SEI TERIALS ENGE& ENGIEERN ELSEVIER Materials Science and Engineering A300(2001)219-225 www.elsevier.com/locate/msea Effect of carbon interlayer on oxidation behavior of C/Sic composites with a coating from room temperature to 1500C Laifei Cheng, Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xian, Shaanxi 710072, People's Republic of China Received 9 May 2000: received in revised form 8 August 2000 Abstract Two C/SiC composites were prepared by AP-CVI and LP-CVI method Oxidation tests of the C/Sic composites and a C/C composite with a coating were conducted in dry air from room temperature to 1500C for 5 h. A continuous series of empirical junctions relating weight change after 5 h oxidation to temperature was found to fit the test results of the three composites quit ell Oxidation behavior of the three composites could be described by a continuous function over the full temperature range. The Sic matrix made the coating cracking temperature of the AP-CVI C/SiC being 100oC lower than that of the C/C, and the Pyc interlayer with a different thickness made the coating cracking temperature and matrix cracking temperature of the LP-CVI C/Sic ing 100@C lower than those of the AP-CVI C/SiC. There was an optimum thickness of the Pyc interlayer for improving the oxidation resistance of C/SiC composites. The thicker the Pyc interlayer, the lower the transition temperature, the higher the mechanical properties, but the larger the maximum weight loss. Below the transition temperatures, the activation energies of reaction for the C/C and C/Sic composites varied little. Above the coating temperatures, the Si-w layer had higher activation energy for diffusion than the Si-Zr layer. C 2001 Elsevier Science B.V. All rights reserved. Keywords: C/SiC composites: Oxidation behavior; Interlayer 1. Introduction carbon felt in a large Cvi furnace oxidation of the fibers caused by the moisture should be taken into An interlayer of pyrolytic carbon(PyC)is very neces- consideration. Thirdly, it can increase strength of the sary for improving the mechanical properties of carbon composites by decreasing damage of the fibers pro- fiber reinforced silicon carbide composites(C/SiC). duced by the thermal stress at the interface owing to the Firstly, it can change fracture behavior of C/SiC com- great thermal expansion mismatch between the fibers posites from brittle to toughened by weakening the and the SiC matrix 14,5]. Both oxidation and damage interfacial bonding of the fibers and the SiC matrix decrease the strength of the fibers. The carbon fibers [1-3]. Secondly, it can increase strength of the com- have a very high strength, and then keeping the posites by protecting the fibers from oxidation in the strength is the most important problem for preparation CVI process. If a large CVI equipment is used, some of C/Sic composites. Obviously, the thicker the PyC oxidation of the carbon fibers is unavoidable although interlayer, the higher the mechanical properties. But the he equipment is clean and displays no leaks. When the gher properties can only be kept in inert atmospheres, furnace is open to load specimens, moisture in air will and will decrease rapidly in oxidizing atmospheres be- be adsorbed by the furnace wall, heating elements and cause they are very sensitive to the oxidation of the carbon felt. The adsorbed moisture can not be removed carbon fibers and the Pyc interlayer [6-8]. The PyC completely by vaccum. In heating process, the moisture interlayer thickness has great effect on the oxidation will be released. Because there large amout of resistance of C/SiC composites [9]. If the PyC interl in C/SiC composites was thick enough, they are 849460 Tesponding author. Tel. +86-29-8494616 fax: +86-2 like C/C composites and have poor oxidation tance. Two C/SiC composites with different PyC inter E-mail address: cmcres(@nwyu educn (L. Cheng layer thickness were prepared, and oxidation behavior 0921-5093/01/s- see front matter o 2001 Elsevier Science B.V. All rights reserved PI:S0921-5093(00)01405-2
Materials Science and Engineering A300 (2001) 219–225 Effect of carbon interlayer on oxidation behavior of C/SiC composites with a coating from room temperature to 1500°C Laifei Cheng *, Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical Uni6ersity, Xi’an, Shaanxi 710072, People’s Republic of China Received 9 May 2000; received in revised form 8 August 2000 Abstract Two C/SiC composites were prepared by AP-CVI and LP-CVI method. Oxidation tests of the C/SiC composites and a C/C composite with a coating were conducted in dry air from room temperature to 1500°C for 5 h. A continuous series of empirical functions relating weight change after 5 h oxidation to temperature was found to fit the test results of the three composites quite well. Oxidation behavior of the three composites could be described by a continuous function over the full temperature range. The SiC matrix made the coating cracking temperature of the AP-CVI C/SiC being 100°C lower than that of the C/C, and the PyC interlayer with a different thickness made the coating cracking temperature and matrix cracking temperature of the LP-CVI C/SiC being 100°C lower than those of the AP-CVI C/SiC. There was an optimum thickness of the PyC interlayer for improving the oxidation resistance of C/SiC composites. The thicker the PyC interlayer, the lower the transition temperature, the higher the mechanical properties, but the larger the maximum weight loss. Below the transition temperatures, the activation energies of reaction for the C/C and C/SiC composites varied little. Above the coating temperatures, the Si–W layer had higher activation energy for diffusion than the Si–Zr layer. © 2001 Elsevier Science B.V. All