CARBON PERGAMON Carbon40(2002)877-882 Corrosion of a 3D-C/ SiC composite in salt vapor environments Laifei Cheng. yongdong xu. litong zhang. Xingang luan State Key laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an Shaanxi 710072, PR China Received 22 May 2001; accepted 27 July 2001 Abstract temperatures from 1000 to 1500%C. The degradation mechanisms for the C/SiC composite could be determined by dividing at complicated function describing the weight change of the composite with temperature into several monotone functions describing the mechanisms, and then identified by the strength change of the composite with temperature. There were three reactions between the C/Sic composite and the Na, SO, vapor. The first one was the passive oxidation of the CND SiC, leading a small weight gain. The second one was the oxidation of the carbon phases, leading a small weight loss. The third one was the active oxidation of the CVD SiC, leading a large weight loss. The threshold temperature for these reactions was, espectively, 1080, 1 100 and 1300.C. The transition temperature from passive to active was 1200C. The activation energy .Dectively, calculated by the weight change with temperature to be 114, 105 and 112 kcal/mol The flexural strength loss of the composite reached its minimum value when the weight gain of the composite reached its maximum value at 1200C, Below 1200C, the C/Sic composite had a higher corrosion resistance to the Na, SO, vapor Above 1300C, the poor corrosion resistance of the CVd SiC made the composite having a poor corrosion resistance to the Na2 SO4 vapor. C 2002 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon composites; B Oxidation; D. Activation energy 1. Introduction to increasing properties of C/Sic composite against the combustion gas environments that either the oxidation Carbon fiber reinforced silicon carbide composite(C resistance or the corrosion resistance controls the degr C)is one of the most promising structural materials for dation process. The third one is the difference between the high-temperature applications. Besides oxidation resist- corrosion behavior of the C/SiC composite and that of the of the composite that shoe the another important property Sic ceramics. It is well known that the carbon phases in ald be taken into consideration the C/SiC composite are very easy to react with oxygen, for long-time use such as components for high thrust/ but it is not clear whether they are also easy to react with weight jet engines because the combustion gas contains salts For answering these questions, it is very necessary to salts [1, 2]. Three problems are mainly concerned with the investigate the corrosion behavior of a C/SiC composite in corrosion resistance of the composite. The first one is the Na, SO, vapor environments with no oxygen at tempera- corrosion behavior of the composite at elevated tempera tures from 1000 to 1500C ture up to 1500.C. At low temperatures, a liquid film of As ceramics. the hot corrosion behavior of SiC and alts will formed on the surface of the composite in the gas si, n has been widely studied for a long time 3-10 due to deposition. At high temperatures, salts in the gas However, this is not enough to understand the hot corro- ion behavior of a C/SiC composite because the composite has different corrosion behavior in different temperature contains fibers as well as interphases besides the SiC ranges. The second one is the cooperation of the oxidation matrix and the Sic ceramics are not prepared by CVd resistance and the corrosion resistance. It is very important Recently, some investigations have been conducted on the hot corrosion behavior of C/SiC and SiC/SiC composite ponding author. Tel:+86-29-849-4616 fax: 86-29. [11, 12]. Hot corrosion of the ceramics and the composites was mostly researched by exposure to thin films of salts E-mailaddress.chenglf@nwpu.edu.cn(LCheng rather than to vapors of salts in environments with oxygen 0008-6223/02/S-see front matter 2002 Elsevier Science Ltd. All rights reserved PII:S0008-6223(01)00203-2
