CARBON PERGAMON Carbon39(2001)1127-113 Effect of glass sealing on the oxidation behavior of three dimensional c/ Sic composites in air Laifei Cheng", Yongdong Xu, Litong Zhang, Rong Gao State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China Received 7 March 2000; accepted 27 May 2000 Abstract a borosilicate glass which consisted of 40-45 mol. B2O, and 55-60 mol. SiO, was used as a sealant for the C/SiC composites with and without a Si-Zr coating, and the oxidation behavior of the C/Sic composites in air was investigated The glass increased greatly the oxidation resistance by decreasing the maximum weight loss and the cracking temperature range below 1000oC as well as by lowering the transition temperature. The effective sealing temperature of the glass was from 700 to 900 C. The glass began losing its sealing ability very rapidly above 900C, and lost this ability completely above 1000.C. After oxidation for 20 h in an air atmosphere with a large temperature gradient from 350 to 1300., the strengths of the composite with both the Si-Zr coating and the glass sealant were maintained over the whole temperature range. The composite with the sealant only lost strength mostly at high temperature and that with only the coating lost almost all its strength at low temperature, and both of the composites could not be used in environments with a temperature gradient. The sealing temperature range of the glass was narrower than the cracking temperature range of the composite. No matter how the composition d, the borosilicate glass could not seal over the whole cracking temperature range of the composite at the same time for long-term use. 2001 Published by Elsevier Science Ltd Keywords: A Carbon composites; B Coating, Cracking: Oxidation 1. Introduction SiC matrix is unable to protect the fibers and the pyrocar bon interphase in C/Si posites with a coating from Carbon fiber reinforced silicon carbide composites(C/ oxidation over a wide temperature range. Although the Sic) have been developed and tested for high-temperature oxidation resistance of a coated C/SiC composite is better and long-time structural applications such as the com- than that of a coated C/C composite, sealants are neces- ponents of turbine engines and for high-temperature an sary to increase the resistance of C/SiC composites for limited-time structural applications such as the reentry long-term use. Because they react rapidly with the carbon thermal protection system of spacecraft. Various inhibitors fiber and matrix above 1000.C, glasses decrease the have been investigated to increase the resistance of C/c working temperature of C/C composites. The Sic matrix composites with a coating at low temperature [1-5]. As in C/SiC composites acts as a barrier layer between the inhibitors, glasses have been found more effective, appar- glasses and the pyrocarbon interlayer, they will not de- ently by blocking the reactive sites of carbon with oxygen crease the working temperature of C/Sic comp [6-8]. Three kinds of glass have been widely used as significantly. Consequently, the glass sealants play more sealants of C/c composites, but they have not been important roles in C/SiC composites than in C/C compos- eported as sealants of C/SiC composites up to now Because there is a large thermal expansion mismatch, the In general, P,O glasses can be effective but are limited to below 600C, while Sio, glasses do not have attractive *Corresponding author. Tel. +86-29-849-127, fax: +86-29. viscosity and wetting characteristics as sealants. Although 49-1000. B, O, glasses combine thermal stability with appropriat E-mailaddress:cmcres(@nwpu.edu.cn(L.Cheng) viscosity and wetting to provide protection over a wide 0008-6223/01/S- see front matter 2001 Published by Elsevier Science Ltd PII:S0008-6223(00)00148-2
PERGAMON Carbon 39 (2001) 1127–1133 Effect of glass sealing on the oxidation behavior of three dimensional C/SiC composites in air Laifei Cheng , Yongdong Xu, Litong Zhang, Rong Gao * State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Received 7 March 2000; accepted 27 May 2000 Abstract A borosilicate glass which consisted of 40–45 mol.% B O and 55–60 mol.% SiO was used as a sealant for the C/SiC 23 2 composites with and without a Si–Zr coating, and the oxidation behavior of the C/SiC composites in air was investigated. The glass increased greatly the oxidation resistance by decreasing the maximum weight loss and the cracking temperature range below 10008C as well as by lowering the transition temperature. The effective sealing temperature of the glass was from 700 to 9008C. The glass began losing its sealing ability very rapidly above 9008C, and lost this ability completely above 10008C. After oxidation for 20 h in an air atmosphere with a large temperature gradient from 350 to 13008C, the strengths of the composite with both the Si–Zr coating and the glass sealant were maintained over the whole temperature range. The composite with the sealant only lost strength mostly at high temperature and that with only the coating lost almost all its strength at low temperature, and both of the composites