rights reserved. Keywords: C/SiC composites; Oxidation behavior; Interlayer www.elsevier.com/locate/msea 1. Introduction An interlayer of pyrolytic carbon (PyC) is very necessary for improving the mechanical properties of carbon fiber reinforced silicon carbide composites (C/SiC). Firstly, it can change fracture behavior of C/SiC composites from brittle to toughened by weakening the interfacial bonding of the fibers and the SiC matrix [1–3]. Secondly, it can increase strength of the composites by protecting the fibers from oxidation in the CVI process. If a large CVI equipment is used, some oxidation of the carbon fibers is unavoidable although the equipment is clean and displays no leaks. When the furnace is open to load specimens, moisture in air will be adsorbed by the furnace wall, heating elements and carbon felt. The adsorbed moisture can not be removed completely by vaccum. In heating process, the moisture will be released. Because there is a large amout of carbon felt in a large CVI furnace, oxidation of the fibers caused by the moisture should be taken into consideration. Thirdly, it can increase strength of the composites by decreasing damage of the fibers produced by the thermal stress at the interface owing to the great thermal expansion mismatch between the fibers and the SiC matrix [4,5]. Both oxidation and damage decrease the strength of the fibers. The carbon fibers have a very high strength, and then keeping the strength is the most important problem for preparation of C/SiC composites. Obviously, the thicker the PyC interlayer, the higher the mechanical properties. But the higher properties can only be kept in inert atmospheres, and will decrease rapidly in oxidizing atmospheres because they are very sensitive to the oxidation of the carbon fibers and the PyC interlayer [6–8]. The PyC interlayer thickness has great effect on the oxidation resistance of C/SiC composites [9]. If the PyC interlayer in C/SiC composites was thick enough, they are more like C/C composites and have poor oxidation resistance. Two C/SiC composites with different PyC interlayer thickness were prepared, and oxidation behavior * Corresponding author. Tel.: +86-29-8494616; fax: +86-29- 8494620. E-mail address: cmcres@nwyu.edu.cn (L. Cheng). 0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)01405-2
L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 of the composites with a coating was investigated and prepared by liquid-reaction method at 1500C for 30 compared with that of a C/C composite with a coating min on the CVD Sic layer. n this paper Oxidation tests of the three coated materials were conducted in dry air for 5 h at different temperatures from400C-1500°C. 2. Experimental Three composites were used in experiments. The first 3. Modelling one, cut and machined off from aircraft brakes, was a 2D-CC. It has been prepared by CVI and treated at If the interfacial reactions produced by oxidation 2000C. It does not contain any oxidation inhibitor. could not be taken account below 1500C, weight The density of the C/C measured by Archimedes' change of coated C/C composites on oxidation is con- method is 1.7 g cm-3, and the total porosity is 15.5% sidered to be influenced by three factors:(1)oxidation (10% open; 5.5% closed). The substrates were 5 x 5x of substrates by diffusion of oxygen through coating 25 mm in size. The three-layer coating prepared on the cracks;(2) sealing of coating cracks when temperature substrates consisted of a conversion SiC layer, a barrier is raised;(3)oxidation of coating by diffusion of oxy- Sic layer and a sealant Si-W layer. The conversion Sic gen through the oxide film. Because a threshold of yer,which could improve the expansion mismatch of temperature and an activation energy were needed the substrates and the coating, was prepared by sili- weight changes produced by these three factors could conization on the surface of the substrates at 1500c be expressed by the following equations separately or 15 min. The dense SiC barrier layer, which was used to prevent infiltration of the outer layer, was prepared △Wc=Ac(1-exp(-BcTo) (1) on the transition layer by CVD at 1100oC for 30 min. AWs =As1-exp(-BsT"), be formed on, was prepared by liquid-reaction at AWF=A (1-exp(-"), l500°Cfor30min where A, B and n are constants, C, S and F represent The second one was a C/Sic composite prepared by y diffusion through cracks, sealing of cracks, diffusion sual pressure chemical vapor deposition method (AP through oxide film and interfacial reactions rest CVI). The carbon fiber was T-300TM. The preforms tively, and Aw is the weight change in percent. Weight vere densified with pyrolysis carbon (Pyc) and Sic change of coated C/C should be the sum of the weight from butane and methyltrichlorosilane(MTS/H2). The losses and weight gains produced by the three factors interfacial layer of Pyc was deposited for one hour at 870°C. The deposition conditions of Sic matrix were as△W=△W+△Ws+△W follow: temperature was 1100C, time was 2 h, flow of Besides these three factors weight change of coated H2 was about 150 mImin, and the molar ratio of H2 C/Sic composites is considered to be influenced by and mTS was 10. The substrates with a size of 4x6x sealing of Sic matrix cracks when temperature is raised 40 mm were cut from the fabricated composite and submitted to a deposition treatment for 2 h to seal the AWs=As(1-exp(- BsTs)), open ends of the fiber. A Si-W coating was prepared where S represent sealing of the matrix cracks. Weight by liquid-reaction at 1500C for 30 min on the Sic change of the coated C/SiC from room temperature to deposition process. The third one was a C/SiC composite prepared by 1500]C should be the sum of the weight losses