Carbon 40 (2002) 877–882 Corrosion of a 3D-C/SiC composite in salt vapor environments Laifei Cheng , Yongdong Xu, Litong Zhang, Xingang Luan * State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi9an Shaanxi 710072, PR China Received 22 May 2001; accepted 27 July 2001 Abstract Corrosion behavior of the three-dimensional C/SiC composite was investigated in a Na SO vapor environment at 2 4 temperatures from 1000 to 15008C. The degradation mechanisms for the C/SiC composite could be determined by dividing a complicated function describing the weight change of the composite with temperature into several monotone functions describing the mechanisms, and then identified by the strength change of the composite with temperature. There were three reactions between the C/SiC composite and the Na SO vapor. The first one was the passive oxidation of the CND SiC, 2 4 leading a small weight gain. The second one was the oxidation of the carbon phases, leading a small weight loss. The third one was the active oxidation of the CVD SiC, leading a large weight loss. The threshold temperature for these reactions was, respectively, 1080, 1100 and 13008C. The transition temperature from passive to active was 12008C. The activation energy for these reactions was, respectively, calculated by the weight change with temperature to be 114, 105 and 112 kcal/mol. The flexural strength loss of the composite reached its minimum value when the weight gain of the composite reached its maximum value at 12008C. Below 12008C, the C/SiC composite had a higher corrosion resistance to the Na SO vapor. 2 4 Above 13008C, the poor corrosion resistance of the CVD SiC made the composite having a poor corrosion resistance to the Na SO vapor. 2002 Elsevier Science Ltd. All rights reserved. 2 4 Keywords: A. Carbon composites; B. Oxidation; D. Activation energy 1. Introduction to increasing properties of C/SiC composite against the combustion gas environments that either the oxidation Carbon fiber reinforced silicon carbide composite (C/ resistance or the corrosion resistance controls the degraSiC) is one of the most promising structural materials for dation process. The third one is the difference between the high-temperature applications. Besides oxidation resist- corrosion behavior of the C/SiC composite and that of the ance, corrosion resistance is the another important property SiC ceramics. It is well known that the carbon phases in of the composite that should be taken into consideration the C/SiC composite are very easy to react with oxygen, for long-time use such as components for high thrust/ but it is not clear whether they are also easy to react with weight jet engines because the combustion gas contains salts. For answering these questions, it is very necessary to salts [1,2]. Three problems are mainly concerned with the investigate the corrosion behavior of a C/SiC composite in corrosion resistance of the composite. The first one is the Na SO vapor environments with no oxygen at tempera- 2 4 corrosion behavior of the composite at elevated tempera- tures from 1000 to 15008C. ture up to 15008C. At low temperatures, a liquid film of As ceramics, the hot corrosion behavior of SiC and salts will formed on the surface of the composite in the gas Si N has been widely studied for a long time [3–10]. 3 4 due to deposition. At high temperatures, salts in the gas However, this is not enough to understand the hot corrowill be in the form of vapor. Consequently, the composite sion behavior of a C/SiC composite because the composite has different corrosion behavior in different temperature contains fibers as well as interphases besides the SiC ranges. The second one is the cooperation of the oxidation matrix and the SiC ceramics are not prepared by CVD. resistance and the corrosion resistance. It is very important Recently, some investigations have been conducted on the hot corrosion behavior of C/SiC and SiC/SiC composite *Corresponding author. Tel.: 186-29-849-4616; fax: 86-29- [11,12]. Hot corrosion of the ceramics and the composites 849-4620. was mostly researched by exposure to thin films of salts E-mail address: chenglf@nwpu.edu.cn (L. Cheng). rather than to vapors of salts in environments with oxygen 0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(01)00203-2
