MATERIALS HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 466(2007)172-177 www.elseviercom/locate/msea Oxidation protection of multilayer CVD Sic/B/Sic coatings for 3D C/SiC composite Yongsheng Liu", Laifei Cheng, Litong Zhang, Shoujun Wu, Duo Li, Yongdong Xu National Key Laboratory of Thermostricture Composite Materials, Northwestern Polytechnical University. Xi'an Shaanxi 710072, People's Republic of China Received 2 December 2006; received in revised form 10 February 2007; accepted 12 February 2007 Abstract A CVD boron coating was introduced between two CVD SiC coating layers. EDS and XRD results showed that the Cvd B coating was a boron crystal without other impurity elements. SEM results indicated that the Cvd B coating was a flake-like or column-like crystal with a compact cross-section. The crack width in the CVd SiC coating deposited on CVD B is smaller than that in a CVD SiC coating deposited on CVD SiC oating. After oxidation at 700C and 1000"C, XRD results indicated that the coating was covered by product B2O3 or B2O3-xSiO2 film. The cracks were sealed as observed by sEM. There was a large amount of fake-like material on hybrid coating surface after oxidation at 1300C Oxidation weight loss and residual flexural strength results showed that hybrid SiC/B/SiC multilayer coating provided better oxidation protection for C/SiC composite than a three layer CVD SiC coating at temperatures from 700 C to 1000C for 600 min, but worse oxidation protection above 1000 C due to the large amount of volatilization of B2 O3 or B,O3xSiOz o 2007 Elsevier B. v. All rights reserved Keywords: CVD SiC; CVD B: C/SiC composite; Oxidation protection Introduction face and the fiber in the composites. Below the CVD SiC coating deposition temperature(usually 1000C), the cracks in the CVD Continuous carbon fiber reinforced silicon carbide(C/SiC) coating are not sealing because of the thermal expansion mis- composites are potential candidates for a variety of applications match between the carbon fiber and SiC matrix as well as the in the aerospace field including rocket nozzles, aeronautic jet limited Sioz from surface oxidation. Above the CVD Sic coat- engines, heat shields and aircraft braking systems [1-3. How- ing deposition temperature(usually 1000oC), the cracks in the ever the oxidation of carbon fiber and interface limits long-term CVD SiC coating could be sealed by thermal expansion and a applications of C/Sic composite in high-temperature oxidizing layer of SiOz film due to surface oxidation. However this effect environments. Therefore the development of reliable oxidation is minor below 1300C. Therefore, it is important that the com- protection by coating is crucial to the composites. Because the posites to be protected by a modified coating from 700C to oxidation of CVD Sic is passive up to 1700C and the formed 1300C SiO2 film has a low oxygen diffusion coefficient, CVD In order to protect carbon articles in the oxidation environ- coating is fundamental coating material for oxidation protection ment, the National Carbon Co has developed a hybrid coating of carbon-based materials [4]. Research on CVD SiC coating which is composed of an inner SiC layer and an outer glaze showed that defects, such as cracks due to the coefficient of based on B2O3 [4]. Since that time, borate glassy materials thermal expansion(CTE)mismatch between the composite and became important constituents of oxidation protection systems the coating are unavoidable in a CVD SiC coating 5]. These for carbon [6-10]. This research was focused on sealing the cracks lead to oxygen diffusion inward and oxidation of inter- coating cracks during oxidation by formation of low viscosity B2O or borosilicate glass. The developed coatings were mainly B2O3 [11], B4C[12]or SiBC [13]. An outer coating of B2 O3 is Corresponding author. Tel : +862988486068x823: fax: +8629. not feasible due to B2O3 volatilization. Multilayer SiC/B4C/SiC E-mail addresses: liuys99067(@ mail nwpu.edu. c or SiC/SiBC/SiC coatings can be fabricated expediently by Liuys99067(@163.com(YLiu) CVD process [12, 13], but the escape of CO and CO2 makes 0921-5093/S-see front matter O 2007 Elsevier B v. All rights reserved doi:10.1016/msea.2007.02059