and low-pressure chemical vapor deposition method (LP weight gains produced by the four factors CVI). The preforms were infiltrated with pyrolysis ca △W=ΔWc+ΔWs+ΔWs+△W Pyc) and Sic from butane methyltrichlorosilane(MTS/H,). The conditions for de- osition of PyC interlayer were as follow: temperature 4. Results and discusion 960C, pressure 5 KPa, time 20 h, Ar flow 200 ml- min- butane flow 15 ml- min- The conditions For the three composites, a continuous function of for deposition of Sic matrix were as follow: tempera eight change with temperature which fitted the test ture 1000oC, pressure 5 KPa, time 120 h, H2 flow 350 results quite well was obtained by selecting proper mI'min-, Ar flow 350 ml-min-, and the molar ratio constants in equations(Eqs. (1)-(3)and(5)through of H2 and MTS was 10. The substrates with a size of trial and error(Fig. 1), and the selected constants are 3 x5x40 mm were machined from the fabricated com- listed in Table l. It was shown that the present model posite with a size of 4 6x 150 mm and treated for 20 represented the different oxidation behavior of the h to deposit a CVD SiC layer. A Si-Zr coating was three composites over the full temperature range, and
220 L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 of the composites with a coating was investigated and compared with that of a C/C composite with a coating in this paper. 2. Experimental Three composites were used in experiments. The first one, cut and machined off from aircraft brakes, was a 2D-C/C. It has been prepared by CVI and treated at 2000°C. It does not contain any oxidation inhibitor. The density of the C/C measured by Archimedes’ method is 1.7 g cm−3 , and the total porosity is 15.5% (10% open; 5.5% closed). The substrates were 5×5× 25 mm in size. The three-layer coating prepared on the substrates consisted of a conversion SiC layer, a barrier SiC layer and a sealant Si–W layer. The conversion SiC layer, which could improve the expansion mismatch of the substrates and the coating, was prepared by siliconization on the surface of the substrates at 1500°C for 15 min. The dense SiC barrier layer, which was used to prevent infiltration of the outer layer, was prepared on the transition layer by CVD at 1100°C for 30 min. The Si–W sealant layer, which a silica glass film could be formed on, was prepared by liquid-reaction at 1500°C for 30 min. The second one was a C/SiC composite prepared by usual pressure chemical vapor deposition method (APCVI). The carbon fiber was T-300™. The preforms were densified with pyrolysis carbon (PyC) and SiC from butane and methyltrichlorosilane (MTS/H2). The interfacial layer of PyC was deposited for one hour at 870°C. The deposition conditions of SiC matrix were as follow: temperature was 1100°C, time was 2 h, flow of H2 was about 150 ml·min−1 , and the molar ratio of H2 and MTS was 10. The substrates with a size of 4×6× 40 mm were cut from the fabricated composite and submitted to a deposition treatment for 2 h to seal the open ends of the fiber. A Si–W coating was prepared by liquid-reaction at 1500°C for 30 min on the SiC layer formed in the deposition process. The third one was a C/SiC composite prepared by low-pressure chemical vapor deposition method (LPCVI). The preforms were infiltrated with pyrolysis carbon (PyC) and SiC from butane and methyltrichlorosilane (MTS/H2). The conditions for deposition of PyC interlayer were as follow: temperature 960°C, pressure 5 KPa, time 20 h, Ar flow 200 ml·min−1 , butane flow 15 ml·min−1 . The conditions for deposition of SiC matrix were as follow: temperature 1000°C, pressure 5 KPa, time 120 h, H2 flow 350 ml·min−1 , Ar flow 350 ml·min−1 , and the molar ratio of H2 and MTS was 10. The substrates with a size of 3×5×40 mm were machined from the fabricated composite with a size of 4×6×150 mm and treated for 20 h to deposit a CVD SiC layer. A Si–Zr coating was prepared by liquid-reaction method at 1500°C for 30 min on the CVD SiC layer. Oxidation tests of the three coated materials were conducted in dry air for 5 h at different temperatures from 400°C–1500°C. 3. Modelling If the interfacial reactions produced by oxidation could not be taken account below 1500°C, weight change of coated C/C composites on oxidation is considered to be influenced by three factors: (1) oxidation of substrates by diffusion of oxygen through coating cracks; (2) sealing of coating cracks when temperature is raised; (3) oxidation of coating by diffusion of oxygen through the oxide film. Because a threshold of temperature and an activation energy were needed, weight changes produced by these three factors could be expressed by the following equations separately DWC=AC(1−exp(−BCTnC)), (1) DWS=AS(1−exp(−BSTnS )), (2) DWF=AF(1−exp(−BFTnF)), (3) where A, B and n are constants, C, S and F represent diffusion through cracks, sealing of cracks, diffusion through oxide film and interfacial reactions respectively, and DW is the weight change in percent. Weight change of coated C/C should be the sum of the weight losses and weight gains produced by the three factors DW=DWC+DWS+DWF, (4) Besides these three factors, weight change of coated C/SiC composites is considered to be influenced by sealing of SiC matrix cracks when temperature is raised DWS%=AS% (1−exp(−BS% TnS% )), (5) where S% represent sealing of the matrix cracks. Weight change of the coated C/SiC from room temperature to 1500°C should be the sum of the weight losses and weight gains produced by the four factors. DW=DWC+DWS+DWS%+DWF. (6) 4. Results and discusion For the three composites, a continuous function of weight change with temperature which fitted the test results quite well was obtained by selecting proper constants in equations (Eqs. (1)–(3) and (5)) through trial and error (Fig. 1), and the selected constants are listed in Table 1. It was shown that the present model represented the different oxidation behavior of the three composites over the full temperature range, and
