Corrosion Science 51 (2009)820-826 Contents lists available at Science Direct Corrosion science ELSEVIER journalhomepagewww.elsevier.com/locate/corsci Preparation and oxidation protection of CVd Sic/a-BC/SiC coatings for 3D C/SiC composites Yongsheng Liu*, Litong Zhang, Laifei Cheng, Wenbin Yang, Weihua Zhang, Yongdong Xu ational Key Laboratory of Thermostructure Composite Materials, Northwesten Polytechnical University, Xi'an Shaanxi 710072, PR China ARTICLE INFO A BSTRACT An amorphous boron carbide(a-BC)coating was prepared by LPCvd process from BCl3-CHa-H2-Ar sys eceived 7 october 2008 tem. xPS result showed that the boron concentration was 15.0.%. and carbon was 82.0 at. % One third of Available online 6 February 2009 boron was distributed to a bonding with carbon and 37.0 at.% in graphite lattice. A multi- ple-layered structure of CVD SiCa-BC/ SiC was coated on 3D C/ tesOxidation tests were con- ucted at 700, 1000, and 1200C in 14 vol% H2O/8 voL% 02/78 ere up to 100 h. The 3D A Ceramic matrix composites C/SiC composites with the modified coating system had a god istance. This resulted in the high strength retained ratio of the composites even after the e 2009 Elsevier Ltd. All rights reserved. C Oxidation 1 Introduction interphase was prepared by Jacques et al. [ 15. The crack in the inter- phase could become a self-seal at 600C in an oxidation environ- Oxidation behaviour of carbon fibre and pyrolytic carbon(PyC) ment. The fatigue life of the composites was higher than that of the in the C/Sic and c/c composites limits their long-term applications composite with Pyc interphase. The oxidation protection of B,C for in high-temperature oxidizing environments for example rocket carbon fibre and carbon/ carbon composites were also researched nozzles and aeronautic jet engines [ 1, 2 ]. To improve the oxidation by Piquero et al. [16 and Tsou et al. [17], which demonstrated the resistant of the C/Sic and C/C composites, boron-bearing species good oxidation protection of B,C for carbon fibre and C/composites 3 were widely used: boron oxide(B2O3), boron carbide(BC) below 900C. Both coatings of B-C and Si-B-C were also used as pa and ternary boron-bearing compound(Si-B-c) tial matrix in C/SiC and Sic/Sic composites for improving oxidation Various boron-bearing inhibitors, coatings, and sealants have resistant by Lamouroux et al. [18 and Viricelle et al. [19] been tested to protect the C/Sic and C/c composites [4-7. McKee Based on the above research, boron carbide was frequently et al.[4,5 studied the oxidation behaviour of graphite impre ntroduced into the C/Sic and c/c composites by CVD methods. nated with aqueous solutions of boric oxide and organo-borates, Crystal boron carbide was synthesized in almost all investigations and reported a remarkable reduction in the rate of oxidation of used with CVD method. The amorphous boron carbide was only the graphite in dry or moist air between 600C and 1000C. A pro- fabricated by Berjonneau et al. [20]. and moreover its oxidation posed mechanism for inhibition was the formation of a glassy bor- behaviour has not been reported for C/Sic or C/C composites. on-oxide residue that blocked active sites on the carbon surface In this paper, an amorphous boron carbide(a-BC)coating w Boron-bearing ceramics except the boron oxide, i.e. boron 8, B-c synthesized from boron trichloride, methane and hydrogen mix [9], and Si-B-C[10 were also important candidates for oxidation tures. Firstly, morphologies and chemical characterization of the protection of C/SiC composites, since the glassy boron oxide would a-BC coating were analyzed. Secondly the a- BC coating was intro- formed due to the oxidation reaction. Multilayer SiC/B/Sic duced into two CVD SiC coatings. The oxidation behaviour of the 11. SiC/B,C/Sic [11 or SiC/SiBC/SiC [10) coatings can be fabri- SiC/a-BC/SiC coatings for was clarified for the 3D C-SiC composite, cated expediently by cvd process. Labruquere et al. 12-14 exam- as compared to the Sic/graphite B-C/Sic coatings. ned an oxidation resistance of B-C, Si-B-C and Sic for C omposites. Their results showed that B-Cdeposit was oxidized rap- 2. Experiment procedure idly and led to the formation of B 203 film which protected carbon from the oxidation at interfacial zones. The multilayer Sic/C(B)SiC 2.1. Fabrication of specimens Corresponding author. Tel:+86 29 8848 6068 823: fax: +86 29 8849 4620. Firstly, the preforms were fabricated from carbon fibre(T-300 Japan Toray), which has a volume fraction in the range of 0010-938XS-see front matter e 2009 Elsevier Ltd. All rights reserved. do:101016/ J- corsi2009.01.026
Preparation and oxidation protection of CVD SiC/a-BC/SiC coatings for 3D C/SiC composites Yongsheng Liu *, Litong Zhang, Laifei Cheng, Wenbin Yang, Weihua Zhang, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China article info Article history: Received 7 October 2008 Accepted 29 January 2009 Available online 6 February 2009 Keywords: A. Ceramic matrix composites B. SEM B. XPS B. Weight loss C. Oxidation abstract An amorphous boron carbide (a-BC) coating was prepared by LPCVD process from BCl3–CH4–H2–Ar system. XPS result showed that the boron concentration was 15.0 at.%, and carbon was 82.0 at.%. One third of boron was distributed to a bonding with carbon and 37.0 at.% was dissolved in graphite lattice. A multiple-layered structure of CVD SiC/a-BC/SiC was coated on 3D C/SiC composites. Oxidation tests were conducted at 700, 1000, and 1200 C in 14 vol.% H2O/8 vol.% O2/78 vol.% Ar atmosphere up to 100 h. The 3D C/SiC composites with the modified coating system had a good oxidation resistance. This resulted in the high strength retained ratio of the composites even after the oxidation. 2009 Elsevier Ltd. All rights reserved. 1. Introduction Oxidation behaviour of carbon fibre and pyrolytic carbon (PyC) in the C/SiC and C/C composites limits their long-term applications in high-temperature oxidizing environments for example rocket nozzles and aeronautic jet engines [1,2]. To improve the oxidation resistant of the C/SiC and C/C composites, boron-bearing species [3] were widely used: boron oxide (B2O3), boron carbide (B4C) and ternary boron-bearing compound (Si–B–C). Various boron-bearing inhibitors, coatings, and sealants have been tested to protect the C/SiC and C/C composites [4–7]. McKee et al. [4,5] studied the oxidation behaviour of graphite impregnated with aqueous solutions of boric oxide and organo-borates, and reported a remarkable reduction in the rate of oxidation of the graphite in dry or moist air between 600 C and 1000 C. A proposed mechanism for inhibition was the formation of a glassy boron-oxide residue that blocked active sites on the carbon surface. Boron-bearing ceramics except the boron oxide, i.e. boron[8], B–C [9], and Si–B–C [10] were also important candidates for oxidation protection of C/SiC composites, since the glassy boron oxide would be formed due to the oxidation reaction. Multilayer SiC/B/SiC [8,11], SiC/B4C/SiC [11] or SiC/SiBC/SiC [10] coatings can be fabricated expediently by CVD process. Labruquere et al. [12–14] examined an oxidation resistance of B–C, Si–B–C and SiC for C/C composites. Their results showed that B–C deposit was oxidized rapidly and led to the formation of B2O3 film which protected carbon from the oxidation at interfacial zones. The multilayer SiC/C(B)/SiC interphase was prepared by Jacques et al.