could not be used in environments with a temperature gradient. The sealing temperature range of the glass was narrower than the cracking temperature range of the composite. No matter how the composition was changed, the borosilicate glass could not seal over the whole cracking temperature range of the composite at the same time for long-term use. 2001 Published by Elsevier Science Ltd. Keywords: A. Carbon composites; B. Coating; Cracking; Oxidation 1. Introduction SiC matrix is unable to protect the fibers and the pyrocarbon interphase in C/SiC composites with a coating from Carbon fiber reinforced silicon carbide composites (C/ oxidation over a wide temperature range. Although the SiC) have been developed and tested for high-temperature oxidation resistance of a coated C/SiC composite is better and long-time structural applications such as the com- than that of a coated C/C composite, sealants are necesponents of turbine engines and for high-temperature and sary to increase the resistance of C/SiC composites for limited-time structural applications such as the reentry long-term use. Because they react rapidly with the carbon thermal protection system of spacecraft. Various inhibitors fiber and matrix above 10008C, glasses decrease the have been investigated to increase the resistance of C/C working temperature of C/C composites. The SiC matrix composites with a coating at low temperature [1–5]. As in C/SiC composites acts as a barrier layer between the inhibitors, glasses have been found more effective, appar- glasses and the pyrocarbon interlayer, they will not deently by blocking the reactive sites of carbon with oxygen crease the working temperature of C/SiC composites [6–8]. Three kinds of glass have been widely used as significantly. Consequently, the glass sealants play more sealants of C/C composites, but they have not been important roles in C/SiC composites than in C/C composreported as sealants of C/SiC composites up to now. ites. Because there is a large thermal expansion mismatch, the In general, P O glasses can be effective but are limited 2 5 to below 6008C, while SiO glasses do not have attractive 2 *Corresponding author. Tel.: 186-29-849-127; fax: 186-29- viscosity and wetting characteristics as sealants. Although 849-1000. B O glasses combine thermal stability with appropriate 2 3 E-mail address: cmcres@nwpu.edu.cn (L. Cheng). viscosity and wetting to provide protection over a wide 0008-6223/01/$ – see front matter 2001 Published by Elsevier Science Ltd. PII: S0008-6223(00)00148-2
L. Cheng et al. Carbon 39(2001)1127-1133 temperature range both as coatings and as sealants, they After preparing the coating, the specimens were treated volatilize rapidly at high temperatures, and furthermore with the sealant. The sealant was a borosilicate glass that tend to adsorb water in air. The volatility of B, O, glasses consisted of 55-60 mol. SiO, and 40-45 mol. B,O3 can be improved by adding Sio, [9. In this paper, a The specimens were infiltrated by Sio, and B, O, separ- borosilicate glass was employed as a sealant, and the effect ately at room temperature, and then the glass was formed of the glass on the oxidation behavior of a three-dimen after sintering for I h at 1000.C in a vacuum furnace sional C/SiC composite was investigated 2.2 Oxidation tests 2. Experimental procedure The oxidation tests for the Imens 2.I. Fabrication of the specimens atmosphere with a temperature gradient were conducted in a tube furnace(Fig. 1). Two thermal couples were employed in the tube, one for measuring the temperature T-300 carbon fiber from Japan Toray was employed The fiber preforms were prepared by three-dimensional gradient and the other for controlling the heating tempera- braid method. The dimension of the preforms was 5 the center of the tube, and the other end was outside the mmX6 mmX150 mm. The volume fraction of fibers was controlled in the range from 40 to 45%. The preforms were tube. A large temperature gradient from 350 to 1300C was formed along the specimens. The oxidation tests for the deposited with a pyrolysis carbon (Pyc) and Sic by short specimens in dry air at different temperatures were w-pressure chemical vapor deposition (LPCVI). The onducted in a box furnace interfacial layer of Pyc was deposited for 20 h at 870C and 5 kPa. The deposition conditions of the Sic matrix were as follows: deposition temperature 1100C,pressure 3. Measurements of the composite 5 kPa, time 20 h, flow of H, about 350 ml min, and the molar ratio of H, and mts (methyltrichlorosilane)10 After oxidation for 20 h in air atmosphere with a Two kinds of specimens were machined from the gradient temperature, the flexural strengths along the prepared composite. The long specimens with a dimension pecimens were measured by the three-point bending of 4 mmx5 mmx 140 mm were used to investigate the method with a span of 20 m g each specimen,a oxidation resistance of the composite with the glass over strength distribution with the distance from the one end the whole temperature range, and the short specimens with were obtained. After oxidation at different temperatures in