Materials Science and Engineering A 466 (2007) 172–177 Oxidation protection of multilayer CVD SiC/B/SiC coatings for 3D C/SiC composite Yongsheng Liu ∗, Laifei Cheng, Litong Zhang, Shoujun Wu, Duo Li, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 2 December 2006; received in revised form 10 February 2007; accepted 12 February 2007 Abstract A CVD boron coating was introduced between two CVD SiC coating layers. EDS and XRD results showed that the CVD B coating was a boron crystal without other impurity elements. SEM results indicated that the CVD B coating was a flake-like or column-like crystal with a compact cross-section. The crack width in the CVD SiC coating deposited on CVD B is smaller than that in a CVD SiC coating deposited on CVD SiC coating. After oxidation at 700 ◦C and 1000 ◦C, XRD results indicated that the coating was covered by product B2O3 or B2O3·xSiO2 film. The cracks were sealed as observed by SEM. There was a large amount of flake-like material on hybrid coating surface after oxidation at 1300 ◦C. Oxidation weight loss and residual flexural strength results showed that hybrid SiC/B/SiC multilayer coating provided better oxidation protection for C/SiC composite than a three layer CVD SiC coating at temperatures from 700 ◦C to 1000 ◦C for 600 min, but worse oxidation protection above 1000 ◦C due to the large amount of volatilization of B2O3 or B2O3·xSiO2. © 2007 Elsevier B.V. All rights reserved. Keywords: CVD SiC; CVD B; C/SiC composite; Oxidation protection 1. Introduction Continuous carbon fiber reinforced silicon carbide (C/SiC) composites are potential candidates for a variety of applications in the aerospace field including rocket nozzles, aeronautic jet engines, heat shields and aircraft braking systems [1–3]. However the oxidation of carbon fiber and interface limits long-term applications of C/SiC composite in high-temperature oxidizing environments. Therefore the development of reliable oxidation protection by coating is crucial to the composites. Because the oxidation of CVD SiC is passive up to 1700 ◦C and the formed SiO2 film has a low oxygen diffusion coefficient, CVD SiC coating is fundamental coating material for oxidation protection of carbon-based materials [4]. Research on CVD SiC coating showed that defects, such as cracks due to the coefficient of thermal expansion (CTE) mismatch between the composite and the coating are unavoidable in a CVD SiC coating [5]. These cracks lead to oxygen diffusion inward and oxidation of inter- ∗ Corresponding author. Tel.: +86 29 8848 6068x823; fax: +86 29 8849 4620. E-mail addresses: liuys99067@mail.nwpu.edu.cn, Liuys99067@163.com (Y. Liu). face and the fiber in the composites. Below the CVD SiC coating deposition temperature (usually 1000 ◦C), the cracks in the CVD coating are not sealing because of the thermal expansion mismatch between the carbon fiber and SiC matrix as well as the limited SiO2 from surface oxidation. Above the CVD SiC coating deposition temperature (usually 1000 ◦C), the cracks in the CVD SiC coating could be sealed by thermal expansion and a layer of SiO2 film due to surface oxidation. However this effect is minor below 1300 ◦C. Therefore, it is important that the composites to be protected by a modified coating from 700 ◦C to 1300 ◦C. In order to protect carbon articles in the oxidation environment, the National Carbon Co. has developed a hybrid coating which is composed of an inner SiC layer and an outer glaze based on B2O3 [4]. Since that time, borate glassy materials became important constituents of oxidation protection systems for carbon [6–10]. This research was focused on sealing the coating cracks during oxidation by formation of low viscosity B2O3 or borosilicate glass. The developed coatings were mainly B2O3 [11], B4C [12] or SiBC [13]. An outer coating of B2O3 is not feasible due to B2O3 volatilization. Multilayer SiC/B4C/SiC or SiC/SiBC/SiC coatings can be fabricated expediently by CVD process [12,13], but the escape of CO and CO2 makes 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.02.059
Y Liu et al. Materials Science and Engineering A 466(2007)172-177 the coatings be porous, and the outer SiC coating may fake observed with a scanning electron microscope(SEM,S-4700) off during oxidation process. Consequently boron coating may And an energy dispersive X-ray spectrum(EDS, EDAX) was be a suitable interlayer as a sealing coating in multilayer Sic performed to identify element species in the as-deposited coat- oatings, since only B2O3 is produced after oxidation. CVD ings echnique is suitable to introduce a boron coating with variable Flexural strengths of the composite specimens before and thickness between a two cvd sic coatings after oxidation were measured by a three-point bending method In this paper, a CVD B coating is synthesized from boron at the room temperature. The span dimension was 20 mm and trichloride and hydrogen and was introduced into two CVd the loading rate was 0.5 mm min Sic coatings. Microstructure and chemical characterization of the CVD B and the hybrid SiC/B/SiC multilayer coating are 3. Results and discussion reported. Effects of the hybrid SiC/B/SiC multilayer coating on oxidation behavior, microstructure and mechanical properties of 3. 1. Morphology and chemical compositions of hybrid the C/SiC composite are also reported. 