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 forY. 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 hydro￾gen 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 fol￾lows: 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 coat￾ings. 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 sim￾ilar 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 par￾ticles (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 morpholo￾gies 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 oxida￾tion 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 coat￾ing 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