dimension of4mm×smm×40 mm were used to air atmosphere with a constant temperature, the flexural strengths were measured by the three-point bending meth- the glass at different temperatures. Both the long speci- od with a span of 30 mm, and weight changes were mens and the short specimens were deposited for 20 h to measured prepare CVD SiC coating after machining. Density of the composite varied from 1.93 to 2.01 g/cm. Taking account of the Sic coating formed on the composite surface in the 3. Results and discussion CVD process, the density was nominal. A Si-Zr coating was prepared by liquid-reaction at 1500C for 30 min on According to the experimental results of oxidation the Sic layer of the long specimens different temperatures, a relation of weight change with 1300°C Specimen Fig. 1. Schematic of oxidation method in air with a temperature gradient
1128 L. Cheng et al. / Carbon 39 (2001) 1127 –1133 temperature range both as coatings and as sealants, they After preparing the coating, the specimens were treated volatilize rapidly at high temperatures, and furthermore with the sealant. The sealant was a borosilicate glass that tend to adsorb water in air. The volatility of B O glasses consisted of 55–60 mol.% SiO and 40–45 mol.% B O . 23 2 23 can be improved by adding SiO [9]. In this paper, a The specimens were infiltrated by SiO and B O separ- 2 2 23 borosilicate glass was employed as a sealant, and the effect ately at room temperature, and then the glass was formed of the glass on the oxidation behavior of a three-dimen- after sintering for 1 h at 10008C in a vacuum furnace. sional C/SiC composite was investigated. 2.2. Oxidation tests 2. Experimental procedure The oxidation tests for the long specimens in dry air atmosphere with a temperature gradient were conducted in 2.1. Fabrication of the specimens a tube furnace (Fig. 1). Two thermal couples were employed in the tube, one for measuring the temperature T-300E carbon fiber from Japan Toray was employed. gradient and the other for controlling the heating tempera- The fiber preforms were prepared by three-dimensional ture which was 13008C. One end of the specimens was at braid method. The dimension of the preforms was 5 the center of the tube, and the other end was outside the mm36 mm3150 mm. The volume fraction of fibers was tube. A large temperature gradient from 350 to 13008C was controlled in the range from 40 to 45%. The preforms were formed along the specimens. The oxidation tests for the deposited with a pyrolysis carbon (PyC) and SiC by short specimens in dry air at different temperatures were low-pressure chemical vapor deposition (LPCVI). The conducted in a box furnace. interfacial layer of PyC was deposited for 20 h at 8708C and 5 kPa. The deposition conditions of the SiC matrix 2.3. Measurements of the composite were as follows: deposition temperature 11008C, pressure 21 5 kPa, time 20 h, flow of H about 350 ml min , and the 2 molar ratio of H and MTS (methyltrichlorosilane) 10. After oxidation for 20 h in air atmosphere with a 2 gradient temperature, the flexural strengths along the Two kinds of specimens were machined from the specimens were measured by the three-point bending prepared composite. The long specimens with a dimension method with a span of 20 mm. Along each specimen, a of 4 mm35 mm3140 mm were used to investigate the strength distribution with the distance from the one end oxidation resistance of the composite with the glass over were obtained. After oxidation at different temperatures in the whole temperature range, and the short specimens with air atmosphere with a constant temperature, the flexural a dimension of 4 mm35 mm340 mm were used to strengths were measured by the three-point bending meth- investigate the oxidation behavior of the composite with od with a span of 30 mm, and weight changes were the glass at different temperatures. Both the long specimens and the short specimens were deposited for 20 h to measured. prepare CVD SiC coating after machining. Density of the 3 composite varied from 1.93 to 2.01 g/cm . Taking account of the SiC coating formed on the composite surface in the 3. Results and discussion CVD process, the density was nominal. A Si–Zr coating was prepared by liquid-reaction at 15008C for 30 min on According to the experimental results of oxidation in air the SiC layer of the long specimens. at different temperatures, a relation of weight change with Fig. 1. Schematic of oxidation method in air with a temperature gradient