878 L. Cheng et al. /Carbon 40 (2002)877-882 below 1300C. Obviously, the corrosion resistance to thin the specimens by the Ar flow. As the tail gas was films of salts was essentially different from that to vapors conducted into the bubble bottle, the salt was dissolved in of salts because the diffusion of the reaction products in the water and collected. The maximum testing temperature corrosion was changed. Coupling of the oxidation and the of the device was limited to below 1500c by the corrosion made it difficulty to see the real corrosion corrosion resistance of alumina. Because the melting point resistance. In order to increase the working temperature of was 850C, a higher vapor pressure of the molten salt can the composite, the testing temperature for hot corrosion be achieved above this temperature. The vapor concen- tration of the salt in the gas tube can be calculated by the olatilization rate, which could be measured by the weight 2. Experimental proced centration at 1000.C was calculated by the measured weight loss to be 300 ppm. Above 1000"C, Na, SO, was 2.I. Fabrication of the composite decomposed into Na,O and sO, The processing of a three-dimensional C/SiC composite Na2 SO,= Na20+SO has been reported [13]. Specimens with a dimension of 4x5x30 mm were machined from the prepared compo- caused by Na, O vapor and SO, gas. Corrosion of the site. The two-layer SiC coating with a thickness of about 40 um was prepared by chemical vapor deposition for 40 h apor to the alumina crucible and the alumina on the composite ing with MTS ube was not serious, the products should have not an (methyltrichlorosilane)at 1000C obvious effect on corrosion behavior of the composite due to flowing of Ar 23. Measurements on the composites Corrosion tests in Na, SO, vapor environments in the temperature range from 1000 to 1500.C were conducted in After corrosion in the salt vapor environments, the a special device made by ourselves(Fig. 1). Two thermo- strengths of the specimens were measured by the couples were employed in the device, one was in the int bending method with a span of 20 mm, and the alumina furnace tube and the other was in the alumina gas hanges were measured by an electronic balance tube. The former was used to measure and control the temperature of the specimens, and the latter was used to measure the temperature of the melted salt. The specimens 3. Results and discussion were placed in the middle area of the gas tube. An alumina crucible containing some Na, SO4 was put before the The weight change of the C/Sic composite with pecimens. Changing the position of the crucible could temperature after corrosion in the Na, SO, environment at change the temperature of the salt due to the temperature a concentration of 300 ppm for different times from 1000 gradient in this area. The salt vapor was transported onto to 1500C is shown in Fig. 2. Below 1100C, the corrosion temperature oooooo goooodoooo oyoooooo thcrmo-co Melt mperature Furnace tub Fig. 1. Schematic of the corrosion test device for a C/SiC composite in salt vapor environments
878 L. Cheng et al. / Carbon 40 (2002) 877 –882 below 13008C. Obviously, the corrosion resistance to thin the specimens by the Ar flow. As the tail gas was films of salts was essentially different from that to vapors conducted into the bubble bottle, the salt was dissolved in of salts because the diffusion of the reaction products in the water and collected. The maximum testing temperature corrosion was changed. Coupling of the oxidation and the of the device was limited to below 15008C by the corrosion made it difficulty to see the real corrosion corrosion resistance of alumina. Because the melting point resistance. In order to increase the working temperature of was 8508C, a higher vapor pressure of the molten salt can the composite, the testing temperature for hot corrosion be achieved above this temperature. The vapor concenshould be increased. tration of the salt in the gas tube can be calculated by the volatilization rate, which could be measured by the weight loss of the crucible with melted salt inside. The con- 2. Experimental procedure centration at 10008C was calculated by the measured weight loss to be 300 ppm. Above 10008C, Na SO was 2 4 2.1. Fabrication of the composite decomposed into Na O and SO 2 3 Na SO 5 Na O 1 SO (1) The processing of a three-dimensional C/SiC composite 24 2 3 has been reported [13]. Specimens with a dimension of Then, corrosion of the Na SO vapor to the composite was 2 4 435330 mm were machined from the prepared compo- caused by Na O vapor and SO gas. Corrosion of the 2 3 site. The two-layer SiC coating with a thickness of about Na SO vapor to the alumina crucible and the alumina 2 4 40 mm was prepared by chemical vapor deposition for 40 h tube was not serious, the products should have not an on the composite after machining with MTS obvious effect on corrosion behavior of the composite due (methyltrchlorosilane) at 10008C. to flowing of Ar. 2.2. Corrosion tests 2.3. Measurements on the composites Corrosion tests in Na SO vapor environments in the 2 4 After corrosion