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L . Cheng et al . / Materials Science and Engineering A300 (2001) 219 –225 221 Table 1 Constants in a model relation of weight change with temperature for oxidation of the coated C/C, AP-CVI C/SiC (U-C/SiC) and LP-CVI C/SiC (L-C/SiC) F CS S% C/C U-C/ SiC L-C/ SiC C/C U-C/ SiC L-C/ SiC C/C U-C/ SiC L-C/ SiC C/C U-C/ SiC L-C/ SiC 8.8 A −10.3 110.6 1.4 −4.7 4.4 3.2 −30.2 −9.8 −10.3 26.2 −2.0×10−9 −1.7×10−17 −4.0×10−14 −1.0×10−25 −3.0×10−23 −2.0×10−10 −6.0×10−28 −1.0×10−9 B −1.0×10−10 −3.0×10−10 −1.0×10−10 n 10.8 11.8 13.0 14.2 20.5 21.3 23.2 14.4 12.8 9.9 10.4
L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 2LPCⅥCsc- AP-CVI C/SiC ature can be defined as the coating cracking tempera ture. It was more dangerous for the composites working at low temperatures than at high temperature The temperatures corresponding to the maximum weight losses were defined as the transition tempera tures. Each curve of the C/SiC composites had a char- acteristic different from that of the C/C composite, the former having two shoulders and the latter having only one produced by sealing of the coating cracks between the transition temperature and the cracking tempera ture. Sealing of the matrix cracks produced the added shoulder on each curve of the C/SiC composites. Below Fig 1. Relation curves of weight change after 5 h oxidation with the matrix deposition temperature, cracks appeared in temperature for the C/C composite with a Si-W outer-layer coating the SiC matrix and got narrow with temperature and the C/SiC composites with a Si-w outer-layer coating and a Si-Zr outer-layer coating. raising. Even though cracks were produced by the thermal stress in the carbon matrix of C/C composites, could be used to explain the different oxidation mecha they had almost no effect on the oxidation behavio nisms in different temperature ranges. This indicated and then no matrix cracking temperature appeared on that oxidation behavior of the carbon fiber reinforced the oxidation behavior curve because they were much carbon or ceramic matrix composites with a coating smaller in width than those in the SiC matrix of C/SiC could be discribed by a continuous function over the composites. The characteristic temperatures of three composites were listed in Table 2. The large thermal full temperature range, no matter how the composites mismatch between the C/C composite and its coating were prepared It can be seen that the relation curves of weight 1250C, although it was decreased by the transition change of the three coated composites with temperature layer prepared using siliconization. The C/SiC com- had two similar characteristics. The composites gained posites had a lower thermal mismatch with their coat lost weight above or below some temperature, and ings, and then a lower coating cracking temperature he weight losses reached their maximum values at owing to the larger coefficient of thermal expansion some temperature. Because of the thermal mismatch (CTE) produced by the SiC matrix. The coatings on between the coating and the composite, cracks would these two composites consisted of a different outer be produced in the coating at some temperature below layer, but they had almost the same CTE because they the deposition temperature. This temperature is called had the same CVD Sic inner layer and the CtEs of the the coating cracking temperature. Above the coating Si-zr and Si-w layer which were mainly determined cracking temperature, the coated composite alway by th silicon matrix were little different The gains weight because oxidation takes place only on the cracking temperature of the LP-CVI C/SiC was 100C coating surface. Below the coating cracking tempera- lower than that of the AP-CVI C/SiC. This indicated ture, the coated composite always losses weight due to that there was a large difference between the Cte of oxidation of the PyC and fibers. If the weight gain the former and that of the latter. Although CtE of the produced by oxidation of the Sic-phase can be ne- C/SiC composites changed with the different CVI tech glected, the transition temperature from weight loss to nologies, the large difference was mainly produced by weight gain should be the theoretical value of the the PyC interlayer with different thickness. When the coating cracking temperature. Comparing with the ratio is higher than 10 and the temperature is proper, weight loss produced by oxidation of the carbon-phase, pure SiC can be deposited from the MTS/H2 system at the weight gain produced by oxidation of Sic-phase is either AP or LP-CVD condition. The Cte of the Sic small and then has a little effect on the coating matrix prepared by AP-CVD should be the same as racking temperature. Therefore, the transition temper- that of the SiC matrix prepared by LP-CVD. Another Table 2 The characteristic temperatures of C/C and C/SiC composites with a coating Composites with a coating Coating cracking temperature(C) Matrix cracking temperature (C) Transition temperature (C) AP-CVI C/SiC l000 LP-CVI C/SiC