L. Cheng et al. Carbon 39(2001)1127-1133 1505 C/Sic -e-C/SiC+Sealant 0 1525.354 1600 Temperature(°C) Fig. 2. Weight changes of the C/SiC and C/SiC+sealant after oxidation at different temperatures for 5h. temperature was obtained(Fig. 2). It could be clearly seen of oxygen in B, O, was much faster than in silica. The that larger the content of B, O, in the glass, the faster the diffusion of oxygen. with increasing temperature, oxygen 1. the maximum weight loss of the sealed specimens was diffusion became faster and above 1000C, the oxygen greatly decreased from about 3.5 to 1% diffusion was so fast that the glass had little resistance to 2. the transition temperature, which was defined as the oxidation. Therefore, the glass had poor sealing ability temperature at which the weight loss reached its above 1000C due to its low viscosity and fast diffusion of maximum value, was lowered from 700 to 600C 3. the cracking temperature range, which was defined as From 700 to 1000C, the diffusion through the matrix the temperature range from the threshold temperature of cracks was the controlling step in oxidation because the oxidation to the cracking temperature, was n pores had a much larger size than the cracks in the from600to300° unsealed composite. The favorable wettability of the glass 4. above 0o C, the weight losses of the sealed speci- on the Sic allowed it to fill the matrix pores(Fig. 3a), the mens were nearly the same with those of the unsealed matrix cracks( Fig. 3b)and the fiber filaments the sealed composite, thus the oxidation was controlled by the oxygen diffusion through both the glass and the cracks For the unsealed C/SiC, the weight loss varied little in the same temperature range. Because the diffusion route from 1000 to 1300C. This indicated that the cracking was increased, the weight loss was greatly decreased. In temperatures of the matrix and the coating were 1000C other words, the self-sealing temperature of the matrix in which was the same as their deposition temperatures. The the unsealed composite was 1000oC, and that in the sealed composite still lost weight above 1000.C because there composite was lowered to be 700C or more. For the were preparation defects in the Cvd Sic coating on the sealed composite, cracks were produced in the glass below pecimens, which acted as the diffusion channels for 700oC. From 600 to 700oC, the oxidation was controlled oxygen. Below 1000C, the weight loss of the composite by the oxygen diffusion through the glass cracks. From as related to the oxygen diffusion through the matrix 400 to 600C, the oxidation was controlled by the reaction cracks, but not to the oxygen diffusion though the defects. of oxygen with carbon. The activation energy was calcu- When the matrix cracks were sealed above 1000C. the lated to be 24 kcal/mol. For the unsealed composite weight loss of the composite was controlled by the defects oxidation was controlled by the reaction of oxygen with The defects led to irregular weight changes with tempera- carbon below 700C. The activation energy was calculated ture above 1000°C, especially from1300°Cto1500°C. to be 28 kcal/mol (Fig 4) The weight losses of the sealed and unsealed specimens After oxidation for 10 h at different temperatur ere nearly the same between 1000"C and 1300.C. In ariation of flexural strength with temperature for the order to seal the cracks and pores on the matrix better unsealed and sealed C/Sic composite were shown in Fi elow 1000C, the glass was designed to have a lower 5. It can be seen that the strengths changed with tempera- viscosity. With increasing temperature, the viscosity was ture in a similar manner to those of the weight changes. It decreased and above 1000C, the viscosity was so low that was indicated that the glass had the same effect on the the glass was unable to seal the defects well. The diffusion strengths as on the weight changes
L. Cheng et al. / Carbon 39 (2001) 1127 –1133 1129 Fig. 2. Weight changes of the C/SiC and C/SiC1sealant after oxidation at different temperatures for 5 h. temperature was obtained (Fig. 2). It could be clearly seen of oxygen in B O was much faster than in silica. The 2 3 that larger the content of B O in the glass, the faster the 2 3 diffusion of oxygen. With increasing temperature, oxygen 1. the maximum weight loss of the sealed specimens was diffusion became faster and above 10008C, the oxygen greatly decreased from about 3.5 to 1%; diffusion was so fast that the glass had little resistance to 2. the transition temperature, which was defined as the oxidation. Therefore, the glass had poor sealing ability temperature at which the weight loss reached its above 10008C due to its low viscosity and fast diffusion of maximum value, was lowered from 700 to 6008C; oxygen. 3. the cracking temperature range, which was defined as From 700 to 10008C, the diffusion through the matrix the temperature range from the threshold temperature of cracks was the controlling step in oxidation because the oxidation to the cracking temperature, was narrowed pores had a much larger size than the cracks in the from 600 to 3008C; unsealed composite. The favorable wettability of the glass 4. above 10008C, the weight losses of the sealed speci- on the SiC allowed it to fill the matrix pores (Fig. 3a), the mens were nearly the same with those of the unsealed matrix cracks (Fig. 3b) and the fiber filaments (Fig. 3c) in specimens. the sealed composite, thus the oxidation was controlled by the oxygen diffusion through both the glass and the cracks For the unsealed C/SiC, the weight loss varied little in the same temperature range. Because the diffusion route from 1000 to 13008C. This indicated that the cracking was increased, the weight loss was greatly decreased. In