2. Experiment procedure Fig. 1 shows the morphology of the CVD B coating. The particle size of CVD B crystal was several micrometers and had 2.1. Fabrication of specimens a distinct flake-like or column-like orientation crystal as shown in Fig. 1(a). Fig. 1(b) shows the cross-section morphology of Firstly, fiber preforms were fabricated from carbon fiber(T- the CVd B coating(deposition time was 10 h). It is clear that 300, Japan Toray), which has a volume fraction in the range the prepared CVd B coating is very dense. Fig. 2(a)shows that of 40-45% and a braiding angle of 20 using a four-step three only boron peaks is observed in an EDS analysis for the coating. dimensional (4-step 3D) braiding method in Nanjing Institute Fig. 2(b)shows the XRD results of the deposited B coating on of Glass Fiber, PR China. Secondly, pyrolytic carbon(PyC) the C/SiC composite. It is clearly that the deposit is crystalline interface and the silicon carbide matrix were deposited by low boron pressure chemical vapor infiltration(LPCVD) process. PyC was ig. 3 shows the morphologies of CVD Sic deposited on deposited on the fiber using C3H6 precursor at 870C for I h at a CVD B and CVD SiC, respectively. The morphologies are sim- reduced pressure of 500 Pa, yielding a thickness of 200 nm. The ilar with each other. The crack width of CVD Sic deposited on SiC matrix was achieved at 1 100C for 120 h at reduced pressure CVd B however seems obviously narrower than that deposited of 2 kPa by using methyltrichlorosilane (MTS, CH3 SiCl3)with a on CVD SiC H2: MTS molar ratio of 10. This was achieved by bubbling hydro- gen in gas phase through the MTS. An argon diluent was used to 3.2. Morphologies and compositions of hybrid coating slow down the chemical reaction rate during deposition. Then, after oxidation the as-received composite was machined and polished, and 3 mm x 4 mm x 30mm substrates were obtained. Finally, the The morphologies of the coatings after oxidation at 700Cfor pecimens were coated with the hybrid CVD Sic/CVD B/CVd 600 min are shown in Fig 4. The interspaces between Sic par- SiC multilayer coatings. The conditions for CVD SiC were the ticles(clusters) in the SiC/B/SiC coating were generally sealed same as the SiC matrix except for the deposition time, which by a glassy material. As shown in Fig 4(b), the crack is filled was 30 h. The deposition conditions for CVD B were as fol- and sealed by the glassy material. Fig. 5 shows the morpholo- lows: temperature 1000C, pressure l kPa, time 10h, BCl3 flow gies of the hybrid coating after oxidation for 600 min at 1000oC. 10 ml min-, H2 flow 60 ml min- and Ar flow 60 ml min-. As temperature increased, the spaces between the Sic particles (clusters)clearly exhibited an improved sealing performance by 2. 2. Oxidation tests the glassy material. However, pores could be observed in the glassy layer after oxidation at 1000C. As shown in Fig. 5(b), The oxidation tests were conducted in a MoSi2 furnace the crack is filled by porous material, and cannot be sealed fully. in static air environments at temperatures range from 700C, Fig. 6 shows the morphologies of the hybrid coating after oxida .000C for 600 min and 1300C for 120 min Three specimens tion for 120 min at 1300C. There are large amount of liquid on put in an alumina tube with a purity of 99.99% were used for the sample surface during oxidation at 1300C, which resulted each experimental condition. The mass of the specimens were in the adhesion between sample and corundum tube as shown recorded after they were oxidized for Oh, 2h, 5 h and 10 h at the in Fig. 6(b). The experiment was terminated after oxidation for desired temperature. They were measured using an electronic 120 min No glassy material can be observed on the hybrid coat- balance(sensitivity =0.01 mg). ing surface. A large amount of flake-like material existed on the 2.3. Measurements of the composites Fig. 7 shows the phases of the hybrid coating before and after oxidation at700°C,1000°cand1300°C. It is apparent Phase identification was obtained with an X-ray diffraction that the phase is B-SiC as for the outer CVD SiC coating before device(XRD, Rigaku D/MAX-2400 with Cu Ko radiation). oxidation. The hybrid coating contains large amount of Sic, The surface and cross-section morphologies of the coating were B2O3 and small amount of Sioz after oxidation at 700C for
Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 173 the coatings be porous, and the outer SiC coating may flake off during oxidation process. Consequently boron coating may be a suitable interlayer as a sealing coating in multilayer SiC coatings, since only B2O3 is produced after oxidation. CVD technique is suitable to introduce a boron coating with variable thickness between a two CVD SiC coatings. In this paper, a CVD B coating is synthesized from boron trichloride and hydrogen and was introduced into two CVD SiC coatings. Microstructure and chemical characterization of the CVD B and the hybrid SiC/B/SiC multilayer coating are reported. Effects of the hybrid SiC/B/SiC multilayer coating on oxidation behavior, microstructure and mechanical properties of the C/SiC composite are also reported. 2. Experiment procedure 2.1. Fabrication of specimens Firstly, fiber preforms were fabricated from carbon fiber (T- 300, Japan Toray), which has a volume fraction in the range of 40–45% and a braiding angle of 20◦ using a four-step three dimensional (4-step 3D) braiding method in Nanjing Institute of Glass Fiber, PR China. Secondly, pyrolytic carbon (PyC) interface and the silicon carbide matrix were deposited by low pressure chemical vapor infiltration (LPCVI) process. PyC was deposited on the fiber 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 diluent was used to slow down the chemical reaction rate during deposition. Then, the as-received composite was machined and polished, and 3 mm × 4 mm × 30 mm substrates were obtained. Finally, the specimens were coated with the hybrid CVD SiC/CVD B/CVD SiC multilayer coatings. The conditions for CVD SiC were the same as the SiC matrix except for the deposition time, which was 30 h. The deposition conditions for CVD B were as follows: temperature 1000 ◦C, pressure 1 kPa, time 10 h, BCl3 flow 10 ml min−1, H2 flow 60 ml min−1 and Ar flow 60 ml min−1. 2.2. Oxidation tests The oxidation tests were conducted in a MoSi2 furnace in static air environments at temperatures range from 700 ◦C, 1000 ◦C for 600 min and 1300 ◦C for 120 min. Three specimens put in an alumina tube with a purity of 99.99% were used for each experimental condition. The mass of the specimens were recorded after they were oxidized for 0 h, 2 h, 5 h and 10 h at the desired temperature. They were measured using an electronic balance (sensitivity = 0.01 mg). 2.3. Measurements of the composites Phase identification was obtained with an X-ray diffraction device (XRD, Rigaku D/MAX-2400 with Cu K radiation). The surface and cross-section morphologies of the coating were observed with a scanning electron microscope (SEM, S-4700). And an energy dispersive X-ray spectrum (EDS, EDAX) was performed to identify element species in the as-deposited coatings. Flexural strengths of the composite specimens before and after oxidation were measured by a three-point bending method at the room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm min−1. 3. Results and discussion 3.1. Morphology and chemical compositions of hybrid coating Fig. 1 shows the morphology of the CVD B coating. The particle size of CVD B crystal was several micrometers and had a distinct flake-like or column-like orientation crystal as shown in Fig. 1(a). Fig. 1(b) shows the cross-section morphology of the CVD B coating (deposition time was 10 h). It is clear that the prepared CVD B coating is very dense. Fig. 2(a) shows that only boron peaks is observed in an EDS analysis for the coating. Fig. 2(b) shows the XRD results of the deposited B coating on the C/SiC composite. It is clearly that the deposit is crystalline boron. Fig. 3 shows the morphologies of CVD SiC deposited on CVD B and CVD SiC, respectively. The morphologies are similar with each other. The crack width of CVD SiC deposited on CVD B however seems obviously narrower than that deposited on CVD SiC. 3.2. Morphologies and compositions of hybrid coating after oxidation The morphologies of the coatings after oxidation at 700 ◦C for 600 min are shown in Fig. 4. The interspaces between SiC particles (clusters) in the SiC/B/SiC coating were generally sealed by a glassy material. As shown in Fig. 4(b), the crack is filled and sealed by the glassy material. Fig. 5 shows the morphologies of the hybrid coating after oxidation for 600 min at 1000 ◦C. As temperature increased, the spaces between the SiC particles (clusters) clearly exhibited an improved sealing performance by the glassy material. However, pores could be observed in the glassy layer after oxidation at 1000 ◦C. As shown in Fig. 5(b), the crack is filled by porous material, and cannot be sealed fully. Fig. 6 shows the morphologies of the hybrid coating after oxidation for 120 min at 1300 ◦C. There are large amount of liquid on the sample surface during oxidation at 1300 ◦C, which resulted in the adhesion between sample and corundum tube as shown in Fig. 6(b). The experiment was terminated after oxidation for 120 min. No glassy material can be observed on the hybrid coating surface. A large amount of flake-like material existed on the coating. Fig. 7 shows the phases of the hybrid coating before and after oxidation at 700 ◦C, 1000 ◦C and 1300 ◦C. It is apparent that the phase is -SiC as for the outer CVD SiC coating before oxidation. The hybrid coating contains large amount of SiC, B2O3 and small amount of SiO2 after oxidation at 700 ◦C for
