SiC/SiC composite 2.1. Fabrication of the composites materials [5-12]. Recently, attention has been focused on three-dimension woven or braided ceramic matrix Hi-Nicalon SiC fiber was employed and each ya aIn composite materials in order to meet mechanical and contained 500 filaments. The three dimensional(3D) thermal properties requirements under most complex fabric perform was braided by four-step processing and loads [13-18. supplied by Nanjing Institute of Glass Fiber in China The present research involved 3D textile Hi-Nicalon The structure of the preform is illustrated in Fig. 1. The SiC/SiC composites prepared by chemical vapor infil- fiber volume fraction was 40%. In the present experi ment, chemical vapor infiltration was employed to deposit a pyrolytic carbon layer and silicon carbide, 4 Corresponding author which has been described previously in detail [ 14, 15].A 8491000. +86-29-8491427:fax: 86-29. thin carbon layer as the interfacial layer was deposited E-mailaddress:ydxu(@nwpu.edu.cn(Y.Xu). on the surface of the hi-Nicalon sic fiber with butane 0272-8842/01/S2000C 2001 Elsevier Science Ltd and Techna S.r. I All rights reserved. PII:S0272-8842(01)00002-5
High performance 3D textile Hi-Nicalon SiC/SiC composites by chemical vapor infiltration Yongdong Xu*, Laifei Cheng, Litong Zhang, Xiaowei Yin, Hongfeng Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi, an, Shaanxi 710072, China Received 11 September 2000; received in revised form 30 October 2000; accepted 18 December 2000 Abstract Three dimensional textile Hi-Nicalon silicon carbide fiber reinforced silicon carbide composites with high toughness and reliability were fabricated by chemical vapor infiltration. The mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g cm3 after the three dimension silicon carbide perform has been infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300C in vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mis-match of thermal expansion coefficients between fiber and matrix. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance and the value of dynamic fracture toughness is 36.0 kJ m2 was measured with Charpy impact tests. # 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: C. Mechanical properties; 3D SiC/SiC composites; Chemical vapor infiltration 1. Introduction Continuous fiber reinforced ceramic matrix composites (CFCCs) show superior performance when superalloy at elevated temperatures and higher toughness wheen compared with monolithic ceramics [1–4]. For these reasons, CFCCs have the most potential to be used in advanced aero-engines, space, and fusion power reactors [1–4]. Among these CFCCs, silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) are promising and have received considerable attention. Many investigations have been conducted on one dimension (1D) and two-dimension (2D) woven SiC/SiC composite materials [5–12]. Recently, attention has been focused on three-dimension woven or braided ceramic matrix composite materials in order to meet mechanical and thermal properties requirements under most complex loads [13–18]. The present research involved 3D textile Hi-Nicalon SiC/SiC composites prepared by chemical vapor infiltration and examined the mechanical properties over the temperature range from room temperature to 1300C. The aims of the current contribution are to develop an understanding of the architecture on the mechanical properties and the damage behavior of the 3D Hi-Nicalon SiC/SiC composites and to expand the experimental knowledge for the three dimensional textile composite materials. 2. Materials and experimental procedures 2.1. Fabrication of the composites Hi-Nicalon SiC fiber was employed and each yarn contained 500 filaments. The three dimensional (3D) fabric perform was braided by four-step processing and supplied by Nanjing Institute of Glass Fiber in China. The structure of the preform is illustrated in Fig. 1. The fiber volume fraction was 40%. In the present experiment, chemical vapor infiltration was employed to deposit a pyrolytic carbon 1ayer and silicon carbide, which has been described previously in detail [14,15]. A thin carbon layer as the interfacial layer was deposited on the surface of the Hi-Nicalon SiC fiber with butane 0272-8842/01/$20.00 # 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(01)00002-5 Ceramics International 27 (2001) 565–570 www.elsevier.com/locate/ceramint * Corresponding author. Tel.: +86-29-8491427; fax: 86-29- 8491000. E-mail address: ydxu@nwpu.edu.cn (Y. Xu).