temperatures of the matrix and the coating were 10008C other words, the self-sealing temperature of the matrix in which was the same as their deposition temperatures. The the unsealed composite was 10008C, and that in the sealed composite still lost weight above 10008C because there composite was lowered to be 7008C or more. For the were preparation defects in the CVD SiC coating on the sealed composite, cracks were produced in the glass below specimens, which acted as the diffusion channels for 7008C. From 600 to 7008C, the oxidation was controlled oxygen. Below 10008C, the weight loss of the composite by the oxygen diffusion through the glass cracks. From was related to the oxygen diffusion through the matrix 400 to 6008C, the oxidation was controlled by the reaction cracks, but not to the oxygen diffusion though the defects. of oxygen with carbon. The activation energy was calcuWhen the matrix cracks were sealed above 10008C, the lated to be 24 kcal/mol. For the unsealed composite, weight loss of the composite was controlled by the defects. oxidation was controlled by the reaction of oxygen with The defects led to irregular weight changes with tempera- carbon below 7008C. The activation energy was calculated ture above 10008C, especially from 13008C to 15008C. to be 28 kcal/mol (Fig. 4). The weight losses of the sealed and unsealed specimens After oxidation for 10 h at different temperatures, the were nearly the same between 10008C and 13008C. In variation of flexural strength with temperature for the order to seal the cracks and pores on the matrix better unsealed and sealed C/SiC composite were shown in Fig. below 10008C, the glass was designed to have a lower 5. It can be seen that the strengths changed with temperaviscosity. With increasing temperature, the viscosity was ture in a similar manner to those of the weight changes. It decreased and above 10008C, the viscosity was so low that was indicated that the glass had the same effect on the the glass was unable to seal the defects well. The diffusion strengths as on the weight changes
130 L. Cheng et al. Carbon 39(2001)1127-1133 938828KU X288188NmWD22 (a) 339528K0×2,891NmWD22 939128KV1,5881mWD22 Fig. 3. SEM micrographs of the glass filled in(a) the matrix pores, (b)the matrix cracks and (c) the fiber filaments ln△W-13711/+16.318 81 80 ln△W=11542/T+13.319 10T(K Fig. 4. Arrhenius relation of oxidation below 700"C for the unsealed and sealed C/SiC composite
1130 L. Cheng et al. / Carbon 39 (2001) 1127 –1133 Fig. 3. SEM micrographs of the glass filled in (a) the matrix pores, (b) the matrix cracks and (c) the fiber filaments. Fig. 4. Arrhenius relation of oxidation below 7008C for the unsealed and sealed C/SiC composite
L. Cheng et al. Carbon 39(2001)1127-1133 1131 1200 1000 C+Sealant 600 200 0 0 400 800 1200 Temperature(°C) Fig. 5. Flexural changes of the C/SiC and C/SiC+sealant after oxidation at different temperatures for 5 h. From the above discussion, it is evident that the sealed glass sealant and the si-zr coating after oxidation for 20 h composite had an excellent oxidation resistance at low in air atmosphere with a large temperature gradient. It temperature for 5 h. The glass increased the oxidation could be seen that the strengths were maintained over the resistance by greatly decreasing the maximum weight loss whole temperature range. The composite was protected by and the cracking temperature range as well as by lowering the coating at high temperature and by the sealant at low the transition temperature, although it could not change the temperature. It was of interest to note that there were two activation energy. The danger temperature range was from peaks on the strength distribution along the specimen. The the threshold temperature of oxidation to the cracking right peak was governed by the coating protection and the temperature. At the danger temperature, the weight loss ft one governed by the sealant resistance. The glass was reached its maximum value volatilized rapidly in the oxidation process above 1300.C The poor sealing ability of the glass above 1000.C could and at the same time reactions took place between the be confirmed by the results of the strength changes along glass and the oxide film formed on the si-zr coating the long specimens after oxidation in air atmosphere with a surface. After oxidation for 20 h, the glass no longer large temperature gradient for 20 h(Fig. 6). Although the remained on the specimen surface. Because of the coating rengths below 1000.C decreased little due to the excel- protection against oxidation, the strength was maintaine ent sealing ability of the glass, they decreased greatly at ear to the high-temperature end although it was decreased high temperature. Therefore, the composite with the glass slightly. Below 700C, the viscosity was so high that sealant could not be applied in the environments with a cracks were produced in the glass, and more serious large temperature gradient that was unfortunately unavoid- oxidation took place in the composite. As a result, the able in the actual applications composite lost more strength near to the low-temperature Besides the sealants, coatings were necessary for the end. There was a transition zone between the two peaks of oxidation protection of the C/SiC composite. Fig. 7 shows trength that was the boundary of two areas governed the strength changes along the long specimens with the separately by the sealant and the coating. The transition 1200 1400 1000 600 600 400 Strength 400 200 Temperature 0 0 10 Distance from the specimen end (mm) Fig. 6. Effect of temperature gradient on the flexural strength of the C/Sic with glass sealing