Y Liu et al. Materials Science and Engineering A 466(2007)172-177 (b)享 CVD B Crystalline boron CVD Sic CⅤDsiC C/SiC 100um Fig 1. Surface and cross-section morphology of CVD B on CVD SiC coating(deposited at 1000 C for 10h):(a)surface:( b)cross-section (a)4.3 21 00200 203040 Energy /k\ 20(deg) Fig. 2. EDS and XRD spectrum of CVD B coating(deposited at 1000C for 10h):(a) EDS;( b)XRD. Fig. 3 gies of CVD SiC deposited on:(a)CVD B; (b)CVD SiC. Fig 4. Morphologies of the SiC/B/SiC coating after oxidized at 700C for 900 min: (a)surface: ( b)crack sealing
174 Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 Fig. 1. Surface and cross-section morphology of CVD B on CVD SiC coating (deposited at 1000 ◦C for 10 h): (a) surface; (b) cross-section. Fig. 2. EDS and XRD spectrum of CVD B coating (deposited at 1000 ◦C for 10 h): (a) EDS; (b) XRD. Fig. 3. Morphologies of CVD SiC deposited on: (a) CVD B; (b) CVD SiC. Fig. 4. Morphologies of the SiC/B/SiC coating after oxidized at 700 ◦C for 900 min: (a) surface; (b) crack sealing
Y Liu et al. Materials Science and Engineering A 466(2007)172-177 Fig. 5. Morphology of the SiC/B/SiC coating after oxidized at 1000 C for 900 min: (a) surface: (b)crack sealing. 100um 1 Omm Fig. 6. Morphology of the SiC/B/SiC coating after oxidized at 1300C for 120 min: (a)surface: (b)conglutination between B2 O3 glass and crucible 600 min As temperature increased, the surface consisted of SiC, SiO,(g)+ B2 O3()1o0ooB2O3xSiO2()) SiO and b2O3-xSio after oxidized at 1000C for 600 min and 1300°Cfor120min. For the composites with hybrid SiC/B/SiC multilayer coating, B2O3·xSiO2①)→B2O3(g)+xSiO2(s) the oxidation reactions in air from 700oC to 1300C are as 2C(s)+O2(g)→→2CO(g) The cracks in the Cvd SiC coating cannot be sealed at 700oC SiC(s)+302(g)→→2SiO2(s)+2COg) because of the low oxidation rate of cvd Sic if no cvd layer existed. A previous report [14] indicated that the B2O3 4B(s)+302(g)→2B2O3() (2) would flow at 630.C. Therefore, B2O, liquid would be produced at 700oC due to the reaction(2), after the CVd B layer was B2O3() 3) introduced into the hybrid coating. The liquid flows through the cracks in the Cvd sic. which lead to the sealed crack On the other hand, B203 would accelerate the oxidation of CVd Sic by forming a melt, according to B2O3-SiO2 system phase diagram [15]. This melt would be expected to have high oxygen 1300C Oxidized diffusion rates and lead to the observed rapid oxidation rates. The formation of the glassy material coating the surface and 000° C Oxidized cracks was proved by observation of SEM. The compositions were SiC, Sioz and a small amount of B2O3 as the XRD results Avk showed. Because of the low temperature at 700C volatilization 700° C Oxidized of B2O3 is faint as shown in reaction At 1000C, the reactions(1)3) would be faster. So the microcracks in the Sic coating would also result from the D SiC would also prompt the crack sealing. Large amount of B2O3 would be formed with increasing temperature. Then the reactions(4) Fig. 7. XRD spectrums of the Sic/B/SiC coating before and after oxidized at face and crack were also both covered with glassy material as 700°C,1000°Cand1300°C observed by SEM in Fig. 5. The compositions were SiC, SiOz
Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 175 Fig. 5. Morphology of the SiC/B/SiC coating after oxidized at 1000 ◦C for 900 min: (a) surface; (b) crack sealing. Fig. 6. Morphology of the SiC/B/SiC coating after oxidized at 1300 ◦C for 120 min: (a) surface; (b) conglutination between B2O3 glass and crucible. 600 min. As temperature increased, the surface consisted of SiC, SiO2 and B2O3·xSiO2 after oxidized at 1000 ◦C for 600 min and 1300 ◦C for 120 min. For the composites with hybrid SiC/B/SiC multilayer coating, the oxidation reactions in air from 700 ◦C to 1300 ◦C are as follows: 2SiC(s) + 3O2(g)800 ◦C −→ 2SiO2(s) + 2CO(g) (1) 4B(s) + 3O2(g)500 ◦C −→ 2B2O3(l) (2) B2O3(l)600−1000 ◦C −→ B2O3(g) (3) Fig. 7. XRD spectrums of the SiC/B/SiC coating before and after oxidized at 700 ◦C, 1000 ◦C and 1300 ◦C. SiO2(g) + B2O3(l)1000 ◦C −→ B2O3 · xSiO2(l) (4) B2O3 · xSiO2(l)≥1000 ◦C −→ B2O3(g) + xSiO2(s) (5) 2C(s) + O2(g)400 ◦C −→ 2CO(g) (6) The cracks in the CVD SiC coating cannot be sealed at 700 ◦C because of the low oxidation rate of CVD SiC if no CVD B layer existed. A previous report [14] indicated that the B2O3 would flow at 630 ◦C. Therefore, B2O3 liquid would be produced at 700 ◦C due to the reaction (2), after the CVD B layer was introduced into the hybrid coating. The liquid flows through the cracks in the CVD SiC, which lead to the sealed crack. On the other hand, B2O3 