[15]. The crack in the interphase could become a self-seal at 600 C in an oxidation environment. The fatigue life of the composites was higher than that of the composite with PyC interphase. The oxidation protection of B4C for carbon fibre and carbon/carbon composites were also researched by Piquero et al. [16] and Tsou et al. [17], which demonstrated the good oxidation protection of B4C for carbon fibre and C/C composites below 900 C. Both coatings of B–C and Si–B–C were also used as partial matrix in C/SiC and SiC/SiC composites for improving oxidation resistant by Lamouroux et al. [18] and Viricelle et al. [19]. Based on the above research, boron carbide was frequently introduced into the C/SiC and C/C composites by CVD methods. Crystal boron carbide was synthesized in almost all investigations used with CVD method. The amorphous boron carbide was only fabricated by Berjonneau et al. [20], and moreover its oxidation behaviour has not been reported for C/SiC or C/C composites. In this paper, an amorphous boron carbide (a-BC) coating was synthesized from boron trichloride, methane and hydrogen mixtures. Firstly, morphologies and chemical characterization of the a-BC coating were analyzed. Secondly, the a-BC coating was introduced into two CVD SiC coatings. The oxidation behaviour of the SiC/a-BC/SiC coatings for was clarified for the 3D C-SiC composite, as compared to the SiC/graphite B–C/SiC coatings. 2. Experiment procedure 2.1. Fabrication of specimens Firstly, the preforms were fabricated from carbon fibre (T-300, Japan Toray), which has a volume fraction in the range of 0010-938X/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.01.026 * Corresponding author. Tel.: +86 29 8848 6068 823; fax: +86 29 8849 4620. E-mail addresses: liuys99067@163.com, yongshengliu@nwpu.edu.cn (Y. Liu). Corrosion Science 51 (2009) 820–826 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Y Liu et aL /Corrosion Science 51(2009)820-826 40-45 voL% and a braiding angle of 20 using a four-step three dimensional (4-step 3D)braiding method in Nanjing Institute of Glass Fibre, China. Secondly, Py c interface and the silicon carbide matrix were deposited by low pressure chemical vapor infiltration (LPCVI)process. The Py c was deposited on the carbon fibre using C3H6 precursor at 870C for 1 h at a reduced pressure of 500 Pa, yielding a thickness of 200 nm. The Sic matrix was achieved 1100C for 120 h at reduced pressure of 2 kPa by using methyl chlorosilane(Mrs, CH3SiCl3)with a H2: MTS molar ratio of 10. This was achieved by bubbling hydrogen in gas phase through the MTS Standard peak of B-C bond An argon dilution was used to slow down the chemical reaction 1100cm1;800cm;470c rate during deposition. Then the test specimens were machined composite to a size of3.0×4.0×400 by polishing. Finally, the specimens were coated with the hybrid CVD SiC/CVd a-BCCVD SiC multilayer coatings. The CVD Sic 4000 were 1000 e same fabrication conditions as the sic matrix except for the Wave numbers deposition time of 30 h. The deposition conditions for CvD a-BC coating were as follows: temperature 900C, pressure 1.0 kPa, Fig. 1. IR spectrum of CVd a-BC (deposited at 900 sC for 20 h). time 20 h, BCl3 flow 50 ml min, H2 flow 60 ml min", CH4 flow 20 ml min- Ar flow 60 ml min 2.2 Oxidation tests a-BC coating include 1086.23, 792. 13 and 48536cm", which are well consistent with the standard peaks of B-c bond which are 1100. 800 and 470 cm-l xPS result shows the element concentra- Oxidation tests were conducted in an alumina tube furnace. The tions of the coating are 15.0 at boron, 82.0 at.% carbon and purity of the alumina tube is 99.99 at. % Gas mixture of 14 voL% 3. 0 at oxygen. Bonding state of boron(1s)in the a-BC coating H20 8 voL%O2/Ar 78 voL% was flowed into it. Five specimens were were also shown in Table 1. There are five types bonding states Ised for each experimental condition. The H20 gas flowing rate of a-BC coating, which are B-C bond, B dissolved in graphite lattice, vas 14 ml/ min The weights of the specimens were recorded afte BC20, BCOz and B-o bond as a B2O3. The major bonding state was each oxidation for 0.5, 1, 2, 5 and 10 h at the desired temperature. B-c bond and boron atom dissolved in the graphitic lattice. The Then the record time sequence was 20, 30, 40, 50, 60, 70, 80, 90, and 100 h. They were measured using an electronic balance former concentration is 33.5 at.% and the later is 37.0 at.% (sensitivity =0.01 mg Fig 2 shows the surface and cross-section morphologies of the hybrid coatings. Both surface morphologies of CVd a-BC and CVD 2.3. Measurements of the composites Sic layers are cauliflower-like glossy as shown in Figs. 2(a) and (c). The cross-section morphology of CVD a-BC layer is homoge- Phases were identified by an X-ray diffraction device(XRD, rig nous and glass-like with 18 um thickness, while each SiC layer is aku D/MAx- 2400 with Cu Ky radiation). Surtace and cross-section Sic layer and a-bc layer is shown in Fig 3 which was observed tron microscope(SEM, S-2700, Hitachi, Japan). X-ray Photoelectron from the coatings before the top coating were deposited. No crack Spectroscopy(XPS)(AXIS ULTRA, KRATOS ANALYTICAL Ltd )was and debonding exist in the interface between CVD SiC and Cvd a- used to analyze the element concentration and bonding states of boron and carbon. Raman micro-spectroscopy(LABRAM, DiLor 3. 2. Oxidation behaviour of 3D C/SiC composites coated with Sic/a-BC SA, France)and IR spectroscopy(Cary 5000, Varian Co.. America) SiC coatings were used to characterize the b-c bond Flexural strengths of the composite specimens were measured by a three-point bending method at the room temperature. The Fig. 4 shows the surface morphologies of CVD SiC/a-BC/SiC coat- span dimension was 30.0 mm for the test specimen of 40.0 mm ngs after oxidation at different temperatures for 100 h in 14 vol% length and the loading rate was 0.5 mm min H208 voL%O2/78 voL% Ar atmosphere. It is obvious that there are great differences among the surface morphologies as follows 3. Results and discussion (1)At 700C, the surface morphologies are almost the same a at of the coatings before oxidation see compare Fig 4(a) 3. 1. Characterization of Cvd Sic/a-BCSiC hybrid coatings to Fig. 2(c). No B2O3 glassy material can be found in the coat gs surface Crack in the surface is also not sealed by glass in oth XRD and Raman micro-spectroscopy did not show any exi Fig. 4(b). These results show that the outer CVD SiC layer is dence for a crystalline boron carbide phase, which is consistent hardly oxidized at 700C. The interim CVD a-BC layer is also with Berjonneau's results [20]. Therefore, the a-BC coating was not obviously oxidized since no large amount of B2O3 glass characterized by IR spectrum and XPS, summarized in Fig. 1 and formation. This may be due to the low oxygen partial pres- Table 1. IR spectrum result shows that the absorb peaks of the sure in atmosphere(8.0 voL%). Table 1 Proportions of B 1s components for the a-BC coating by XPS onding energy (ev) 1889 1900 ontent(at% 33.5 Element concentration(at % Boron: 15.0%6: carbon: 82.0%: oxygen: 3.0%