L. Cheng et al. / Carbon 39 (2001) 1127 –1133 1131 Fig. 5. Flexural changes of the C/SiC and C/SiC1sealant after oxidation at different temperatures for 5 h. From the above discussion, it is evident that the sealed glass sealant and the Si–Zr coating after oxidation for 20 h composite had an excellent oxidation resistance at low in air atmosphere with a large temperature gradient. It temperature for 5 h. The glass increased the oxidation could be seen that the strengths were maintained over the resistance by greatly decreasing the maximum weight loss whole temperature range. The composite was protected by and the cracking temperature range as well as by lowering the coating at high temperature and by the sealant at low the transition temperature, although it could not change the temperature. It was of interest to note that there were two activation energy. The danger temperature range was from peaks on the strength distribution along the specimen. The the threshold temperature of oxidation to the cracking right peak was governed by the coating protection and the temperature. At the danger temperature, the weight loss left one governed by the sealant resistance. The glass was reached its maximum value. volatilized rapidly in the oxidation process above 13008C, The poor sealing ability of the glass above 10008C could and at the same time reactions took place between the be confirmed by the results of the strength changes along glass and the oxide film formed on the Si–Zr coating the long specimens after oxidation in air atmosphere with a surface. After oxidation for 20 h, the glass no longer large temperature gradient for 20 h (Fig. 6). Although the remained on the specimen surface. Because of the coating strengths below 10008C decreased little due to the excel- protection against oxidation, the strength was maintained lent sealing ability of the glass, they decreased greatly at near to the high-temperature end although it was decreased high temperature. Therefore, the composite with the glass slightly. Below 7008C, the viscosity was so high that sealant could not be applied in the environments with a cracks were produced in the glass, and more serious large temperature gradient that was unfortunately unavoid- oxidation took place in the composite. As a result, the able in the actual applications. composite lost more strength near to the low-temperature Besides the sealants, coatings were necessary for the end. There was a transition zone between the two peaks of oxidation protection of the C/SiC composite. Fig. 7 shows strength that was the boundary of two areas governed the strength changes along the long specimens with the separately by the sealant and the coating. The transition Fig. 6. Effect of temperature gradient on the flexural strength of the C/SiC with glass sealing
L. Cheng et al. Carbon 39(2001)1127-1133 150 Distance from the specimen end(mm) Fig. 7. Effect of temperature gradient on the flexural strength of the C/Sic with glass sealing and a Si-Zr coating. zone was formed by the oxidation at the boundary which capacity of all the three composites was the same as each place because the sealing ability of the glass was poor other. Because the coating could not bear load and had a e 900C and the coating begun losing oxidation thickness of about 150 um, the strength was decreased protection ability below the cracking temperature which after preparing the coating. The composite with only the was 1000.C. Obviously, the transition zone was formed at coating was more dangerous than that with only the the area where the temperature was in the range from 900 sealant. Either the sealant or the coating was needed for the to 1000C. With increasing the oxidation time, the transi- oxidation protection of the C/Sic composite over the tion zone moved toward the high-temperature area because whole temperature range. Although the oxidation protec. the glass in this area lost sealing ability at high tempera- tion property was greatly increased by combining the tures and gained sealing ability at low temperatures. After sealant and the coating, new problems were introduced oxidation for 20 h, the transition zone was in the region when the sealant was present. Firstly, there was volatilize- where the temperature was about 1200C tion of the glass and the higher the temperature, the more e The composite with only sealant could not be used at serious the volatilization. Certainly, repairing technology gh temperature and that with only coating could not be of the glass film could be used to pro used at low temperature, and then