would accelerate the oxidation of CVD SiC by forming a melt, according to B2O3–SiO2 system phase diagram [15]. This melt would be expected to have high oxygen diffusion rates and lead to the observed rapid oxidation rates. The formation of the glassy material coating the surface and cracks was proved by observation of SEM. The compositions were SiC, SiO2 and a small amount of B2O3 as the XRD results showed. Because of the low temperature at 700 ◦C volatilization of B2O3 is faint as shown in reaction (3). At 1000 ◦C, the reactions (1)–(3) would be faster. So the microcracks in the SiC coating would also result from the glassy materials. The thermal expansion of CVD SiC would also prompt the crack sealing. Large amount of B2O3 would be formed with increasing temperature. Then the reactions (4) and (5) would also much more serious. The hybrid coating surface and crack were also both covered with glassy material as observed by SEM in Fig. 5. The compositions were SiC, SiO2
Y Liu et al. Materials Science and Engineering A 466(2007)172-177 4.03 1000°C .05 0·1300°C Oxidation time(h) As-received 00°C Fig 8. Oxidation weight loss of the coated 3D C/SiC composites after oxidized re(C) for 600 min Fig. 9. Residual flexural strength of the coated C/SiC before and after oxidized for 900 min: ('d) Sic/B/SiC coating; sic/Sic/SiC coating and B2O3xSiO2 by XRD results. However, volatilization of the B203 was much more extensive as the temperature increased posite showed a large strength loss(64% retained strength)after [8-10].Then the high partial pressure would create pores in the oxidation at 700C, lager than that of SiC/B/SiC coated com- coating surface. The cracks cannot be completely sealed, and posites after oxidation at the same time. The residual strengths the porous material remained due to B2O3 volatilization of the SiC/SiC/SiC coated composite after oxidation at 1000C Above 1300C, the experiment was terminated because is nearly as same as that of SiC/B/SiC coated composites after of the extensive production and volatilization of B2O3 and oxidation at the same time. After oxidation at 1300oC. the resid B2O3xSiO2, which was verified by the adhesion between the ual strength of the Sic/Sic/SiC coated composite oxidized for samples and the corundum tube. There was not a continuous 10 h is higher than that of SiC/B/SiC coated composites only glassy material on the coating surface. The outer layer CVD SiC oxidized for 2h coating faked off because of the high partial pressure of B20 These results indicate that the Sic/B/Sic coating could and B203 xSiOz. Therefore, the hybrid coating surface seemed provide better oxidation protection for C/SiC composite than loose and flake-like according to SEMresults The compositions Sic/SiC/SiC coating between 700C and 1000C for 600min were SiC, SiO2 and B2O3 xSio by EDS and XRD results due to cracks sealed by B2O3 or B2O3 x Sio2 liquid. But above 1000C, the protection effect of the SiC/B/SiC coating for C/SiC 3.3. Effect of oxidation on the weight change and composite decreases due to the extensive volatilization of B2O3 mechanical properties of the coated composites or B2O3 xSiO2 liquid. The protection effect of the SiC/B/SiC coating for C/SiC composite is worse than that of Sic/sic/SiC Fig8 shows the weight change of the coated C/SiC com- coating at the temperatures greater than 1000C posite oxidized at700°C,1000°cfor600 min and1300°C for 120 min. The weight change of the SiC/SiC/SiC coated 3D- 4. Conclusions C/SiC composites oxidized at the same time and temperature are also shown in Fig 8, in order to compare the oxidation behav- (1)ACVD B coating for C/SiC was synthesized from hydrogen ior of he SiC/B/SiC coated composites. Both the coated C/SiC and boron trichloride. The b coating was found to exhibit a composites showed weight loss. As for SiC/SiC/SiC coated crystal structure. The morphology of CVD SiC coating on composites, the weight loss almost increase linearly with the the CVd B layer is similar to that of CVD SiC coating on xidation time increase from 700C to 1300C. which showed the CVD SiC layer, but the crack width at room temperature that the cracks are not fully healed below 1300 C. However, the seems smaller weight loss is nonlinear with increasing oxidation time, which (2) The coating surface was covered by B2O3 or B2O3xSio can be described as follows: at 700C. the weight loss increases glassy material after oxidation at 700C and 1000C, how for the initial 2 h, then decrease with increasing oxidation time, ever the morphology of coating surface seems iake-like at 1000"C, the weight loss increases dramatically for the initial ter oxidation at 1300C. The coating cracks were fully 2h, then decrease rapidly for 3 h, and finally increases rapidly sealed at700°C, and partially