40–45 vol.% and a braiding angle of 20 using a four-step three dimensional (4-step 3D) braiding method in Nanjing Institute of Glass Fibre, China. Secondly, PyC interface and the silicon carbide matrix were deposited by low pressure chemical vapor infiltration (LPCVI) process. The PyC was deposited on the carbon fibre using C3H6 precursor at 870 C for 1 h at a reduced pressure of 500 Pa, yielding a thickness of 200 nm. The SiC matrix was achieved at 1100 C for 120 h at reduced pressure of 2 kPa by using methyltrichlorosilane (MTS, CH3SiCl3) with a H2:MTS molar ratio of 10. This was achieved by bubbling hydrogen in gas phase through the MTS. An argon dilution was used to slow down the chemical reaction rate during deposition. Then the test specimens were machined from the C/SiC composite to a size of 3.0 4.0 40.0 mm, followed by polishing. Finally, the specimens were coated with the hybrid CVD SiC/CVD a-BC/CVD SiC multilayer coatings. The CVD SiC were the same fabrication conditions as the SiC matrix except for the deposition time of 30 h. The deposition conditions for CVD a-BC coating were as follows: temperature 900 C, pressure 1.0 kPa, time 20 h, BCl3 flow 50 ml min1 , H2 flow 60 ml min1 , CH4 flow 20 ml min1 , Ar flow 60 ml min1 . 2.2. Oxidation tests Oxidation tests were conducted in an alumina tube furnace. The purity of the alumina tube is 99.99 at.%. Gas mixture of 14 vol.% H2O/8 vol.% O2/Ar 78 vol.% was flowed into it. Five specimens were used for each experimental condition. The H2O gas flowing rate was 14 ml/min. The weights of the specimens were recorded after each oxidation for 0.5, 1, 2, 5 and 10 h at the desired temperature. Then the record time sequence was 20, 30, 40, 50, 60, 70, 80, 90, and 100 h. They were measured using an electronic balance (sensitivity = 0.01 mg). 2.3. Measurements of the composites Phases were identified by an X-ray diffraction device (XRD, Rigaku D/MAX-2400 with Cu Kr´ radiation). Surface and cross-section morphologies of the coating were observed using a scanning electron microscope (SEM, S-2700, Hitachi, Japan). X-ray Photoelectron Spectroscopy (XPS) (AXIS ULTRA, KRATOS ANALYTICAL Ltd.) was used to analyze the element concentration and bonding states of boron and carbon. Raman micro-spectroscopy (LABRAM, DiLor SA, France) and IR spectroscopy (Cary 5000, Varian Co., America) were used to characterize the B–C bond. Flexural strengths of the composite specimens were measured by a three-point bending method at the room temperature. The span dimension was 30.0 mm for the test specimen of 40.0 mm length and the loading rate was 0.5 mm min1 . 3. Results and discussion 3.1. Characterization of CVD SiC/a-BC/SiC hybrid coatings Both XRD and Raman micro-spectroscopy did not show any exidence for a crystalline boron carbide phase, which is consistent with Berjonneau’s results [20]. Therefore, the a-BC coating was characterized by IR spectrum and XPS, summarized in Fig. 1 and Table 1. IR spectrum result shows that the absorb peaks of the a-BC coating include 1086.23, 792.13 and 485.36 cm1 , which are well consistent with the standard peaks of B–C bond which are 1100, 800, and 470 cm1 . XPS result shows the element concentrations of the coating are 15.0 at.% boron, 82.0 at.% carbon and 3.0 at.% oxygen. Bonding state of boron (1s) in the a-BC coating were also shown in Table 1. There are five types bonding states of a-BC coating, which are B–C bond, B dissolved in graphite lattice, BC2O, BCO2 and B–O bond as a B2O3. The major bonding state was B–C bond and boron atom dissolved in the graphitic lattice. The former concentration is 33.5 at.% and the later is 37.0 at.%. Fig. 2 shows the surface and cross-section morphologies of the hybrid coatings. Both surface morphologies of CVD a-BC and CVD SiC layers are cauliflower-like glossy as shown in Figs. 2(a) and (c). The cross-section morphology of CVD a-BC layer is homogenous and glass-like with 18 lm thickness, while each SiC layer is 25 lm has shown in Fig. 2(b). The interfacial morphology between SiC layer and a-BC layer is shown in Fig. 3 which was observed from the coatings before the top coating were deposited. No crack and debonding exist in the interface between CVD SiC and CVD aBC coating. 3.2. Oxidation behaviour of 3D C/SiC composites coated with SiC/a-BC/ SiC coatings Fig. 4 shows the surface morphologies of CVD SiC/a-BC/SiC coatings after oxidation at different temperatures for 100 h in 14 vol.% H2O/8 vol.% O2/78 vol.% Ar atmosphere. It is obvious that there are great differences among the surface morphologies as follows: (1) At 700 C, the surface morphologies are almost the same as that of the coatings before oxidation see compare Fig. 4(a) to Fig. 2(c). No B2O3 glassy material can be found in the coatings surface. Crack in the surface is also not sealed by glass in Fig. 4(b). These results show that the outer CVD SiC layer is hardly oxidized at 700 C. The interim CVD a-BC layer is also not obviously oxidized since no large amount of B2O3 glass formation. This may be due to the low oxygen partial pressure in atmosphere (8.0 vol.%). Wave numbers 485.36 792.13 1086.23 4000 3000 2000 1000 Standard peak of B-C bond: 1100cm-1; 800cm-1; 470cm-1 Fig. 1. IR spectrum of CVD a-BC (deposited at 900 sC for 20 h). Table 1 Proportions of B 1s components for the a-BC coating by XPS. Substance B–C bond B dissolved in graphite lattice BC2O BCO2 B2O3 Bonding energy (eV) 187.8 188.9 190.0 192.0 193.2 Content (at.%) 33.5 37.0 15.4 10.0 4.0 Element concentration (at.%) Boron: 15.0%; carbon: 82.0%; oxygen: 3.0% Y. Liu et al. / Corrosion Science 51 (2009) 820–826 821