Y. Xu et al./ Ceramics International 27(2001)565-570 (C3H6) prior to densification. Methytrichosilane(MTS, 3. Results and discussion CH3 SiCl3) was used for deposition of Sic and carried by bubbling hydrogen(H2). Typical conditions used for 3. 1. Flexural loading ne densification of silicon carbide matrix are 1100C. a hydrogen to MTS mol ratio of 10, and a pressure of 2-3 The density of the composites was 2.5 g cm-s after the kPa. Argon (Ar) was employed as a diluent gas to slow three dimensional silicon carbide preform was chemical down the chemical reaction rate of deposition vapor infiltrated for 30 h. Fig. 2a showed the typical failure behavior of 3D Hi-Nicalon SiC/Sic textile com- 2. 2. Mechanical properties measurement posites at room temperature. The mechanical behavior was initially linear elastic. Then, a nonlinear region was Mechanical properties of the composite materials observed, reflecting matrix damage which induced sig were characterized under flexural shear, and impact nificantly compliance, and residual displacement. Finally, ding. Flexural strength was measured with a three- the fiber failed, initiating at the maxim load, causing the point-bending method at temperatures ranging from unstable fracture of the composites. As the temperature room temperature up to 1300oC in vacuum. Shear was increased from room temperature to 1300C, the strength was measured by the short beam bending flexural strength of the 3D Hi-Nicalon SiC/SiC compo- method with a span of 15 mm. Fracture toughness was sites was slightly increased but not decreased. The aver determined with single edged-notched beam method. age values of the flexural strength were 920 MPa at The impact tests were performed with an instrumented room temperature and 1010 MPa at 1300oC in vacuum Charpy equipment for the test. The sample size was The failure behavior of the composites changed with 3.0x20x70 mm, and the impact velocity of 3 m s-I was increase of the temperature. At room temperature the imposed for the test stress drop was very gradual after the maximum stress point. However, the failure behavior became brittle and 2.3. Microstructure observation and surface analysis he composites exhibited steep stress drops after the maximum stress point at high temperatures(Fig. 2b) The density of the samples was determined by the The variation of failure behavior of the composites water displacement method. The microstructure of the was attributed to the interfacial bonding between fiber fracture surface was observed by a scanning electron and matrix. Fig 3 showed the typical microstructure of mIcroscope the Hi-Nicalon SiC/SiC composite materials. The pyr- olysis carbon interfacial layer was very uniform and the thickness was 300 nm. It was this layer that ensured the proper interfacial bonding between fiber and matrix as well as the load transfer from the silicon carbide matrix to the Hi-Nicalon SiC fiber. Moreover, it has been reported that the thermal expansion coefficients of the silicon car bide matrix and the Hi-Nicalon fiber were 22x10-6K-I and 5.3x10-6K-, respectively [19-21]. After the com- posites were cooled down from the infiltration tempera braiding ture to room temperature, a tensile stress was generated Fig. I. Structure of a three dimension preform. cross the interfacial layer. As a result, it was easy for the 1000 4.0 2040.6081.01.2141.6 Displacement, mm Fig. 2. Stress-deflection curve under flexural loading
(C3H6) prior to densification. Methytrichosilane (MTS, CH3SiCl3) was used for deposition of SiC and carried by bubbling hydrogen (H2). Typical conditions used for the densification of silicon carbide matrix are 1100C, a hydrogen to MTS mol ratio of 10, and a pressure of 2–3 kPa. Argon(Ar) was employed as a diluent gas to slow down the chemical reaction rate of deposition. 2.2. Mechanical properties measurement Mechanical properties of the composite materials were characterized under flexural shear, and impact loading. Flexural strength was measured with a threepoint-bending method at temperatures ranging from room temperature up to 1300C in vacuum. Shear strength was measured by the short beam bending method with a span of 15 mm. Fracture toughness was determined with single edged-notched beam method. The impact tests were performed with an instrumented Charpy equipment for the test. The sample size was 3.02070 mm, and the impact velocity of 3 m s1 was imposed for the test. 2.3. Microstructure observation and surface analysis The density of the samples was determined by the water displacement method. The microstructure of the fracture surface was observed by a scanning electron microscope. 