sealed at I000°C. for the last several hours; at 1300C, the weight loss increases (3) The residual strength of the SiC/SiC/SiC coated compos dramatically before the experiment was stopped ite showed a strength loss(76.9% retained strength) after The residual flexural strengths are compared as shown in xidation at 700C. larger than that of SiC/B/SiC coated Fig 9. The residual strength of the SiC/B/SiC coated composite composites after oxidation. The residual strength of the had a small strength loss(87.7% retained strength) after oxida- Sic/SiC/SiC coated composite after oxidation at 1000C tion for 600 min at 700oC and 1000C, and a biggish strength is nearly as same as that of Sic/B/Sic coated composites loss(76.9% retained strength) after oxidation at 1300C for after oxidation at the same time. After oxidation at 1300oC 120 min. The residual strength of the SiC/SiC/Sic coated com- the residual strength of the SiC/Sic/SiC coated composite
176 Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 Fig. 8. Oxidation weight loss of the coated 3D C/SiC composites after oxidized for 600 min. and B2O3·xSiO2 by XRD results. However, volatilization of the B2O3 was much more extensive as the temperature increased [8–10]. Then the high partial pressure would create pores in the coating surface. The cracks cannot be completely sealed, and the porous material remained due to B2O3 volatilization. Above 1300 ◦C, the experiment was terminated because of the extensive production and volatilization of B2O3 and B2O3·xSiO2, which was verified by the adhesion between the samples and the corundum tube. There was not a continuous glassy material on the coating surface. The outer layer CVD SiC coating flaked off because of the high partial pressure of B2O3 and B2O3·xSiO2. Therefore, the hybrid coating surface seemed loose and flake-like according to SEM results. The compositions were SiC, SiO2 and B2O3·xSiO2 by EDS and XRD results. 3.3. Effect of oxidation on the weight change and mechanical properties of the coated composites Fig. 8 shows the weight change of the coated C/SiC composite oxidized at 700 ◦C, 1000 ◦C for 600 min and 1300 ◦C for 120 min. The weight change of the SiC/SiC/SiC coated 3DC/SiC composites oxidized at the same time and temperature are also shown in Fig. 8, in order to compare the oxidation behavior of he SiC/B/SiC coated composites. Both the coated C/SiC composites showed weight loss. As for SiC/SiC/SiC coated composites, the weight loss almost increase linearly with the oxidation time increase from 700 ◦C to 1300 ◦C, which showed that the cracks are not fully healed below 1300 ◦C. However, the weight loss is nonlinear with increasing oxidation time, which can be described as follows: at 700 ◦C, the weight loss increases for the initial 2 h, then decrease with increasing oxidation time; at 1000 ◦C, the weight loss increases dramatically for the initial 2 h, then decrease rapidly for 3 h, and finally increases rapidly for the last several hours; at 1300 ◦C, the weight loss increases dramatically before the experiment was stopped. The residual flexural strengths are compared as shown in Fig. 9. The residual strength of the SiC/B/SiC coated composite had a small strength loss (87.7% retained strength) after oxidation for 600 min at 700 ◦C and 1000 ◦C, and a biggish strength loss (76.9% retained strength) after oxidation at 1300 ◦C for 120 min. The residual strength of the SiC/SiC/SiC coated comFig. 9. Residual flexural strength of the coated C/SiC before and after oxidized for 900 min: ( ) SiC/B/SiC coating; ( ) SiC/SiC/SiC coating. posite showed a large strength loss (64% retained strength) after oxidation at 700 ◦C, lager than that of SiC/B/SiC coated composites after oxidation at the same time. The residual strengths of the SiC/SiC/SiC coated composite after oxidation at 1000 ◦C is nearly as same as that of SiC/B/SiC coated composites after oxidation at the same time. After oxidation at 1300 ◦C, the residual strength of the SiC/SiC/SiC coated composite oxidized for 10 h is higher than that of SiC/B/SiC coated composites only oxidized for 2 h. These results indicate that the SiC/B/SiC coating could provide better oxidation protection for C/SiC composite than SiC/SiC/SiC coating between 700 ◦C and 1000 ◦C for 600min due to cracks sealed by B2O3 or B2O3·xSiO2 liquid. But above 1000 ◦C, the protection effect of the SiC/B/SiC coating for C/SiC composite decreases due to the extensive volatilization of B2O3 or B2O3·xSiO2 liquid. The protection effect of the SiC/B/SiC coating for C/SiC composite is worse than that of SiC/SiC/SiC coating at the temperatures greater than 1000 ◦C. 4. Conclusions (1) A CVD B coating for C/SiC was synthesized from hydrogen and boron trichloride. The B coating was found to exhibit a crystal structure. The