22 Y Liu et al /Corrosion Science 51 (2009)820-826 CVD a-BC surface CVD SiC CVD a-BC 100m CVD SIC 500um c CVDSiC surface Fig. 2. Surface and cross section of CVD SiC/a-BCSiC coatings (a)CVD a-BC surface(b)Cross-section morphology of hybrid coatings (c)CVD SiC surface. 700C only Sic phase was identified. The cristobalite occurs after oxidation at 1000C. the obvious cristobalite exists after oxidation at 1200C which was corresponding wi morphologies as shown in Fig. 4(f). Fig 6 shows the weight change curves of the coated C/SiC com posite after oxidation at 700, 1000, and 1200C up to 100 h in 14 voL% H20/8 voL% O2/78 voL% Ar atmosphere. Clearly showing the weight changes are small at all the test temperatures. The max- Sic imum of weight loss is only 0.49 wt% after 100 h-oxidation at 700C. The weight change curves of difference each temperature have great differences, which can be described detailed as Fig 3. Interfacial morphology between CVD a-BC and CVD SiC coating. (1)At 700C, the weight of the composites lost continuously. During the first 10h, the weight loss is very obvious with 0.0072 wt %/h rate. It is small with 0.0032 wt %/h rate from (2)At 1000C, the surface morphologies have changed obvi- 10 to 68 h. At the last 32 h, it is obvious again with ously compared with that of coatings before oxidation. 0.0074 wt %h rate ccording to surface morphologies in the Fig 4(c)and(d 2)At 1000C, the weight of the composites increased continu luch glass occur on the coatings surface. The crack in coat- ously at the first 41 h, then the weight decreased with ings is sealed by glass. The positions among CVD SiC coa increasing oxidation time. The weight gain has a maximum lations are also filled by glass. Some new cracks occur in the of 0.11 wt% after oxidation for 41 h. Finally, the weight loss glass surface layer after oxidation which due to the mis is 0011 wt% after oxidation for 100 h match of thermal expansion coefficient between coatings (3)At 1200C. the weight of the composites increased continu and glass layer, though no spallation of the glass occurred ously after oxidation for 100 h. The maximum of weight gain at this temperature. is 0. 155 wt%, and the final weight gain is 0. 14 wt. %. The sus- (3)At 1200C, the surface was fully covered with tained weight gain indicates that the coating is healed an shown in Fig 4(e)and(f). There are many pores an no oxygen and water intrude into the interface and fibre. n the glass layer. The cracks resulted from the misr layer. The pores in the glass layer could be formed due to coated with Sic/Sic/Sic, sic B/SiC or Sic/graphitic B-C/SiC had its the volatilization of B203 glass. The crystallization phenom- weight loss at 700-1300C for 10 or 15 h in air atmosphere. At ena can be found in the glass layer as shown in Fig 4(f). The 700C oxidation, the weights of Sic/Sic/SiC, Sic/graphitic B-C/ glass crystallized owing to the effects of high temperature SiC, and Sic/B/SiC lost above 0.71 wt% for 15 h, 0. 19 wt% for (1200C)and long time(100 h), which was proven by XRD 15 h, and 0.003 wt% for 10 h, respectively. At 1100C oxidation, easurement Fig. 5 shows the XRD patterns measured on the weights of Sic/Sic/Sic and Sic/graphitic B-C/Sic lost above the test specimens oxidized at different temperatures. At 0.16 wt% for 15 h, 0.51 wt% for 15 h, respectively. At 1000C oxi-
(2) At 1000 C, the surface morphologies have changed obviously compared with that of coatings before oxidation. According to surface morphologies in the Fig. 4(c) and (d), much glass occur on the coatings surface. The crack in coatings is sealed by glass. The positions among CVD SiC coagulations are also filled by glass. Some new cracks occur in the glass surface layer after oxidation which due to the mismatch of thermal expansion coefficient between coatings and glass layer, though no spallation of the glass occurred at this temperature. (3) At 1200 C, the surface was fully covered with glass as shown in Fig. 4(e) and (f). There are many pores and cracks in the glass layer. The cracks resulted from the mismatch of thermal expansion coefficient between coatings and glass layer. The pores in the glass layer could be formed due to the volatilization of B2O3 glass. The crystallization phenomena can be found in the glass layer as shown in Fig. 4(f). The glass crystallized owing to the effects of high temperature (1200 C) and long time (100 h), which was proven by XRD measurement. Fig. 5 shows the XRD patterns measured on the test specimens oxidized at different temperatures. At 700 C only SiC phase was identified. The cristobalite occurs after oxidation at 1000 C. The obvious cristobalite exists after oxidation at 1200 C which was corresponding with morphologies as shown in Fig. 4(f). Fig. 6 shows the weight change curves of the coated C/SiC composite after oxidation at 700, 1000, and 1200 C up to 100 h in 14 vol.% H2O/8 vol.% O2/78 vol.% Ar atmosphere. Clearly showing the weight changes are small at all the test temperatures. The maximum of weight loss is only 0.49 wt.% after 100 h-oxidation at 700 C. The weight change curves of difference each temperature have great differences, which can be described detailed as follows: (1) At 700 C, the weight of the composites lost continuously. During the first 10 h, the weight loss is very obvious with 0.0072 wt.%/h rate. It is small with 0.0032 wt.%/h rate from 10 to 68 h. At the last 32 h, it is obvious again with 0.0074 wt.%/h rate. (2) At 1000 C, the weight of the composites increased continuously at the first 41 h, then the weight decreased with increasing oxidation time. The weight gain has a maximum of 0.11 wt.% after oxidation for 41 h. Finally, the weight loss is 0.011 wt.% after oxidation for 100 h. (3) At 1200 C, the weight of the composites increased continuously after oxidation for 100 h. The maximum of weight gain is 0.155 wt.%, and the final weight gain is 0.14 wt.%. The sustained weight gain indicates that the coating is healed and no oxygen and water intrude into the interface and fibre. According to the previous results [8,9], the C/SiC composites coated with SiC/SiC/SiC, SiC/B/SiC or SiC/graphitic B–C/SiC had its weight loss at 700–1300 C for 10 or 15 h in air atmosphere. At 700 C oxidation, the weights of SiC/SiC/SiC, SiC/graphitic B–C/ SiC, and SiC/B/SiC lost above 0.71 wt.% for 15 h, 0.19 wt.% for 15 h, and 0.003 wt.% for 10 h, respectively. At 1100 C oxidation, the weights of SiC/SiC/SiC and SiC/graphitic B–C/SiC lost above 0.16 wt.% for 15 h, 0.51 wt.% for 15 h, respectively. At 1000 C oxiFig. 2. Surface and cross section of CVD SiC/a-BC/SiC coatings. (a) CVD a-BC surface. (b) Cross-section morphology of hybrid coatings. (c) CVD SiC surface. Fig. 3. Interfacial morphology between CVD a-BC and CVD SiC coating. 822 Y. Liu et al. / Corrosion Science 51 (2009) 820–826