222 L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 Fig. 1. Relation curves of weight change after 5 h oxidation with temperature for the C/C composite with a Si–W outer-layer coating and the C/SiC composites with a Si–W outer-layer coating and a Si–Zr outer-layer coating. ature can be defined as the coating cracking temperature. It was more dangerous for the composites working at low temperatures than at high temperature. The temperatures corresponding to the maximum weight losses were defined as the transition temperatures. Each curve of the C/SiC composites had a characteristic different from that of the C/C composite, the former having two shoulders and the latter having only one produced by sealing of the coating cracks between the transition temperature and the cracking temperature. Sealing of the matrix cracks produced the added shoulder on each curve of the C/SiC composites. Below the matrix deposition temperature, cracks appeared in the SiC matrix and got narrow with temperature raising. Even though cracks were produced by the thermal stress in the carbon matrix of C/C composites, they had almost no effect on the oxidation behavior, and then no matrix cracking temperature appeared on the oxidation behavior curve because they were much smaller in width than those in the SiC matrix of C/SiC composites. The characteristic temperatures of three composites were listed in Table 2. The large thermal mismatch between the C/C composite and its coating made the cracking temperature being higher up to 1250°C, although it was decreased by the transition layer prepared using siliconization. The C/SiC composites had a lower thermal mismatch with their coatings, and then a lower coating cracking temperature owing to the larger coefficient of thermal expansion (CTE) produced by the SiC matrix. The coatings on these two composites consisted of a different outer layer, but they had almost the same CTE because they had the same CVD SiC inner layer and the CTEs of the Si–Zr and Si–W layer which were mainly determined by the silicon matrix were little different. The coating cracking temperature of the LP-CVI C/SiC was 100°C lower than that of the AP-CVI C/SiC. This indicated that there was a large difference between the CTE of the former and that of the latter. Although CTE of the C/SiC composites changed with the different CVI technologies, the large difference was mainly produced by the PyC interlayer with different thickness. When the ratio is higher than 10 and the temperature is proper, pure SiC can be deposited from the MTS/H2 system at either AP or LP-CVD condition. The CTE of the SiC matrix prepared by AP-CVD should be the same as that of the SiC matrix prepared by LP-CVD. Another could be used to explain the different oxidation mechanisms in different temperature ranges. This indicated that oxidation behavior of the carbon fiber reinforced carbon or ceramic matrix composites with a coating could be discribed by a continuous function over the full temperature range, no matter how the composites were prepared. It can be seen that the relation curves of weight change of the three coated composites with temperature had two similar characteristics. The composites gained or lost weight above or below some temperature, and the weight losses reached their maximum values at some temperature. Because of the thermal mismatch between the coating and the composite, cracks would be produced in the coating at some temperature below the deposition temperature. This temperature is called as the coating cracking temperature. Above the coating cracking temperature, the coated composite always gains weight because oxidation takes place only on the coating surface. Below the coating cracking temperature, the coated composite always losses weight due to oxidation of the PyC and fibers. If the weight gain produced by oxidation of the SiC-phase can be neglected, the transition temperature from weight loss to weight gain should be the theoretical value of the coating cracking temperature. Comparing with the weight loss produced by oxidation of the carbon-phase, the weight gain produced by oxidation of SiC-phase is very small and then has a little effect on the coating cracking temperature. Therefore, the transition temperTable 2 The characteristic temperatures of C/C and C/SiC composites with a coating Composites with a coating Coating cracking temperature (°C) Matrix cracking temperature (°C) Transition temperature (°C) /C 1250C 800 AP-CVI C/SiC 76010001150 /SiC 900 680 1050LP-CVI C
L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 Fig. 2. TEM micrograph of Pyc interlayer in(a) LP-CVI C/SiC,(b)AP-CVI C/SiC actor which affect the coating cracking temperature is ture because the lower coating cracking temperature led the deposition temperature. The deposition temperature to a less slow diffusion of oxygen. in AP-CVD was 100C higher than that in LP-CVD It should be noted that the maximum weight loss of The higher the deposition temperature, the larger the the C/C composite was nearly half an order higher than thermal mismatch between the fibers and the matrix, that of the AP-CVI C/SiC composite, and a little higher the lower the CtE of the composite, the larger the than that of the LP-CVI C/SiC composite. In other mismatch between the coating and the substrates, the words, oxidation resistance of the AP-CvI C/SiC at the higher the coating cracking temperature. From the transition temperature was much better than that of the TEM micrographs of the composites(Fig. 2), it could LP-CVI C/SiC. Lower transition temperature and max be seen that thickness of the Pyc interlayer in the imum weight loss were required for the C/Sic com- LP-CVI C/SiC was about 0.3 um(Fig. 2 a)and that in posites with a better resistance. Unfortunately, it was he AP-CVI C/SiC was about 80 nm(Fig. 2 b). The very difficult to achieve this at the same time. In order PyC interlayer improved the properties of a C/SiC decrease the maximum weight loss. thickness of the composite in two ways. First, it increased the strengh PyC interlayer should be decreased, and in order to by decreasing the damage of the fibers caused by