L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 of the composites with a coating was investigated and prepared by liquid-reaction method at 1500C for 30 compared with that of a C/C composite with a coating min on the CVD Sic layer. n this paper Oxidation tests of the three coated materials were conducted in dry air for 5 h at different temperatures from400C-1500°C. 2. Experimental Three composites were used in experiments. The first 3. Modelling one, cut and machined off from aircraft brakes, was a 2D-CC. It has been prepared by CVI and treated at If the interfacial reactions produced by oxidation 2000C. It does not contain any oxidation inhibitor. could not be taken account below 1500C, weight The density of the C/C measured by Archimedes' change of coated C/C composites on oxidation is con- method is 1.7 g cm-3, and the total porosity is 15.5% sidered to be influenced by three factors:(1)oxidation (10% open; 5.5% closed). The substrates were 5 x 5x of substrates by diffusion of oxygen through coating 25 mm in size. The three-layer coating prepared on the cracks;(2) sealing of coating cracks when temperature substrates consisted of a conversion SiC layer, a barrier is raised;(3)oxidation of coating by diffusion of oxy- Sic layer and a sealant Si-W layer. The conversion Sic gen through the oxide film. Because a threshold of yer,which could improve the expansion mismatch of temperature and an activation energy were needed the substrates and the coating, was prepared by sili- weight changes produced by these three factors could conization on the surface of the substrates at 1500c be expressed by the following equations separately or 15 min. The dense SiC barrier layer, which was used to prevent infiltration of the outer layer, was prepared △Wc=Ac(1-exp(-BcTo) (1) on the transition layer by CVD at 1100oC for 30 min. AWs =As1-exp(-BsT"), be formed on, was prepared by liquid-reaction at AWF=A (1-exp(-"), l500°Cfor30min where A, B and n are constants, C, S and F represent The second one was a C/Sic composite prepared by y diffusion through cracks, sealing of cracks, diffusion sual pressure chemical vapor deposition method (AP through oxide film and interfacial reactions rest CVI). The carbon fiber was T-300TM. The preforms tively, and Aw is the weight change in percent. Weight vere densified with pyrolysis carbon (Pyc) and Sic change of coated C/C should be the sum of the weight from butane and methyltrichlorosilane(MTS/H2). The losses and weight gains produced by the three factors interfacial layer of Pyc was deposited for one hour at 870°C. The deposition conditions of Sic matrix were as△W=△W+△Ws+△W follow: temperature was 1100C, time was 2 h, flow of Besides these three factors weight change of coated H2 was about 150 mImin, and the molar ratio of H2 C/Sic composites is considered to be influenced by and mTS was 10. The substrates with a size of 4x6x sealing of Sic matrix cracks when temperature is raised 40 mm were cut from the fabricated composite and submitted to a deposition treatment for 2 h to seal the AWs=As(1-exp(- BsTs)), open ends of the fiber. A Si-W coating was prepared where S represent sealing of the matrix cracks. Weight by liquid-reaction at 1500C for 30 min on the Sic change of the coated C/SiC from room temperature to deposition process. The third one was a C/SiC composite prepared by 1500]C should be the sum of the weight losses and low-pressure chemical vapor deposition method (LP weight gains produced by the four factors CVI). The preforms were infiltrated with pyrolysis ca △W=ΔWc+ΔWs+ΔWs+△W Pyc) and Sic from butane methyltrichlorosilane(MTS/H,). The conditions for de- osition of PyC interlayer were as follow: temperature 4. Results and discusion 960C, pressure 5 KPa, time 20 h, Ar flow 200 ml- min- butane flow 15 ml- min- The conditions For the three composites, a continuous function of for deposition of Sic matrix were as follow: tempera eight change with temperature which fitted the test ture 1000oC, pressure 5 KPa, time 120 h, H2 flow 350 results quite well was obtained by selecting proper mI'min-, Ar flow 350 ml-min-, and the molar ratio constants in equations(Eqs. (1)-(3)and(5)through of H2 and MTS was 10. The substrates with a size of trial and error(Fig. 1), and the selected constants are 3 x5x40 mm were machined from the fabricated com- listed in Table l. It was shown that the present model posite with a size of 4 6x 150 mm and treated for 20 represented the different oxidation behavior of the h to deposit a CVD SiC layer. A Si-Zr coating was three composites over the full temperature range, and220 L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 of the composites with a coating was investigated and compared with that of a C/C composite with a coating in this paper. 