Y. Liu et aL/Corrosion Science 51(2009)820-826 Coating defect 500m Cracking self-sealing Surface 0 Surface pores Glass crystallization 100um 500m Fig 4. Surface morphologies of CVD Sic/a-BC/ SiC coatings after oxidized at different temperatures. (a)and (b)at 700C, (c)and(d)at 1000C, and (e)and(n)at 1200.C, 50 1200°c 1000° 2-。m 多-03-01000 A-1200℃ 700°c 102030405060708090100 Time(h) Fig. 6. Weight changes of the coated composites oxidized at different temperatures Fig. 5. XRD patterns of multilayer SiC/a-BCSiC coating surface after oxidation. SiC/SiC. 二m ic lost above 0.2 wt. 15 h and 0 r only 2 h, respectively ation, the weights of SiC/b/SiC lost 0.011 wt% for 10 h. However, The susta indicates that the inter- the weights of SiC/a-BC/SiC lost 0.49 wt% at 700C, 0.011 wt% at face and carbon are damaged by penetration of oxygen 1000C for 100 h. Upon a 1300C oxidation, the weights of Sic an ater. However the f SiC/a-BC/SiC gained
dation, the weights of SiC/B/SiC lost 0.011 wt.% for 10 h. However, the weights of SiC/a-BC/SiC lost 0.49 wt.% at 700 C, 0.011 wt.% at 1000 C for 100 h. Upon a 1300 C oxidation, the weights of SiC/ SiC/SiC, SiC/graphitic B–C/SiC, and SiC/B/SiC lost above 0.2 wt.% for 10 h, 2.5 wt.% for 15 h, and 0.05 wt.% for only 2 h, respectively. The sustained weight loss of the composite indicates that the interface and carbon are damaged by owing to a penetration of oxygen and/or water. However, the weights of SiC/a-BC/SiC gained Fig. 4. Surface morphologies of CVD SiC/a-BC/SiC coatings after oxidized at different temperatures. (a) and (b) at 700 C, (c) and (d) at 1000 C, and (e) and (f) at 1200 C, respectively. Fig. 5. XRD patterns of multilayer SiC/a-BC/SiC coating surface after oxidation. Weight Change (%) Time (h) -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0 10 20 30 40 50 60 70 80 90 100 700 1000 1200 Fig. 6. Weight changes of the coated composites oxidized at different temperatures up to 100 h. Y. Liu et al. / Corrosion Science 51 (2009) 820–826 823
Y Liu et al/ Corrosion Science 51(2009)820-826 0. 14 wt% at 1200C. The oxidation behaviour of the Sic/a-BCSic Table 2 ating differs from that of the Sicsic/SiC coating. For Sic/Sic) Compare of coatings system on oxidation protection for 3D C/Sic composite O3 glass formation. Only the oxidations trength retained ratio of Tested condition of Sic, carbon fibre and Pyc were referred The oxidation 3D C/SiC composite/6 of the SiCa-BC/SiC coating should be similar as that of SiC/a-BC/SiC 83.0 t%H2O8%O2/78 phitic B-C/SiC, and Sic/B/ Sic coating systems in the same Ar therm 15 times atmosphere. The weights gain of SiC/a-BC/SiC coating can SiC/graphitic 1300°C 02/78% Ar, no uted to the special H20 and O2 atmosphere. The detail protection mechanism would be discussed in next part. Sic/B/SiC [8] 76.9 1300C, 2 h, air, thermal shock 3 The residual flexural strengths of 3D C/SiC composites with three-layers CVD Sic coatings or Sic/a-BC/SiC coatings are shown SiC/SiCsIc 73.0 1300°C15h,22%02/78%Ar,no 7. Apparently the sic thermal shock than that of SiC/SiC/SiC coatings. The strength retained ratios of 3D C/Sic composites coated with SiC/a-BC/SiC coatings oxidized at 00, 1000, and 1200C were 83.2%, 96.4%, and 82.6%, respectively. temperature would decrease when H20 exists [3, 22]. During the On the other hand, those for Sic/Sic/SiC coating were 65.8%, 94.9%, oxidation process, the following reactions would occur. rial, and lead to the strength loss besides oxidation, which proven 2BC5s Oo2(S)+12.302(8)-B2030)+11CO2( 8) from the results of the composites coated with SiC/SiC/SiC coating )+O2(g)→CO2(g) in reference [8] and [21]. Furthermore, strength loss increases with B203 (1)+H2O(8)-2HBO2(g) thermal shock times increasing at previous 20-30 times thermal B203(1)+3H, O(g)-2H3 BO3(g) after 20 times thermal shock. Then, the strength loss ratio would Sic(S)+302(8)-+SiO2(s)+2C0(g) not increase SIC(S)+3H2O(8)-SiO2(s)+CO(g)+3H,(g) (6) The C/Sic composites with several coating systems indicated SiO2(s)+2H20(g) 600-1000°C the low strength retained ratio as listed in Table 2. Though the strength retained ratio of the composite coated with Sic/gra- B203 0)+B203(g) phitic-BCxSiC was also 83.0%, thermal shock has not been applied to the composite. Therefore, the strength loss should mainly be SiO2(8)+B203(1)-0B2O3XSiO2(1) attributed to the thermal shock since the weight increased during B203 XSio2( 21000C B203(g)+Sio2(s) the oxidation process. According to the above results. lude that the Sic/a-BC/SiC coating has a good oxidation protection Reactions (1)-(4) would occur below 700C, leading to the for 3D C/SiC composite up to 1200C for 100 weight loss of the composites. Reactions(5),(6) would occur above700C, leading to the weight gain of the composites Reae 3.3. Protection mechanism of sic/a-BCSiC coatings on 3D C/Sic tion(7)also leads to the weight loss. According to the morphologies weight changes and strength changes, the oxidation process were shown in Fig 8. The main pro- As presented in XPS results(see Table 1). the boron concentra- cess can be described as follows: tion of a-BC was 15 at %. The carbon and oxygen concentration were 82 at. and 3 at.%, respectively. Therefore the a-Bc can be (1)Due to the thermal expansion mismatch between SiC coat- composed of BC55O0.2. A melting point of B2O3 is 450C, and the ing and a-BC coating, were many cracks in as-prepared volatilization point is 900C[22]. Furthermore, the volatilization SiCa-BC/SiC coating as presented in Fig. 8(a). 900 SiC/SiC/Sic 国 SiC/a-BC/SiC 50 100 As-received 700 1200 Temperature/C g. 7. Residual strength for 3D C/SiC composites coated with conventional CVD layer and modified one
0.14 wt.% at 1200 C. The oxidation behaviour of the SiC/a-BC/SiC coating differs from that of the SiC/SiC/SiC coating. For SiC/SiC/ SiC coating, there was no B2O3 glass formation. Only the oxidations of SiC, carbon fibre and PyC were referred. The oxidation behaviour of the SiC/a-BC/SiC coating should be similar as that of SiC/graphitic B–C/SiC, and SiC/B/SiC coating systems in the same oxidation atmosphere. The weights gain of SiC/a-BC/SiC coating can be attributed to the special H2O and O2 atmosphere. The detail protection mechanism would be discussed in next part. The residual flexural strengths of 3D C/SiC composites with three-layers CVD SiC coatings or SiC/a-BC/SiC coatings are shown in Fig. 7. Apparently the SiC/a-BC/SiC coatings have higher strength than that of SiC/SiC/SiC coatings. The strength retained ratios of 3D C/SiC composites coated with SiC/a-BC/SiC coatings oxidized at 700, 1000, and 1200 C were 83.2%, 96.4%, and 82.6%, respectively. On the other hand, those for SiC/SiC/SiC coating were 65.8%, 94.9%, and 88.6%, respectively. Thermal shock would also damage material, and lead to the strength loss besides oxidation, which proven from the results of the composites coated with SiC/SiC/SiC coating in reference [8] and [21]. Furthermore, strength loss increases with thermal shock times increasing at previous 20–30 times thermal shock. According to reference [21], the strength loss ratio is 10% after 20 times thermal shock. Then, the strength loss ratio would not increase. The C/SiC composites with several coating systems indicated the low strength retained ratio as listed in Table 2. Though the strength retained ratio of the composite coated with SiC/graphitic-BCx/SiC was also 83.0%, thermal shock has not been applied to the composite. Therefore, the strength loss should mainly be attributed to the thermal shock since the weight increased during the oxidation process. According to the above results, we can conclude that the SiC/a-BC/SiC coating has a good oxidation protection for 3D C/SiC composite up to 1200 C for 100 h. 3.3. Protection mechanism of SiC/a-BC/SiC coatings on 3D C/SiC composites As presented in XPS results (see Table 1), the boron concentration of a-BC was 15 at.