both of the composites protection time. Secondly, the sealing temperature range of could not be used in the environments with temperature the glass was too narrow because the viscosity of the glass gradient(Fig. 8). It was known that the strengths of the decreased too rapidly with increasing temperature. Whe composite with the sealant was much higher than those of increasing the content of B, O, in the glass, the viscosity the composite with the coating or with both the coating was lowered and the cracking zone was increased as the and sealant before oxidation. In fact, the load-bearing transition zone was decreased. When decreasing the con- 1200 800 0000oo00 0 16 Distance from the specimen end (mm) Fig. 8. Effect of temperature gradient on the flexural strengths of the C/Sic (a) with coating, (b)with sealant and (c)with sealant and
1132 L. Cheng et al. / Carbon 39 (2001) 1127 –1133 Fig. 7. Effect of temperature gradient on the flexural strength of the C/SiC with glass sealing and a Si–Zr coating. zone was formed by the oxidation at the boundary which capacity of all the three composites was the same as each took place because the sealing ability of the glass was poor other. Because the coating could not bear load and had a above 9008C and the coating begun losing oxidation thickness of about 150 mm, the strength was decreased protection ability below the cracking temperature which after preparing the coating. The composite with only the was 10008C. Obviously, the transition zone was formed at coating was more dangerous than that with only the the area where the temperature was in the range from 900 sealant. Either the sealant or the coating was needed for the to 10008C. With increasing the oxidation time, the transi- oxidation protection of the C/SiC composite over the tion zone moved toward the high-temperature area because whole temperature range. Although the oxidation protecthe glass in this area lost sealing ability at high tempera- tion property was greatly increased by combining the tures and gained sealing ability at low temperatures. After sealant and the coating, new problems were introduced oxidation for 20 h, the transition zone was in the region when the sealant was present. Firstly, there was volatilizawhere the temperature was about 12008C. tion of the glass and the higher the temperature, the more The composite with only sealant could not be used at serious the volatilization. Certainly, repairing technology high temperature and that with only coating could not be of the glass film could be used to prolong the oxidation used at low temperature, and then both of the composites protection time. Secondly, the sealing temperature range of could not be used in the environments with temperature the glass was too narrow because the viscosity of the glass gradient (Fig. 8). It was known that the strengths of the decreased too rapidly with increasing temperature. When composite with the sealant was much higher than those of increasing the content of B O in the glass, the viscosity 2 3 the composite with the coating or with both the coating was lowered and the cracking zone was increased as the and sealant before oxidation. In fact, the load-bearing transition zone was decreased. When decreasing the conFig. 8. Effect of temperature gradient on the flexural strengths of the C/SiC (a) with coating, (b) with sealant and (c) with sealant and coating
L. Cheng et al. Carbon 39(2001)1127-1133 tent of B, O, in the glass, the viscosity was lowered and composite. There was still a cracking zone below 700C the transition zone was increased as the cracking zone was and a transition zone from 900 to 1000C where the decreased. Therefore, the sealing temperature range of the ating lost its protection ability as the glass lost its present glass was always narrower than the cracking sealing ability. No matter how its composition was temperature range of the composite. It is clear that to be ar changed, the borosilicate glass could not seal the tw ideal sealant, the viscosity of the glass must be appropriate zones at the same time for long-term use. and must change as slowly as possible with increasing temperature. If the cracking temperatures of the matrix and the coating could be lowered to below 900%C, however, a Acknowledgements glass with a lower viscosity than that used in this work could seal the composite well over the whole temperature The authors acknowledge the National Foundation for natural No. 59772023 and the Chinese ; of the Chinese under Contract Foundation Sciences under Contract No. 99J12.5.2 4. Conclusions References ld not change the activation energy of the C/Sic composite be [1] LoBiondo n. Halogenated glass systems for the protection of 1000C, it increased greatly the oxidation resistance by arbon-carbon decreasing the maximum weight loss and 1995:33(4):499-508 range as well as by lower the danger 2] Kobayashi K. Formation and oxidation resistance of the oating formed on carbon material composed of B,C-SiC 2. The effective sealing temperature of the glass was from powders. Carbon 1995: 33(4):397-403 700 to 900 C. The glass begun losing sealing ability B3] Cameron G, Jiames E Oxidation and hydrolytic stability of very rapidly from 900C, and had this ability no longer above1000°C resistance of diffusion of oxygen as well as rapid volatilization. 