3. Results and discussion 3.1. Flexural loading The density of the composites was 2.5 g cm3 after the three dimensional silicon carbide preform was chemical vapor infiltrated for 30 h. Fig. 2a showed the typical failure behavior of 3D Hi-Nicalon SiC/SiC textile composites at room temperature. The mechanical behavior was initially linear elastic. Then, a non1inear region was observed, reflecting matrix damage which induced significantly compliance, and residual displacement. Finally, the fiber failed, initiating at the maxim load, causing the unstable fracture of the composites. As the temperature was increased from room temperature to 1300C, the flexural strength of the 3D Hi-Nicalon SiC/SiC composites was slightly increased but not decreased. The average values of the flexural strength were 920 MPa at room temperature and 1010 MPa at l300C in vacuum. The failure behavior of the composites changed with increase of the temperature. At room temperature the stress drop was very gradual after the maximum stress point. However, the failure behavior became brittle and the composites exhibited steep stress drops after the maximum stress point at high temperatures (Fig. 2b). The variation of failure behavior of the composites was attributed to the interfacial bonding between fiber and matrix. Fig. 3 showed the typical microstructure of the Hi-Nicalon SiC/SiC composite materials. The pyrolysis carbon interfacial layer was very uniform and the thickness was 300 nm. It was this layer that ensured the proper interfacial bonding between fiber and matrix as well as the load transfer from the silicon carbide matrix to the Hi-Nicalon SiC fiber. Moreover, it has been reported that the thermal expansion coefficients of the silicon carbide matrix and the Hi-Nicalon fiber were 2.2106 K1 and 5.3106 K1 , respectively [19–21]. After the composites were cooled down from the infiltration temperature to room temperature, a tensile stress was generated Fig. 1. Structure of a three dimension preform. cross the interfacial layer. As a result, it was easy for the Fig. 2. Stress–deflection curve under flexural loading. 566 Y. Xu et al. / Ceramics International 27 (2001) 565–570
661715Ky 02115Ky沁:643 Fig 3. Microstructure of 3D Hi-Nicalon SiC/SiC composites. 268320KUx14810wmW03 220KU X330100 261328KV X221WD38 Fig 4. Fracture surface of 3D Hi-Hicalon fiber to be pulled out from the silicon carbide matrix. If CFCCs, the bundle/bundle interfacial bonding is the temperature was increased above the silicon carbide usually considered as a kind of weak interfacial bonding infiltration temperature (1100oC), a compressive stress because of the pores residual in the composites caused was created across the interfacial layer. Hence, the fiber by the"bottom neck effect"during the chemical vapor was very difficult to pull out. The pull-out length at infiltration process. Accordingly, fiber bundle pullout 600C was much shorter than at room temperature, as was a]ways observed at both room temperature and hown in Fig. 4a, b. For the three dimensional textile high temperature(Fig. 4c, d)
fiber to be pulled out from the silicon carbide matrix. If the temperature was increased above the silicon carbide infiltration temperature (1100C), a compressive stress was created across the interfacial layer. Hence, the fiber was very difficult to pull out. The pull-out length at l300C was much shorter than at room temperature, as shown in Fig. 4a,b. For the three dimensional texti1e CFCCs, the bundle/bundle interfacial bonding is usually considered as a kind of weak interfacial bonding because of the pores residual in the composites caused by the ‘‘bottom neck effect’’ during the chemical vapor infiltration process. Accordingly, fiber bundle pullout was a]ways observed at both room temperature and high temperature (Fig. 4c,d). Fig. 3. Microstructure of 3D Hi-Nicalon SiC/SiC composites. Fig. 4. Fracture surface of 3D Hi-Hicalon SiC/SiC composites. Y. Xu et al. / Ceramics International 27 (2001) 565–570 567
Y. Xu et al./ Ceramics International 27(2001)565-570 3. 2. Failure behavior under shear loading The shear failure behavior of the stress-deflection curve was similar to that of bending failure behavior and The shear strength of the 3D Hi-Nicalon SiC/Sic shown in Fig. 5. The obtained value of shear strength composites was measured by the short bending beam was 67.5 MPa which is much higher than that of 3D C method, and the span was 15 mm. Shear strength was SiC textile composites. Different from two-dimensional calculated by the following equation CFCCs and the other laminated composites, interlayer debonding was not observed in the present composites () The results indicated that the present 3D Hi-Nicalon Sic/SiC composites exhibited good shear resistant and where p is the maximum fracture load(N), b and h are isotropic properties the width and height of the sample respectively 3.3. Failure behavior of notched specimen under flexural loading In order to determine the fracture toughness of the composites, samples were notched and tested in a three-point-bending sample with a span of 40 mm and a loading rate of 0.05 mm mm-. The width of the notch was 0.01 mm. The value of fracture toughness(Kls) was calculated by using of the following expression 0.204060.8101 KIc 3-3.072+145()2-25.07 Displacement, mm +25.80 ig. 5. Stress-deflection curve under shear loading Fig. 6. Microstructure differences between(a) Hi-Nicalon SiC/SiC and (b)C/SiC composites Fig. 7. Failure behavior of the 3D Hi-Nicalon SiC/SiC composites with a notch