%. The carbon and oxygen concentration were 82 at.% and 3 at.%, respectively. Therefore the a-BC can be composed of BC5.5O0.2. A melting point of B2O3 is 450 C, and the volatilization point is 900 C [22]. Furthermore, the volatilization temperature would decrease when H2O exists [3,22]. During the oxidation process, the following reactions would occur. 2BC5:5O0:2ðsÞ þ 12:3O2ðgÞ ! B2O3ðlÞ þ 11CO2ðgÞ ð1Þ CðsÞ þ O2ðgÞ ! CO2ðgÞ ð2Þ B2O3ðlÞ þ H2OðgÞ ! 2HBO2ðgÞ ð3Þ B2O3ðlÞ þ 3H2OðgÞ ! 2H3BO3ðgÞ ð4Þ SiCðsÞ þ 3O2ðgÞ ! SiO2ðsÞ þ 2COðgÞ ð5Þ SiCðsÞ þ 3H2OðgÞ ! SiO2ðsÞ þ COðgÞ þ 3H2ðgÞ ð6Þ SiO2ðsÞ þ 2H2OðgÞ ! 6001000 C SiðOHÞ4ðgÞ ð7Þ B2O3ðlÞ ! 6001000 C B2O3ðgÞ ð8Þ SiO2ðgÞ þ B2O3ðlÞ ! 1000 C B2O3 xSiO2ðlÞ ð9Þ B2O3 xSiO2ðlÞ ! 1000 C B2O3ðgÞ þ SiO2ðsÞ ð10Þ Reactions (1)–(4) would occur below 700 C, leading to the weight loss of the composites. Reactions (5), (6) would occur above700 C, leading to the weight gain of the composites. Reaction (7) also leads to the weight loss. According to the morphologies weight changes and strength changes, the oxidation process were shown in Fig. 8. The main process can be described as follows: (1) Due to the thermal expansion mismatch between SiC coating and a-BC coating, there were many cracks in as-prepared SiC/a-BC/SiC coating as presented in Fig. 8(a). 0 100 200 300 400 500 600 700 800 900 007 1000 0021 Residual Strength /MPa Temperature / o C As-received SiC/SiC/SiC SiC/a-BC/SiC Fig. 7. Residual strength for 3D C/SiC composites coated with conventional CVD layer and modified one. Table 2 Compare of coatings system on oxidation protection for 3D C/SiC composites. Coating system Strength retained ratio of 3D C/SiC composite/% Tested condition SiC/a-BC/SiC 83.0 1200 C, 100 h, 14% H2O/8% O2/78% Ar, thermal shock 15 times SiC/graphitic B–C/SiC [9] 83.0 1300 C, 15 h, 22% O2/78% Ar, no thermal shock SiC/B/SiC [8] 76.9 1300 C, 2 h, air, thermal shock 3 times SiC/SiC/SiC [9] 73.0 1300 C, 15 h, 22% O2/78% Ar, no thermal shock 824 Y. Liu et al. / Corrosion Science 51 (2009) 820–826
Y Liu et aL/Corrosion Science 51(2009)820-826 825 H3 BO3/HBO2/Si(OH)4/ O2/H2O +CO2/CO/H Sic b,o sid, b C/Sic O/H,O HaBO /HBO SI(OH)4/ H3BO3/HBO2/Si(OH)4 O2/H2o +CO2/CO/H d . C/SiC 3 C/SiC 2 Fig 8. Oxidation model of multilayer Sic/a-BC/SiC coating after oxidtion. (a) before oxidation, (b) prophase, (c)metaphase. (d)anaphase. (2)At oxidation prophase, ntio increased below 1000oC. but decreased at 1200 oC. byproduct gases such as CO2, CO, H2, H3BO3, HBO2, Si( Apparently, the oxidation temperature has important influ- scape out of the crack Reactions(1)and (2)would be ce on protection mechanism of SiC/a-BC/SiC coatings. inant, and the weights of the composites lost rapidly nd Sio glass materials formed partially, and the cracks The de were sealed partially C/SiC composites can be explained as follows based on oxidation (3)At oxidation metaphase, more B203 and SiOz glass materials temperature: formed and cracks were fully sealed. The Oz and H20 can not intrude into the composites because the full cracks sealing.(1)At 700C, the reactions(1)and(2)would occur, which lead At the same time, B203 and Sio2 glass volatilized owing to to relatively high weight loss of C/Sic composites at the first the reactions (3).(4).(7), and(8). The weights lost slowly Oh. Formation of a large amount of B203 inhibits the ingress of O2/H20 incursion into the composites between 4)At oxidation anaphase, a large amount of B203 and Sioz glass 0 and 68 h. Therefore, weight loss rate becomes slowly materials volatilized therefore the cracks were partially After that, the volatilization of H3 BO3 and HBo would occur ealed Reactions (1)and (2)would be dominant again. the pon the reactions (3)and(4). Therefore the O2/H20 would weights lost rapidly again. Fig. 9 shows the curves of ntrude into the composites again, which led to the relative strength retained ratio and weight change for the coated high weight loss of C/Sic composites again at last 32 h. Com- composites as a function of temperature. Weight cha bined with Fig. 7 and Fig. 9, the strength loss should be ncreased with increasing temperature. Strength retained mainly attributed oxidation of the composites. 2)At 1000C, reactions(1)-(4)occur strongly, which leads to the weight loss. At the same time, reactions(5).(6)also ccurs and lead to weight gain. Cristobalite SiOz would reduce the volatilization. Therefore, weight gain occurred during oxidation process. Finally, the weight loss occurred after oxidized for 100 h. The fast formations of B2O3 a Sioz glass at the initial stage of oxidation lead to the good xidation protection for 3D C/Sic composites. Therefore 892 the strength retained ratio is the highest (3)At 1200C. reactions(1)(4).(7), and (8)would occur strongly which led to weight loss. At the same time, reactions (5).(6) would also occur strongly which led to weight gain. According to results of Wu 21. the maximum of weight gain for CVD SiC occurred at 1400-1500C, which could explain le weight gain occurred at 1200C. The rapid formation of O3 and sioz glass also led to the good oxidation protection 6007008009001000110012001300 Oxidation temperature /C 3D C/SiC comp oxidation mechanism, the residual stre should be Fig. 9. Curves of strength retained ratio an ht change of the coated than that of SiC/SiC/SiC. However, the strength retained composites as a function of temperat is 82.6% for SiC/a-BC/SiC, which is lower than that of SiC/SiC/