1995:33(4):389-95 3. After oxidation for 20 h in air atmosphere with a 4 McKee D. Oxidation behavior and protection of carbon- carbon composites. Carbon 1987, 25(4): 551-7 temperature gradient from 350 to 1300.C, the strengths [] William C. Synthesis and characterization of boron-doped of the composite with both the Si-Zr coating and the carbons. Carbon 1995: 33(4): 367-74 glass sealant were maintained over the whole tempera- [6] McKee D. Borate treatment of carbon fibers and carbon/ ure range because it was protected by the coating at carbon composites Carbon 1988: 26(2 high temperature and was resisted by the sealant at low [7] McKee D rate treatment of carbon fibers and carbon/ temperature. The composite with only the sealant lost carbon composites for improved oxidation resistance. Carbon strength mostly at high temperature and that with only 198624(6:737-4 the coating lost almost all its strength at low tempera- [8]Buchanan F, Little J Particulate-containing glass sealants for ture, and both of the composites could not be used carbon-carbon composites Carbon 1995, 33(4): 481-97. the environments with temperature gradient. [9] Strife J, Sheehan J. Ceramic coatings for carbon-carbon composites. Ceram Bull 1988: 67(2): 369-74 4. The sealing temperature range of the glass w rower rature range of the
L. Cheng et al. / Carbon 39 (2001) 1127 –1133 1133 tent of B O in the glass, the viscosity was lowered and composite. There was still a cracking zone below 700 2 3 8C the transition zone was increased as the cracking zone was and a transition zone from 900 to 10008C where the decreased. Therefore, the sealing temperature range of the coating lost its protection ability as the glass lost its present glass was always narrower than the cracking sealing ability. No matter how its composition was temperature range of the composite. It is clear that to be an changed, the borosilicate glass could not seal the two ideal sealant, the viscosity of the glass must be appropriate zones at the same time for long-term use. and must change as slowly as possible with increasing temperature. If the cracking temperatures of the matrix and the coating could be lowered to below 9008C, however, a Acknowledgements glass with a lower viscosity than that used in this work could seal the composite well over the whole temperature The authors acknowledge the support of the Chinese range. National Foundation for Natural Sciences under Contract No. 59772023 and the Chinese Defense Foundation for Sciences under Contract No. 99J12.5.2. 4. Conclusions References 1. Although the borosilicate glass could not change the activation energy of the C/SiC composite below [1] LoBiondo N. Halogenated glass systems for the protection of 10008C, it increased greatly the oxidation resistance by structural carbon–carbon composites. Carbon decreasing the maximum weight loss and the danger 1995;33(4):499–508. temperature range as well as by lowering the danger [2] Kobayashi K. Formation and oxidation resistance of the temperature. coating formed on carbon material composed of B C–SiC 4 2. The effective sealing temperature of the glass was from powders. Carbon 1995;33(4):397–403. [3] Cameron G, Jiames E. Oxidation and hydrolytic stability of 700 to 9008C. The glass begun losing sealing ability boron nitride — a new approach to improving the oxidation very rapidly from 9008C, and had this ability no longer resistance of carbonaceous structures. Carbon above 10008C because it had low viscosity and fast 1995;33(4):389–95. diffusion of oxygen as well as rapid volatilization. [4] McKee D. Oxidation behavior and protection of carbon– 3. After oxidation for 20 h in air atmosphere with a carbon composites. Carbon 1987;25(4):551–7. temperature gradient from 350 to 13008C, the strengths [5] William C. Synthesis and characterization of boron-doped of the composite with both the Si–Zr coating and the carbons. Carbon 1995;33(4):367–74. glass sealant were maintained over the whole tempera- [6] McKee D. Borate treatment of carbon fibers and carbon/ ture range because it was protected by the coating at carbon composites. Carbon 1988;26(2):551–7. high temperature and was resisted by the sealant at low [7] McKee D. Borate treatment of carbon fibers and carbon/ carbon composites for improved oxidation resistance. Carbon temperature. The composite with only the sealant lost 1986;24(6):737–41. strength mostly at high temperature and that with only [8] Buchanan F, Little J. Particulate-containing glass sealants for the coating lost almost all its strength at low tempera- carbon–carbon composites. Carbon 1995;33(4):481–97. ture, and both of the composites could not be used in [9] Strife J, Sheehan J. Ceramic coatings for carbon–carbon the environments with temperature gradient. composites. Ceram Bull 1988;67(2):369–74. 4. The sealing temperature range of the glass was narrower than the cracking temperature range of the