in the salt vapor environments, the temperature range from 1000 to 15008C were conducted in flexural strengths of the specimens were measured by the a special device made by ourselves (Fig. 1). Two thermothree-point bending method with a span of 20 mm, and the couples were employed in the device, one was in the weight changes were measured by an electronic balance. alumina furnace tube and the other was in the alumina gas tube. The former was used to measure and control the temperature of the specimens, and the latter was used to measure the temperature of the melted salt. The specimens 3. Results and discussion were placed in the middle area of the gas tube. An alumina crucible containing some Na SO was put before the The weight change of the C/SiC composite with 2 4 specimens. Changing the position of the crucible could temperature after corrosion in the Na SO environment at 2 4 change the temperature of the salt due to the temperature a concentration of 300 ppm for different times from 1000 gradient in this area. The salt vapor was transported onto to 15008C is shown in Fig. 2. Below 11008C, the corrosion Fig. 1. Schematic of the corrosion test device for a C/SiC composite in salt vapor environments
L. Cheng et al./ Carbon 40(2002)877-882 loss produced by any mechanism should be related to 1,c=0 temperature by the Arrhenius' law. Actually, the Ar- AW,=A(1-exp(-B T") Here A, n, and B, are constants. Aw, represents weigh gain or weight loss. The three functions could be rewritten I △W1=A1(1-exp(-B17") (4) 17△H2=A2(1-exp(-B22) △W3=A3(1-exp(-B37) Fig. 2. Weight change of the C/SiC composite with temperature ter corrosion in a salt vapor environment for different times. gh the change of the composite in the corrosion enviro as a complicated continuous had nearly no effect on the weight change of the compo- function of temperature, it was the sum of three simple site. The composite gained weight from 1 100 to 1300oC functions and lost weight above 1300.C. The weight gain changed little with corrosion time and reached its maximum value The continuous function of weight change of the compo- site with temperature which fitted the test results quite well at 1200.C. The weight loss increased rapidly with increas- ing temperature and with increasing corrosion time was obtained by selecting proper constants in Eqs. (4)-(6) through trail and error. After the constants were selected. Fig. 3 showed that the weight change of the the weight gain or weight loss produced by all the could be related to temperature by three functions after 5 h corrosion over the whole mechanisms over the full temperature range was separately the complicated mechanisms which affect the weight change of the composite in the co environment could be determined and made clear W=△W.+△W+△W The first mechanism led to a little weight gain AW. This confirmed that the passive oxidation of the CVd Sic took This indicated that the weight change could be considered place to be produced by three mechanisms. Because each mechanism could only lead to weight gain or weight loss, Na20+4SO,+ SiC= SiO2+ 4S02+CO2+ Na,O(7) the three functions should be monotone decreasing or increasing with temperature. The weight gain or weight It could be seen from Fig. 3 that the temperature range Aw 1 1.5 3 10 14 15 Fig. 3. Weight change of the C/SiC composite with temperature after corrosion for 5h
L. Cheng et al. / Carbon 40 (2002) 877 –882 879 loss produced by any mechanism should be related to temperature by the Arrhenius’ law. Actually, the Arrhenius’ relation is equal to n DW 5 i A (1 2 exp(2B T )) (3) ii i Here A , n and B are constants. DW represents weight ii i i gain or weight loss. The three functions could be rewritten as n DW 5 1 A (1 2 exp(2B T )) (4) 11 1 n DW 5 2 A (1 2 exp(2B T )) (5) 22 2 n DW 5 3 A (1 2 exp(2B T )) (6) 33 3 Fig. 2. Weight change of the C/SiC composite with temperature after corrosion in a salt vapor environment for different times. Although the weight change of the composite in the corrosion environment was a complicated continuous had nearly no effect on the weight change of the compo- function of temperature, it was the sum of three simple site. The composite gained weight from 1100 to 13008C, functions. and lost weight above 13008C. The weight gain changed The continuous function of weight change of the compo- little with corrosion time and reached its maximum value site with temperature which fitted the test results quite well at 12008C. The weight loss