3.2. Failure behavior under shear loading The shear strength of the 3D Hi-Nicalon SiC/SiC composites was measured by the short bending beam method, and the span was 15 mm. Shear strength was calculated by the following equation: ¼ 3p 4bh ð1Þ where p is the maximum fracture load (N), b and h are the width and height of the sample respectively. The shear failure behavior of the stress–deflection curve was similar to that of bending failure behavior and shown in Fig. 5. The obtained value of shear strength was 67.5 MPa which is much higher than that of 3D C/ SiC textile composites. Different from two-dimensional CFCCs and the other laminated composites, inter1ayer debonding was not observed in the present composites. The results indicated that the present 3D Hi-Nicalon SiC/SiC composites exhibited good shear resistant and isotropic properties. 3.3. Failure behavior of notched specimen under flexural loading In order to determine the fracture toughness of the composites, samples were notched and tested in a three-point-bending sample with a span of 40 mm and a loading rate of 0.05 mm mm1 . The width of the notch was 0.01 mm. The value of fracture toughness (K1c) was calculated by using of the following expression: K1c ¼ 3pl 2bw2 1:93 3:07 a w þ 1:45 a w 2 25:07 a w 3 þ 25:80 a w 4 ð2Þ Fig. 5. Stress–deflection curve under shear loading. Fig. 6. Microstructure differences between (a) Hi-Nicalon SiC/SiC and (b) C/SiC composites. Fig. 7. Failure behavior of the 3D Hi-Nicalon SiC/SiC composites with a notch. 568 Y. Xu et al. / Ceramics International 27 (2001) 565–570
Y. Xu et al. Ceramics International 27(2001)565-570 Fig 8. Impact fracture surface of Hi-Nicalon Sic/SiC composites. where p is the fracture load, I is the span, and a is the super-alloy (ak=80-160 kJ m). The impact fracture notch depth, b and w are thickness and width of the surface was brush-like in Fig 8a. It was very interesting imple respectively. to observe that the present composite materials could The fracture toughness calculated from Eq(2)was withstand the hitting impact of a steel nail(Fig. 8b) 41.5 MPa m /2. which was 10 times that of monolithic These results revealed that the 3d textile hi-nicalon ceramic materials(3-5 MPa m /)and two times that of SiC/SiC composites exhibited the excellent resistance 3D C/SiC composites(20. 3 MPa m/2)(22]. The fracture against dynamic impact toughness differences between 3D C/SiC and 3D H icalon SiC/SiC composites could be illustrated from the microstructure difference of these two kinds of 4. Conclusions materials. In Fig. 6, it was observed that the surface of Hi-Nicalon sic fiber was much smoother than that of High performance three dimensional textile Hi-Nica T300 carbon fiber. Consequently, it was very easy for lon SiC fiber reinforced silicon carbide composites were Hi-Nicalon SiC fiber to be pulled out from the Sic fabricated by chemical vapor infiltration. The density of matrix, leading to higher fracture toughness the composites was 2.5g cm-3 after the three-dimen- Here, the work of fracture was introduced to repre- sional carbon preform was infiltrated for 30 h. The sent the toughness of the 3D Hi-Nicalon SiC/Sic textile values of flexural strength were 860 MPa at room tem- composite materials. The work of fracture was obtained perature and 1010 MPa at 1300 C in vacuum. Above the from the characteristic area under the load-displacement infiltration temperature, the failure behavior of the com- curve divided by the cross-section of the specimen. In posites became brittle because a compressive stress was order to determine the work of fracture effectively, we generated cross the interfacial layer caused by the mis defined the characteristic area(Ac)which started from match of thermal expansion coefficients between fiber the initial point to the 10% drop of the curve(Fig. 7). and matrix. The obtained value of the shear strength This gives an average work of fracture as high as 28. 1 kJ was 67.5 MPa. The fracture toughness and work of m-, which is the nearly three times that of 3D C/Sic fracture were as high as 41. 5 MPa m/2 and 28. 