the lower the transition temperature, thickness of the Pyc matrix because the mismatch between the fibers and the interlayer should be increased. Nothing could be done Sic matrix was lowered. Second. it increased the tough but the PyC interlayer with an optimum thickness was ness by weakening the interfacial bonding between the prepared to balance the transition temperature and the fibers and the SiC matrix. The thicker the interlayer, maximum weight loss. the higher the strengh and the toughness of the com- The oxidation in the c/Sic composites begins at the posite. As a result, strength of the AP-CVI C/SiC was fiber-PyC interface and progress to the PyC interlayer in the range from 300-400 MPa and that of the LP- rather than to the carbon fibers, and that in the C/C CVI C/SiC was in the range from 700-800 MPa. At the composite began at the fiber-carbon matrix and pro- same time, the thicker the interlayer, the larger the Cte gress to the carbon matrix rather than to the carbon of the composite, and then the lower the mismatch fibers because the fibers had a higher activation energy between the composite and its coating. As a result, than the PyC interlayer and the carbon matrix. The cracking temperature of the coating on the C/SiC com- activation energy of T300 fiber was reported to be posites was lowered, no matter which CVI technology larger than 30 kcal mol-I[1], and that of the PyC was was used for preparing the composite. The effect of the calculated to be 26 kcal mol-(Table 3). The former is nterlayer on the mismatch between the fibers and the a little higher than the later, then oxidation of the Pyc matrix was confirmed by the matrix cracking tempera- will take place prior to that of the T300 fiber. The ture of the composites. The matrix cracking tempera- activation energies of reaction for the three composites ture of the LP-CVI C/SiC was also 100oC lower than determined by the Arrhenius relations of weight change that of the AP-CVI C/SiC with temperature varied little(Table 3). This showed There was a transition temperature for oxidation that the carbon matrix had nearly the same oxidation behavior of the C/C and the C/Sic composites. The resistance as the PyC interlayer. The maximum weight oxidation was seperately controled by the oxygen diffu- loss of the C/Sic composites was related to the Pyc sion above this temperature and by the reaction of interlayer thickness and the oxidation time. The thicke oxygen with carbon below this temperature. At the the interlayer, the larger the maximum weight loss, the transition temperatures, weight losses reached their longer the oxidation time, the larger the maximum values. The composite with a lower coating weight loss. The weight loss of the C/Sic composit cracking temperature had a lower transition tempera- increased with the oxidation time more rapidly before
L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 223 Fig. 2. TEM micrograph of PyC interlayer in (a) LP-CVI C/SiC, (b) AP-CVI C/SiC. factor which affect the coating cracking temperature is the deposition temperature. The deposition temperature in AP-CVD was 100°C higher than that in LP-CVD. The higher the deposition temperature, the larger the thermal mismatch between the fibers and the matrix, the lower the CTE of the composite, the larger the mismatch between the coating and the substrates, the higher the coating cracking temperature. From the TEM micrographs of the composites (Fig. 2), it could be seen that thickness of the PyC interlayer in the LP-CVI C/SiC was about 0.3 mm (Fig. 2 a) and that in the AP-CVI C/SiC was about 80 nm (Fig. 2 b). The PyC interlayer improved the properties of a C/SiC composite in two ways. First, it increased the strengh by decreasing the damage of the fibers caused by the matrix because the mismatch between the fibers and the SiC matrix was lowered. Second, it increased the toughness by weakening the interfacial bonding between the fibers and the SiC matrix. The thicker the interlayer, the higher the strengh and the toughness of the composite. As a result, strength of the AP-CVI C/SiC was in the range from 300–400 MPa and that of the LPCVI C/SiC was in the range from 700–800 MPa. At the same time, the thicker the interlayer, the larger the CTE of the composite, and then the lower the mismatch between the composite and its coating. As a result, cracking temperature of the coating on the C/SiC composites was lowered, no matter which CVI technology was used for preparing the composite. The effect of the interlayer on the mismatch between the fibers and the matrix was confirmed by the matrix cracking temperature of the composites. The matrix cracking temperature of the LP-CVI C/SiC was also 100°C lower than that of the AP-CVI C/SiC. There was a transition temperature for oxidation behavior of the C/C and the C/SiC composites. The oxidation was seperately controled by the oxygen diffusion above this temperature and by the reaction of oxygen with carbon below this temperature. At the transition temperatures, weight losses reached their maximum values. The composite with a lower coating cracking temperature had a lower transition temperature because the lower coating cracking temperature led to a less slow diffusion of oxygen. It should be noted that the maximum weight loss of the C/C composite was nearly half an order higher than that of the AP-CVI C/SiC composite, and a little higher than that of the LP-CVI C/SiC composite. In other words, oxidation resistance of the AP-CVI C/SiC at the transition temperature was much better than that of the LP-CVI C/SiC. Lower transition temperature and maximum weight loss were required for the C/SiC composites with a better resistance. Unfortunately, it was very