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 methyltri￾chlorosilane (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, Rig￾aku D/MAX-2400 with Cu Kr´ radiation). Surface and cross-section morphologies of the coating were observed using a scanning elec￾tron 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 exi￾dence 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 concentra￾tions 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 homoge￾nous 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 a￾BC 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 coat￾ings 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 coat￾ings 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 pres￾sure 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