increased rapidly with increas- was obtained by selecting proper constants in Eqs. (4)–(6) ing temperature and with increasing corrosion time. through trail and error. After the constants were selected, Fig. 3 showed that the weight change of the composite the weight gain or weight loss produced by all the could be related to temperature by three continuous mechanisms over the full temperature range was separately functions after 5 h corrosion over the whole temperature obtained, and then the complicated mechanisms which range affect the weight change of the composite in the corrosion environment could be determined and made clear. DW5 DW 1 DW 1 DW (2) 123 The first mechanism led to a little weight gain DW . This 1 confirmed that the passive oxidation of the CVD SiC took This indicated that the weight change could be considered place to be produced by three mechanisms. Because each mechanism could only lead to weight gain or weight loss, Na O 1 4SO 1 SiC 5 SiO 1 4SO 1 CO 1 Na O (7) 2 3 2 2 22 the three functions should be monotone decreasing or increasing with temperature. The weight gain or weight It could be seen from Fig. 3 that the temperature range Fig. 3. Weight change of the C/SiC composite with temperature after corrosion for 5 h
L. Cheng et al. /Carbon 40 (2002)877-882 for the passive of the CVd Sic was from 1080 the oxidation of the carbon phases. As a result, the weight 1200°C.The mechanism led to a little weight loss produced by the oxidation of the carbon phases did loss AW. This nly be due to the oxidation of the not increase with increasing temperature, although there carbon phases in the composite were still defects in the coating above 1300.C. the weight Na,O+SO, +C=SO, + CO+ Na,O gain of the composite reached its maximum value at 1200C Below 1200C, the weight gain produced by the It could be seen from Fig 3 that the temperature range by the oxidation of carbon phases, then the composite for oxidation of the carbon phases was from 1 100 to showed weight gain. Above 1200.C, the weight gain 1300C. Above 1200C. the oxidation of the cvd sic produced by passive oxidation did not increase and the transformed from passive to active weight loss produced by the oxidation of the carbon phases Na2 0+SO,+2 SiC=2 SiO Na,S+2 CO increased with increasing temperature, then weight gain of the composite decreased. Fig. 4 shows the Arrhenius Therefore, the third mechanism led to a large weight loss relations of the weight gain or weight loss produced by the AW,. It could be seen from Fig 3 that the weight loss three mechanisms with temperature for the C/Sic compo increased very rapidly with increasing temperature site after corrosion for 5 h. The activation energy for the From the discussion above, it was known that there were passive oxidation of the CVD SiC, the oxidation of the two kinds of reaction between the composite and carbon phases and the active oxidation of the CVD Si Na, SO, vapor environment. The first kind of reaction was were, respectively, calculated by the Arrhenius'relations oxidation of the Sic phases, including passive oxidation to be 114. 105 and 112 kcal/ mol. It could be seen that the and active oxidation. The threshold temperature for the activation energy for the reaction of the carbon phases with former was about 1080 C, and that for the latter was about SO, was much higher than that for the reaction of the carbon phases with was 1300C. The second reaction was the oxidation of the Oxidation of the carbon phases was produced by oxyge carbon phases, and the threshold temperature for this in air, and that was produced by So, in salt vapor oxidation was about 1100.C. The SO, gas diffused into the environments with on oxygen. Because the partial pressure composite through defects in the CVd Sic coating [14 of the oxidizing gas in air was much higher than that in the and reacted with the carbon phases. Because the diffusion salt vapor, oxidation of carbon phases in air is much more of the SO, gas in the defects was slow, the weight loss rapid than in the salt vapor. More importantly, the thres- produced by the oxidation hold temperature (400.C)and the activation energy(24 Above 1300C. active oxi kcal/mol)for oxidation of carbon with oxygen were much of the Cvd Sic coating lower than those (1100.C, 105 kcal/mol) for oxidation ln(-△W2)=-52636T+33.674 (-△W3)=-55957T+32.599 105m(K Fig. 4. Anhenius' relations of the weight gain or weight loss produced by the three mechanisms with temperature for the C/SiC composi fter corrosion for 5 h