morphology of CVD SiC coating on the CVD B layer is similar to that of CVD SiC coating on the CVD SiC layer, but the crack width at room temperature seems smaller. (2) The coating surface was covered by B2O3 or B2O3·xSiO2 glassy material after oxidation at 700 ◦C and 1000 ◦C, however the morphology of coating surface seems flake-like after oxidation at 1300 ◦C. The coating cracks were fully sealed at 700 ◦C, and partially sealed at 1000 ◦C. (3) The residual strength of the SiC/SiC/SiC coated composite showed a strength loss (76.9% retained strength) after oxidation at 700 ◦C, larger than that of SiC/B/SiC coated composites after oxidation. The residual strength of the SiC/SiC/SiC coated composite after oxidation at 1000 ◦C is nearly as same as that of SiC/B/SiC coated composites after oxidation at the same time. After oxidation at 1300 ◦C, the residual strength of the SiC/SiC/SiC coated composite
Y Liu et al. Materials Science and Engineering A 466(2007)172-177 oxidized for 600 min is higher than that of Sic/B/SiC coated Referen composites only oxidized for 120 min (4) The SiC/B/SiC coating could provide better oxidation pro- [1 R. Naslain, A Guette, F. Rebillat, et al., J. Solid State Chem. 177(2004) tection for C/SiC composite than SiC/SiC/SiC coating between 700oC and 1000oC for 600 min due to the crack [2]S Schmidt, S. Beyer, H. Knabe, et al., Acta Astronaut. 55(2004)409-420 [3 W. Krenkel, B. Heidenreich, R Renz, Adv Eng Mater. 4(2002)427-436 self-healing by B2O or B2O3xSioz liquid. But above [4] J.R. Strife. I.E. Sheehan, Ceram. Bull. 67(1988)369-374 1000C, the protection effect of the SiC/B/SiC coating for [5] L.F. Cheng. Y.D. Xu, L.T. Zhang, et al,J Mater. Sci. 37(2002)5339-5344 tion of B2O3 or B2O3-xSiO2 liquid The protection effect of m- yashi, K Maeda, H. Sano, Y. Uchiyama, Carbon 33(1995) C/SiC composite decrease due to the extensive volatilize- the SiC/B/SiC coating for C/SiC composite is worse than [7 L.F. Cheng, Y.D. Xu, L.T. Zhang, et al, Carbon 39(2001)1127-1133 [8 H.T. Tsou, w. Kowbel, Carbon 33(1995)1289-1292 that of Sic/Sic/Sic coating above 1000oc [9]J. Schulte-Fischedick, J. Schmidt, R. Tamme, et al., Mater. Sci. Eng. A 386 (2004)428-434. [10] C. Isola, P. Appendino, F. Bosco, et al, Carbon 36(1998)1213-1218. [11 R. Naslain, Ceram. Trans. 58(1995)23-39 he authors would thank to the support of the Key Foundation [12] R. Naslain, Compos. Part A: Appls 29A(1998)1145-1155 tional Science in China(90405015), National Elitist Youth [13] E Lamouroux, SBertrand,R. Pailler,et al,Compos. Sci.Technol (1999)1073-1085 Foundation in China(50425208)and the Doctorate Foundation [14]R. Naslain, Compos. Sci. Technol. 64(2004)155-170 of Northwestern Polytechnical University(CX200505) [15 T.. Rocket, W.R. Foster, J. Am. Ceram Soc. 48(1965)78-85
Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 177 oxidized for 600 min is higher than that of SiC/B/SiC coated composites only oxidized for 120 min. (4) The SiC/B/SiC coating could provide better oxidation protection for C/SiC composite than SiC/SiC/SiC coating between 700 ◦C and 1000 ◦C for 600 min due to the crack self-healing by B2O3 or B2O3·xSiO2 liquid. But above 1000 ◦C, the protection effect of the SiC/B/SiC coating for C/SiC composite decrease due to the extensive volatilization of B2O3 or B2O3·xSiO2 liquid. The protection effect of the SiC/B/SiC coating for C/SiC composite is worse than that of SiC/SiC/SiC coating above 1000 ◦C. Acknowledgments The authors would thank to the support of the Key Foundation of National Science in China (90405015), National Elitist Youth Foundation in China (50425208) and the Doctorate Foundation of Northwestern Polytechnical University (CX200505). References [1] R. Naslain, A. Guette, F. Rebillat, et al., J. Solid State Chem. 177 (2004) 449–456. [2] S. Schmidt, S. Beyer, H. Knabe, et al., Acta Astronaut. 55 (2004) 409–420. [3] W. Krenkel, B. Heidenreich, R. Renz, Adv. Eng. Mater. 4 (2002) 427–436. [4] J.R. Strife, J.E. Sheehan, Ceram. Bull. 67 (1988) 369–374. [5] L.F. Cheng, Y.D. Xu, L.T. Zhang, et al., J. Mater. Sci. 37 (2002) 5339–5344. [6] K. Kobayashi, K. Maeda, H. Sano, Y. Uchiyama, Carbon 33 (1995) 397–403. [7] L.F. Cheng, Y.D. Xu, L.T. Zhang, et al., Carbon 39 (2001) 1127–1133. [8] H.T. Tsou, W. Kowbel, Carbon 33 (1995) 1289–1292. [9] J. Schulte-Fischedick, J. Schmidt, R. Tamme, et al., Mater. Sci. Eng. A 386 (2004) 428–434. [10] C. Isola, P. Appendino, F. Bosco, et al., Carbon 36 (1998) 1213–1218. [11] R. Naslain, Ceram. Trans. 58 (1995) 23–39. [12] R. Naslain, Compos. Part A: Appls. 29A (1998) 1145–1155. [13] F. Lamouroux, S. Bertrand, R. Pailler, et al., Compos. Sci. Technol. 59 (1999) 1073–1085. [14] R. Naslain, Compos. Sci. Technol. 64 (2004) 155–170. [15] T.J. Rocket, W.R. Foster, J. Am. Ceram. Soc. 48 (1965) 78–85