2. Experimental Three composites were used in experiments. The first one, cut and machined off from aircraft brakes, was a 2D-C/C. It has been prepared by CVI and treated at 2000°C. It does not contain any oxidation inhibitor. The density of the C/C measured by Archimedes’ method is 1.7 g cm−3 , and the total porosity is 15.5% (10% open; 5.5% closed). The substrates were 5×5× 25 mm in size. The three-layer coating prepared on the substrates consisted of a conversion SiC layer, a barrier SiC layer and a sealant Si–W layer. The conversion SiC layer, which could improve the expansion mismatch of the substrates and the coating, was prepared by sili￾conization on the surface of the substrates at 1500°C for 15 min. The dense SiC barrier layer, which was used to prevent infiltration of the outer layer, was prepared on the transition layer by CVD at 1100°C for 30 min. The Si–W sealant layer, which a silica glass film could be formed on, was prepared by liquid-reaction at 1500°C for 30 min. The second one was a C/SiC composite prepared by usual pressure chemical vapor deposition method (AP￾CVI). The carbon fiber was T-300™. The preforms were densified with pyrolysis carbon (PyC) and SiC from butane and methyltrichlorosilane (MTS/H2). The interfacial layer of PyC was deposited for one hour at 870°C. The deposition conditions of SiC matrix were as follow: temperature was 1100°C, time was 2 h, flow of H2 was about 150 ml·min−1 , and the molar ratio of H2 and MTS was 10. The substrates with a size of 4×6× 40 mm were cut from the fabricated composite and submitted to a deposition treatment for 2 h to seal the open ends of the fiber. A Si–W coating was prepared by liquid-reaction at 1500°C for 30 min on the SiC layer formed in the deposition process. The third one was a C/SiC composite prepared by low-pressure chemical vapor deposition method (LP￾CVI). The preforms were infiltrated with pyrolysis car￾bon (PyC) and SiC from butane and methyltrichlorosilane (MTS/H2). The conditions for de￾position of PyC interlayer were as follow: temperature 960°C, pressure 5 KPa, time 20 h, Ar flow 200 ml·min−1 , butane flow 15 ml·min−1 . The conditions for deposition of SiC matrix were as follow: tempera￾ture 1000°C, pressure 5 KPa, time 120 h, H2 flow 350 ml·min−1 , Ar flow 350 ml·min−1 , and the molar ratio of H2 and MTS was 10. The substrates with a size of 3×5×40 mm were machined from the fabricated com￾posite with a size of 4×6×150 mm and treated for 20 h to deposit a CVD SiC layer. A Si–Zr coating was prepared by liquid-reaction method at 1500°C for 30 min on the CVD SiC layer. Oxidation tests of the three coated materials were conducted in dry air for 5 h at different temperatures from 400°C–1500°C. 3. Modelling If the interfacial reactions produced by oxidation could not be taken account below 1500°C, weight change of coated C/C composites on oxidation is con￾sidered to be influenced by three factors: (1) oxidation of substrates by diffusion of oxygen through coating cracks; (2) sealing of coating cracks when temperature is raised; (3) oxidation of coating by diffusion of oxy￾gen through the oxide film. Because a threshold of temperature and an activation energy were needed, weight changes produced by these three factors could be expressed by the following equations separately DWC=AC(1−exp(−BCTnC)), (1) DWS=AS(1−exp(−BSTnS )), (2) DWF=AF(1−exp(−BFTnF)), (3) where A, B and n are constants, C, S and F represent diffusion through cracks, sealing of cracks, diffusion through oxide film and interfacial reactions respec￾tively, and DW is the weight change in percent. Weight change of coated C/C should be the sum of the weight losses and weight gains produced by the three factors DW=DWC+DWS+DWF, (4) Besides these three factors, weight change of coated C/SiC composites is considered to be influenced by sealing of SiC matrix cracks when temperature is raised DWS%=AS% (1−exp(−BS% TnS% )), (5) where S% represent sealing of the matrix cracks. Weight change of the coated C/SiC from room temperature to 1500°C should be the sum of the weight losses and weight gains produced by the four factors. DW=DWC+DWS+DWS%+DWF. (6) 4. Results and discusion For the three composites, a continuous function of weight change with temperature which fitted the test results quite well was obtained by selecting proper constants in equations (Eqs. (1)–(3) and (5)) through trial and error (Fig. 1), and the selected constants are listed in Table 1. It was shown that the present model represented the different oxidation behavior of the three composites over the full temperature range, and