1 kJ m-2 composites and six times that of laminated SiC ceramic respectively. The value of dynamic fracture toughness matrix composites (4625 J m -), respectively [22, 23] was 36. kJ m-- 3.4. Impact loading Acknowledgements Instrumented Charpy impact tests on un-notched amples were conducted to determine the energy The authors wish to thank the National natural sci- absorbing capability and dynamic fracture behavior of entific Foundation of China, Chinese Aeronautics Foun- the composite materials. The dynamic fracture toughn dation and National Defense Foundation of China for (ak)was calculated by using the following equation: the financial support ak=△w/bh References where w is the absorbing energy of materials during impact processing, b and h are the thickness and width [1 T M. Besmann R.A. Lowden, of specimen, respectively tration, in:R. Naslain(Ed. ) perature Ceramic Matrix Composites, Woodhead Public The value of ak is 36 kJ m- for 3D Hi-Nicalon SiC/ (23S. Jacques, A.Guette,F.La bordeaux, 1993, pp. 215 Naslain. S. GowJard Sic composite materials, and is lower than that of Preparation and characterization of Sic/Sic composites with
where p is the fracture load, l is the span, and a is the notch depth, b and w are thickness and width of the simp1e respectively. The fracture toughness calculated from Eq. (2) was 41.5 MPa m1/2, which was 10 times that of monolithic ceramic materials (3–5 MPa m1/2) and two times that of 3D C/SiC composites (20.3 MPa m1/2)[22]. The fracture toughness differences between 3D C/SiC and 3D HiNicalon SiC/SiC composites could be i1lustrated from the microstructure difference of these two kinds of materials. In Fig. 6, it was observed that the surface of Hi-Nicalon SiC fiber was much smoother than that of T300 carbon fiber. Consequently, it was very easy for Hi-Nicalon SiC fiber to be pulled out from the SiC matrix, leading to higher fracture toughness. Here, the work of fracture was introduced to represent the toughness of the 3D Hi-Nicalon SiC/SiC textile composite materials. The work of fracture was obtained from the characteristic area under the load-displacement curve divided by the cross-section of the specimen. In order to determine the work of fracture effectively, we defined the characteristic area (Ac) which started from the initial point to the 10% drop of the curve (Fig. 7). This gives an average work of fracture as high as 28.1 kJ m2 , which is the nearly three times that of 3D C/SiC composites and six times that of laminated SiC ceramic matrix composites (4625 J m2 ), respectively [22,23]. 3.4. Impact loading Instrumented Charpy impact tests on un-notched samples were conducted to determine the energy absorbing capability and dynamic fracture behavior of the composite materials. The dynamic fracture toughness (k) was calculated by using the following equation: k ¼ w=bh ð3Þ where w is the absorbing energy of materials during impact processing, b and h are the thickness and width of specimen, respectively. The value of k is 36 kJ m2 for 3D Hi-Nicalon SiC/ SiC composite materials, and is lower than that of super-alloy (k=80–160 kJ m2 ). The impact fracture surface was brush-1ike in Fig. 8a. It was very interesting to observe that the present composite materials could withstand the hitting impact of a steel nail (Fig. 8b). These results revealed that the 3D textile Hi-Nicalon SiC/SiC composites exhibited the excellent resistance against dynamic impact. 4. Conclusions High performance three dimensional textile Hi-Nicalon SiC fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. The density of the composites was 2.5g cm3 after the three-dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300C in vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because a compressive stress was generated cross the interfacial 1ayer caused by the mismatch of thermal expansion coefficients between fiber and matrix. The obtained value of the shear strength was 67.5 MPa. The fracture toughness and work of fracture were as high as 41.5 MPa m1/2 and 28.1 kJ m2 respectively. The value of dynamic fracture toughness was 36.0 kJ m2 . Acknowledgements The authors wish to thank the National Natural Scientific Foundation of China, Chinese Aeronautics Foundation, and National Defense Foundation of China for the financial support. References [1] T.M. Besmann, R.A. Lowden, Overview of chemical vapor infiltration, in: R. Naslain (Ed.), High Temperature Ceramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 215. [2] S. Jacques, A. Guette, F. Langlais, R. Naslain, S. GouJard, Preparation and characterization of SiC/SiC composites with Fig. 8. Impact fracture surface of Hi-Nicalon SiC/SiC composites. Y. Xu et al. / Ceramics International 27 (2001) 565–570 569
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