difficult to achieve this at the same time. In order to decrease the maximum weight loss, thickness of the PyC interlayer should be decreased, and in order to lower the transition temperature, thickness of the PyC interlayer should be increased. Nothing could be done but the PyC interlayer with an optimum thickness was prepared to balance the transition temperature and the maximum weight loss. The oxidation in the C/SiC composites begins at the fiber–PyC interface and progress to the PyC interlayer rather than to the carbon fibers, and that in the C/C composite began at the fiber–carbon matrix and progress to the carbon matrix rather than to the carbon fibers because the fibers had a higher activation energy than the PyC interlayer and the carbon matrix. The activation energy of T300 fiber was reported to be larger than 30 kcal mol−1 [1], and that of the PyC was calculated to be 26 kcal mol−1 (Table 3). The former is a little higher than the later, then oxidation of the PyC will take place prior to that of the T300 fiber. The activation energies of reaction for the three composites determined by the Arrhenius relations of weight change with temperature varied little (Table 3). This showed that the carbon matrix had nearly the same oxidation resistance as the PyC interlayer. The maximum weight loss of the C/SiC composites was related to the PyC interlayer thickness and the oxidation time. The thicker the interlayer, the larger the maximum weight loss, the longer the oxidation time, the larger the maximum weight loss. The weight loss of the C/SiC composites increased with the oxidation time more rapidly before
L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 Table 3 Arrhenius relations of oxidation in different temperature ranges for C/C and C/SiC compoistes C/C or C/SiC composites with a coating Arrhenius relations of weight change with temperature Diffusion through cracks diffusion through oxide ln△W= +20.776 AP-CVI C/SiC +15.984ln△W=128l5/T-9.5335In△W= +13.818 LFCⅵIC/SiC ln△W=-1 +17.667ln△W=15786/T-13.416lnAW= +1389 the PyC interlayer was oxidized out than after Consid- 5. Conclusions ering that the oxidation led to a porosity change and rated the reaction of carbon with with oxygen, the Oxidation behavior of the carbon fiber renfo C/Sic composite with a thicker PyC interlayer lost carbon or ceramic matrix composites with a coi weight more rapidly than that with a thinner one with could be discribed by a continuous function over the increasing the oxidation time. Thus, it could be easily full temperature range, no matter how the composites understood that the maximum weight loss of the LP- were prepared CVI C/SiC composite was much higher than that of the The Sic matrix made the coating cracking tempera- AP-CVI C/SiC composite. The LP-CVI C/SiC com- ture of the AP-CVI C/SiC being 100 C lower than that posite with a thicker PyC interlayer had a better oxida of the C/C, and the PyC interlayer with a different tion resistance than the C/c composite because th thickness made the coating cracking temperature and maximum weight loss of the latter would be much matrix cracking temperature of the LP-CVI C/SiC be- larger than that of the former if the oxidation time was ing 100oC lower than those of the AP-CVI C/SIC. increased furthermore There was an optimum thickness of the Pyc inter- tyer for improving the oxidation resistance of C/Sic composites. On one hand, the thicker the PyC inter- cracking temperature, the activation energy of diffusion layer, the lower the transition temperature, but the the relation of weight change with temperature for the larger the maximum weight loss. On the other hand, the thicker the PyC interlayer, the higher the mechanical C/C composite because it was nonlinear. The activation properties energy for diffusion through the matrix cracks in LP- Below the transition temperatures, the activation en- VI C/SiC composite was calculated to be higher than ergies of oxidation through reaction for the C/C and that in the AP-CVI C/SiC composite. This indicated the C/Sic composites determined by the Arrhenius that the matrix cracks in the former was smaller in relations of weight change with temperature varied width than those in the latter at the same temperature. little Above the coating cracking temperature, the C/C Above the coating temperatures, the Si-W layer and AP-CVI C/SiC composite should exhibited the gher activation energy for diffusion than the si same oxidation behavior because their coating had the layer because the silica film formed on the former had same outer layer and the oxidation took place only on a higher resistance to oxygen diffusion than the oxide their coating surfaces. However, the activation energy film formed on the latter for diffusion through the oxide film on the former coating surface was a little higher than that on the latter coating surface. Because the AP-CVI C/SiC sub- References strates were not machined, their surface was very rough and then the prepared Si-w layer was non-uniform [1] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 31(1993)1273- When the si-w layer was oxidized for 5 h, the CVD 21 amouroux. G. Camus. J. Amer. Ceram. Soc. 77(1994) Sic layer was oxidized at the area where the Si-W 2057 layer was too thin. The oxidation behavior of the B] F. Lamouroux, G. Camus, J. Amer. Ceram Soc. 77(1994) 2058-2068. AP-CVI C/SiC composite above its coating cracking (4)RJ Kerans, Control of fiber-matrix interface properties in ce- temperature was considered to be changed by oxidation amic composites, in: R. Naslain(Ed h Temperatur of the CVD SiC layer. The Si-W layer had higher ramic Matrix Composites. Woodhead Publications, Bordeaux, activation energy for diffusion than the Si-Zr layer 1993 301-312. This indicated that the silica film formed on the former 5R.A. Lowden, K L More, O.J. Schwarz, N L. Vaughn, Improved fiber-matrix interlayers for Nicalon/ Sic composites, in: R had a higher resistance to oxygen diffusion than the Naslain(Ed. ) High Temperature Ceramic Matrix Compos oxide film formed he latter Woodhead Publications, Bordeaux, 1993, pp. 345-352