(2) At oxidation prophase, all above reactions would occur. The byproduct gases such as CO2, CO, H2, H3BO3, HBO2, Si(OH)4, escape out of the crack. Reactions (1) and (2) would be dominant, and the weights of the composites lost rapidly. B2O3 and SiO2 glass materials formed partially, and the cracks were sealed partially. (3) At oxidation metaphase, more B2O3 and SiO2 glass materials formed and cracks were fully sealed. The O2 and H2O can not intrude into the composites because the full cracks sealing. At the same time, B2O3 and SiO2 glass volatilized owing to the reactions (3), (4), (7), and (8). The weights lost slowly at this stage. (4) At oxidation anaphase, a large amount of B2O3 and SiO2 glass materials volatilized, therefore the cracks were partially sealed. Reactions (1) and (2) would be dominant again. The weights lost rapidly again. Fig. 9 shows the curves of strength retained ratio and weight change for the coated composites as a function of temperature. Weight change increased with increasing temperature. Strength retained ratio increased below 1000 C, but decreased at 1200 C. Apparently, the oxidation temperature has important influence on protection mechanism of SiC/a-BC/SiC coatings. The detail protection mechanisms of SiC/a-BC/SiC coatings on C/SiC composites can be explained as follows based on oxidation temperature: (1) At 700 C, the reactions (1) and (2) would occur, which leads to relatively high weight loss of C/SiC composites at the first 10 h. Formation of a large amount of B2O3 inhibits the ingress of O2/H2O incursion into the composites between 10 and 68 h. Therefore, weight loss rate becomes slowly. After that, the volatilization of H3BO3 and HBO2 would occur upon the reactions (3) and (4). Therefore the O2/H2O would intrude into the composites again, which led to the relative high weight loss of C/SiC composites again at last 32 h. Combined with Fig. 7 and Fig. 9, the strength loss should be mainly attributed oxidation of the composites. (2) At 1000 C, reactions (1)–(4) occur strongly, which leads to the weight loss. At the same time, reactions (5), (6) also occurs and lead to weight gain. Cristobalite SiO2 would reduce the volatilization. Therefore, weight gain occurred during oxidation process. Finally, the weight loss occurred after oxidized for 100 h. The fast formations of B2O3 and SiO2 glass at the initial stage of oxidation lead to the good oxidation protection for 3D C/SiC composites. Therefore, the strength retained ratio is the highest. (3) At 1200 C, reactions (1)–(4), (7), and (8) would occur strongly which led to weight loss. At the same time, reactions (5), (6) would also occur strongly which led to weight gain. According to results of Wu [21], the maximum of weight gain for CVD SiC occurred at 1400–1500 C, which could explain the weight gain occurred at 1200 C. The rapid formation of B2O3 and SiO2 glass also led to the good oxidation protection for 3D C/SiC composites. According to weight gain and above oxidation mechanism, the residual strength should be higher than that of SiC/SiC/SiC. However, the strength retained ratio is 82.6% for SiC/a-BC/SiC, which is lower than that of SiC/SiC/ 82 84 86 88 90 92 94 96 98 600 700 800 900 1000 1100 1200 1300 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 Strength retained ratio /% Weight change /wt.% Oxidation temperature /o C Fig. 9. Curves of strength retained ratio and weight change of the coated composites as a function of temperature. Fig. 8. Oxidation model of multilayer SiC/a-BC/SiC coating after oxidtion. (a) before oxidation, (b) prophase, (c) metaphase, (d) anaphase. Y. Liu et al. / Corrosion Science 51 (2009) 820–826 825
826 Y Liu et al/ Corrosion Science 51(2009)820-826 SiC. 88.6%. According to previous reference [21, thermal [31 R. Naslain Rebillat. R. Pailler. F s.x. Bourrat, Boron-bearing nock would lead to the strength loss for C/SiC composites. species in ceramic matrix composites for long-term aerospace applications, J. Moreover, strength loss from thermal shock increased due 14] D.W. Mckee, Borate treatment of carbon fibres and carbon/carbon composites to the increasing temperature. Therefore, the strength los for improved oxidation resistance, Carbon 24(1986)737-741 of composites at 1200C should be mainly attributed the [5 D W. Mckee, C L Spiro, E.]. Lamby, The effects of boron additives on the mal shock influence on the composites [6IP. [7 A Derre, L Filipozzi, Ne have demonstrated the preparation and oxidation protec multilayer CVD SiC/B/SiC coatings for 3D C/SiC composite, Mater. Sci. Eng. A tion of Sic/ a-BC/SiC coatings for 3D C/SiC composites. Th [91 S J. Wu, LF. Cheng, W.B. Yang, Y.S. Liu, LT. Zhang, Y D. Xu, Oxidation prot and cross-section morphologies of the a-BC coating are ultilayer CVD SiC coatings modified by a graphitic B-Cinterlayer for 3D C/Sic nous and amorphous. The boron concentration of a-BC 5.0at%.The carbon concentration of this coating is 82.0 at.%. [10]S. Goujard andenbulcke oxida The main bonding states of boron are B-C bond and b dissolve materials protected by Si-B-C CVD coatings, Thin Solid Films 24 in graphite lattice, which condensations are 33.5 at.% and 37 at. % 111S. Goujard, L Vandenbulcke. The oxidation behaviour of two- and three- respectively. The thickness of a-BC layer is 18 um, and the thick poly layers coatings, ]. Mater. Sci. 29( ness of each Sic layer is about 25 um. [12IS. Lab H. Blanchard. R Pailler. R Naslain. En After oxidation 700. 1000. and 1200oC for 100 h in 14 vol% oxidation re of interfacial area in C/C composites. Part 1: oxidation H20/8 vol% O2/78 vol% Ar atmosphere, the weight change ratios esistance of B-C, Si-B-C and Si-C coated carbon fibres, Eur Ceram Soc. 22 e-0.49, -0.011 and 0. 14 wt%, respectively. The strength re- [131 S. Labruquere, H. Blanchard, R. Pailler. R. Naslain, Enhancement of the tained ratios of 3D C/Sic composites coated with Sic/a-BC/SiC coat- xidation resistance of interfacial area in C/C composites. Part ll: oxidation 88.6%, respectively, for SiC/SiC/Sic coatings. The strength loss /14/S. Labr. Eur. Ceram Soc. 22(2002),bon ings are 83. 2%, 96.4% and 82.6%, respectively, 65.8%, 94.9%, and comes from oxidation and thermal shock of the composites. The resistance of interfacial area in C/ c composites. Part strength loss can mainly be attributed to oxidation at 700C. The oxidation in dry or wet air on mechanical pro composites with ns.JEur. Ceram.Soc.22(2002)1023-1030. strength loss can mainly be attributed to thermal shock at [15]. Jacques, A. Guette, F. Langlais, R. in, S. Goujard, Preparation and 1200C. Therefore, the maximum value of strength retained ratio occurs at1000° Key Eng Mater.127(1997)543-550 [16] T Piquero, H. Vincent, C. Vincent, J. Boui oxidation behaviour of carbon fibres, Carbon 33(1995)455-46 Acknowledgments [17] H.T. Tsou, W. Kowbel, Design of multilayer assisted cvd coatings fo the oxidation protection of composite materials, Surf. Coat. Tech 139-150. This work was supported by the National Science Foundation in (18) E. Lamouroux, S. Bertrand, R. Pailler, R. Naslain, M Cataldi, Oxidation-resistal China(No.90405015,No.50672076,N0.50425208,No.50642039, 50820145202, 50802076). This work was also supported by the Doctorate Foundation of Northwestern Polytechnical University [191 J.P. Viricelle, P. Goursat, D. Bahloul-Hourlier, Oxidation behaviour of a multi (CX200505) [201 J. Berjonneau, G. Cholon, F. Langlais, Deposition process o Referenc arbide from CH4/BCl3/H precursor, Electrochem. Soc. 153(2006)C795- [211 S. Wu, Thermochemical environmental behaviours of 3D SiC/SiC composites, [1 R. Naslain, Design, preparation and properties of non-oxide CMcs for D thesis, North Western Polytechnical University, Xi'an, China, 2006 and nuclear reactors: an overvie chnol64(2004)155-17 [22IP. Vinicelle, P Goursat, D. Bahloul-Hourlier, Oxidation behaviour of a boron Preparation and application bases of B-c ceramic by CVD/CVL PhD arbide based material in dry and wet oxygen, Therm. Anal. Calorim. 63 thesis, North Western Polytechnical University, XI'an, China, 2008 (in Chinese). 2001)507-515
SiC, 88.6%. According to previous reference [21], thermal shock would lead to the strength loss for C/SiC composites. Moreover, strength loss from thermal shock increased due to the increasing temperature. Therefore, the strength loss of composites at 1200 C should be mainly attributed thermal shock influence on the composites. 4. Conclusions We have demonstrated the preparation and oxidation protection of SiC/a-BC/SiC coatings for 3D C/SiC composites. The surface and cross-section morphologies of the a-BC coating are homogenous and amorphous. The boron concentration of a-BC coating is 15.0 at.%. The carbon concentration of this coating is 82.0 at.%. The main bonding states of boron are B–C bond and B dissolved in graphite lattice, which condensations are 33.5 at.% and 37 at.%, respectively. The thickness of a-BC layer is 18 lm, and the thickness of each SiC layer is about 25 lm. After oxidation 700, 1000, and 1200 C for 100 h in 14 vol.% H2O/8 vol.% O2/78 vol.% Ar atmosphere, the weight change ratios are 0.49, 0.011 and 0.14 wt.%, respectively. The strength retained ratios of 3D C/SiC composites coated with SiC/a-BC/SiC coatings are 83.2%, 96.4% and 82.6%, respectively, 65.8%, 94.9%, and 88.6%, respectively, for SiC/SiC/SiC coatings. The strength loss comes from oxidation and thermal shock of the composites. The strength loss can mainly be attributed to oxidation at 700 C. The strength loss can mainly be attributed to thermal shock at 1200 C. Therefore, the maximum value of strength retained ratio occurs at 1000 C. Acknowledgments This work was supported by the National Science Foundation in China (No. 90405015, No. 50672076, No. 50425208, No. 50642039, 50820145202, 50802076). This work was also supported by the Doctorate Foundation of Northwestern Polytechnical University (CX200505). References [1] R. Naslain, Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview, Compos. Sci. Technol. 64 (2004) 155–170. [2] Y.S. Liu, Preparation and application bases of B–C ceramic by CVD/CVI. PhD thesis, North Western Polytechnical University, Xi’an, China, 2008 (in Chinese). [3] R. Naslain, A. Guette, F. Rebillat, R. Pailler, F. Langlais, X. Bourrat, Boron-bearing species in ceramic matrix composites for long-term aerospace applications, J. Solid State Chem. 117 (2004) 449–456. [4] D.W. Mckee, Borate treatment of carbon fibres and carbon/carbon composites for improved oxidation resistance, Carbon 24 (1986) 737–741. [5] D.W. Mckee, C.L. Spiro, E.J. Lamby, The effects of boron additives on the oxidation behaviour of carbons, Carbon 22 (1984) 507–511. [6] P. Ehrburger, P. Baranne, J. Lahaye, Inhibition of the oxidation of carbon/carbon composite by boron oxide, Carbon 24 (1986) 495–499. [7] A. Derre, L. Filipozzi, F. Peron, High temperature behaviour and oxidation resistance of carbon–boron–nitrogen compounds obtained by LPCVD, J. Phys. IV C3 3 (1993) 195–202. [8] Y.S. Liu, L.F. Cheng, L.T. Zhang, S.J. Wu, D. Li, Y.D. Xu, Oxidation protection of multilayer CVD SiC/B/SiC coatings for 3D C/SiC composite, Mater. Sci. Eng. A Struct. 466 (2007) 172–177. [9] S.J. Wu, L.F. Cheng, W.B. Yang, Y.S. Liu, L.T. Zhang, Y.D. Xu, Oxidation protective multilayer CVD SiC coatings modified by a graphitic B–C interlayer for 3D C/SiC composite, Appl. Compos. Mater. 13 (2006) 397–406. [10] S. Goujard, L. Vandenbulcke, Oxidation tests of carbonaceous composite materials protected by Si–B–C CVD coatings, Thin Solid Films 245 (1994) 86– 97. [11] S. Goujard, L. Vandenbulcke, The oxidation behaviour of two- and threedimensational C/SiC thermostructural materials protected by chemicalvapour-deposition polylayers coatings, J. Mater. Sci. 29 (1994) 6212–6220. [12] S. Labruquere, H. Blanchard, R. Pailler, R. Naslain, Enhancement of the oxidation resistance of interfacial area in C/C composites. Part I: oxidation resistance of B–C, Si–B–C and Si–C coated carbon fibres, J. Eur. Ceram. Soc. 22 (2002) 1001–1009. [13] S. Labruquere, H. Blanchard, R. Pailler, R. Naslain, Enhancement of the oxidation resistance of interfacial area in C/C composites. Part II: oxidation resistance of B–C, Si–B–C and Si–C coated carbon preforms densified with carbon, J. Eur. Ceram. Soc. 22 (2002) 1011–1021. [14] S. Labruquere, H. Blanchard, R. Pailler, R. Naslain, Enhancement of the oxidation resistance of interfacial area in C/C composites. Part III: the effect of oxidation in dry or wet air on mechanical properties of C/C composites with internal protections, J. Eur. Ceram. Soc. 22 (2002) 1023–1030. [15] S. Jacques, A. Guette, F. Langlais, R. Naslain, S. Goujard, Preparation and characterization of SiC/SiC micro-composites with composition graded C(B) interphases, Key Eng. Mater. 127 (1997) 543–550. [16] T. Piquero, H. Vincent, C. Vincent, J. Bouix, Influence of carbide coatings on the oxidation behaviour of carbon fibres, Carbon 33 (1995) 455–467. [17] H.T. Tsou, W. Kowbel, Design of multilayer plasma-assisted CVD coatings for the oxidation protection of composite materials, Surf. Coat. Tech. 79 (1996) 139–150. [18] F. Lamouroux, S. Bertrand, R. Pailler, R. Naslain, M. Cataldi, Oxidation-resistant carbon fibre reinforced ceramic-matrix composites, Compos. Sci. Technol. 59 (1999) 1073–1085. [19] J.P. Viricelle, P. Goursat, D. Bahloul-Hourlier, Oxidation behaviour of a multilayered ceramic matrix composite (SiC)f/C/(SiBC)m, Compo. Sci. Technol. 61 (2001) 607–614. [20] J. Berjonneau, G. Chollon, F. Langlais, Deposition process of amorphous boron carbide from CH4/BCl3/H2 precursor, J. Electrochem. Soc. 153 (2006) C795– C800. [21] S.J. Wu, Thermochemical environmental behaviours of 3D SiC/SiC composites, PhD thesis, North Western Polytechnical University, Xi’an, China, 2006 (in Chinese). [22] J.P. Viricelle, P. Goursat, D. Bahloul-Hourlier, Oxidation behaviour of a boron carbide based material in dry and wet oxygen, J. Therm. Anal. Calorim. 63 (2001) 507–515. 826 Y. Liu et al. / Corrosion Science 51 (2009) 820–826