224 L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 Table 3 Arrhenius relations of oxidation in different temperature ranges for C/C and C/SiC compoistes C/C or C/SiC composites with a coating Arrhenius relations of weight change with temperature Reaction Diffusion through cracks Diffusion through oxide C/C ln DW=−13790/T+16.086 ln DW=−38147/T+20.776 AP-CVI C/SiC ln DW=−13141/T+15.984 ln DW=12815/T−9.5335 ln DW=−23102/T+13.818 LP-CVI C/SiC ln DW=−13906/T+17.667 ln DW=15786/T−13.416ln DW=−23040/T+13.898 the PyC interlayer was oxidized out than after. Considering that the oxidation led to a porosity change and accelerated the reaction of carbon with oxygen, the C/SiC composite with a thicker PyC interlayer lost weight more rapidly than that with a thinner one with increasing the oxidation time. Thus, it could be easily understood that the maximum weight loss of the LPCVI C/SiC composite was much higher than that of the AP-CVI C/SiC composite. The LP-CVI C/SiC composite with a thicker PyC interlayer had a better oxidation resistance than the C/C composite because the maximum weight loss of the latter would be much larger than that of the former if the oxidation time was increased furthermore. From the transition temperature to the coating cracking temperature, the activation energy of diffusion through the coating cracks could not be calculated by the relation of weight change with temperature for the C/C composite because it was nonlinear. The activation energy for diffusion through the matrix cracks in LPCVI C/SiC composite was calculated to be higher than that in the AP-CVI C/SiC composite. This indicated that the matrix cracks in the former was smaller in width than those in the latter at the same temperature. Above the coating cracking temperature, the C/C and AP-CVI C/SiC composite should exhibited the same oxidation behavior because their coating had the same outer layer and the oxidation took place only on their coating surfaces. However, the activation energy for diffusion through the oxide film on the former coating surface was a little higher than that on the latter coating surface. Because the AP-CVI C/SiC substrates were not machined, their surface was very rough and then the prepared Si–W layer was non-uniform. When the Si–W layer was oxidized for 5 h, the CVD SiC layer was oxidized at the area where the Si–W layer was too thin. The oxidation behavior of the AP-CVI C/SiC composite above its coating cracking temperature was considered to be changed by oxidation of the CVD SiC layer. The Si–W layer had higher activation energy for diffusion than the Si–Zr layer. This indicated that the silica film formed on the former had a higher resistance to oxygen diffusion than the oxide film formed on the latter. 5. Conclusions Oxidation behavior of the carbon fiber renforced carbon or ceramic matrix composites with a coating could be discribed by a continuous function over the full temperature range, no matter how the composites were prepared. The SiC matrix made the coating cracking temperature of the AP-CVI C/SiC being 100°C lower than that of the C/C, and the PyC interlayer with a different thickness made the coating cracking temperature and matrix cracking temperature of the LP-CVI C/SiC being 100°C lower than those of the AP-CVI C/SiC. There was an optimum thickness of the PyC interlayer for improving the oxidation resistance of C/SiC composites. On one hand, the thicker the PyC interlayer, the lower the transition temperature, but the larger the maximum weight loss. On the other hand, the thicker the PyC interlayer, the higher the mechanical properties. Below the transition temperatures, the activation energies of oxidation through reaction for the C/C and the C/SiC composites determined by the Arrhenius relations of weight change with temperature varied little. Above the coating temperatures, the Si–W layer had higher activation energy for diffusion than the Si–Zr layer because the silica film formed on the former had a higher resistance to oxygen diffusion than the oxide film formed on the latter. References [1] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 31 (1993) 1273– 1288. [2] F. Lamouroux, G. Camus, J. Amer. Ceram. Soc. 77 (1994) 2049–2057. [3] F. Lamouroux, G. Camus, J. Amer. Ceram. Soc. 77 (1994) 2058–2068. [4] R.J. Kerans, Control of fiber-matrix interface properties in ceramic composites, in: R. Naslain (Ed.), High Temperature Ceramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 301–312. [5] R.A. Lowden, K.L. More, O.J. Schwarz, N.L. Vaughn, Improved fiber-matrix interlayers for Nicalon/SiC composites, in: R. Naslain (Ed.), High Temperature Ceramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 345–352
L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 [6 W.H. Glime, J.D. Cawley, Carbon 33(8)(1995)1053-106 [8L. Filipuzzi, R. Naslain, J. Am. Ceram. Soc. 77(2)(1994) [7 L. Filipuzzi, G. Gamus, R. Naslain, J. Am. Ceram. Soc. 77(2) 459-466. (1994)467-4 9R. Naslain, Composites Interface 1(3)(1993)253-286
L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 225 [6] W.H. Glime, J.D. Cawley, Carbon 33 (8) (1995) 1053–1060. [7] L. Filipuzzi, G. Gamus, R. Naslain, J. Am. Ceram. Soc. 77 (2) (1994) 467–480. [8] L. Filipuzzi, R. Naslain, J. Am. Ceram. Soc. 77 (2) (1994) 459–466. [9] R. Naslain, Composites Interface 1 (3) (1993) 253–286.