880 L. Cheng et al. / Carbon 40 (2002) 877 –882 for the passive oxidation of the CVD SiC was from 1080 the oxidation of the carbon phases. As a result, the weight to 12008C. The second mechanism led to a little weight loss produced by the oxidation of the carbon phases did loss DW . This could only be due to the oxidation of the not increase with increasing temperature, although there 2 carbon phases in the composite were still defects in the coating above 13008C. The weight gain of the composite reached its maximum value at Na O 1 SO 1 C 5 SO 1 CO 1 Na O (8) 23 2 2 12008C. Below 12008C, the weight gain produced by the passive oxidation was larger than the weight loss produced It could be seen from Fig. 3 that the temperature range by the oxidation of carbon phases, then the composite for oxidation of the carbon phases was from 1100 to showed weight gain. Above 12008C, the weight gain 13008C. Above 12008C, the oxidation of the CVD SiC produced by passive oxidation did not increase and the transformed from passive to active weight loss produced by the oxidation of the carbon phases increased with increasing temperature, then weight gain of Na O 1 SO 1 2 SiC 5 2 SiO 1 Na S 1 2 CO (9) 23 2 the composite decreased. Fig. 4 shows the Arrhenius’ Therefore, the third mechanism led to a large weight loss relations of the weight gain or weight loss produced by the DW . It could be seen from Fig. 3 that the weight loss three mechanisms with temperature for the C/SiC compo- 3 increased very rapidly with increasing temperature. site after corrosion for 5 h. The activation energy for the From the discussion above, it was known that there were passive oxidation of the CVD SiC, the oxidation of the two kinds of reaction between the composite and the carbon phases and the active oxidation of the CVD SiC Na SO vapor environment. The first kind of reaction was were, respectively, calculated by the Arrhenius’ relations 2 4 oxidation of the SiC phases, including passive oxidation to be 114, 105 and 112 kcal/mol. It could be seen that the and active oxidation. The threshold temperature for the activation energy for the reaction of the carbon phases with former was about 10808C, and that for the latter was about SO was much higher than that for the reaction of the 3 13008C. The transition temperature from passive to active carbon phases with O . 2 was 13008C. The second reaction was the oxidation of the Oxidation of the carbon phases was produced by oxygen carbon phases, and the threshold temperature for this in air, and that was produced by SO in salt vapor 3 oxidation was about 11008C. The SO gas diffused into the environments with on oxygen. Because the partial pressure 3 composite through defects in the CVD SiC coating [14] of the oxidizing gas in air was much higher than that in the and reacted with the carbon phases. Because the diffusion salt vapor, oxidation of carbon phases in air is much more of the SO gas in the defects was slow, the weight loss rapid than in the salt vapor. More importantly, the thres- 3 produced by the oxidation of the carbon phases was small. hold temperature (4008C) and the activation energy (24 Above 13008C, active oxidation took place on the surface kcal/mol) for oxidation of carbon with oxygen were much of the CVD SiC coating and was strong enough to control lower than those (11008C, 105 kcal/mol) for oxidation Fig. 4. Anhenius’ relations of the weight gain or weight loss produced by the three mechanisms with temperature for the C/SiC composite after corrosion for 5 h
L. Cheng et al. /Carbon 40 (2002)877-882 tron det O.. Above 900C. the oxidation in air was con- Below 1200.C, the C/SiC composite had by oxygen diffusion which leads to cavity forma- corrosion resistance to the Na, SO4 vapor because eath the coating in air [14]. In the temperature the oxidation of the CVd Sic was passive, but range from 1 100 to 1300C, the oxidation in the salt vapor activation energy for the oxidation of the carbon phases was controlled by the reaction of carbon with SO, which was as high as that for the oxidation of the CVD SiC. It leads to uniform oxidation of carbon phases. Even though could be concluded that the Sic/Sic composite would not cracking and no spalling on the coating, SO, can get in have a higher corrosion resistance than the C/SiC compo- the composite by diffusion in the coating defects. There- site did to the Na, SO, vapor, even though Hi-Nicalon fore, the weight loss due to the carbon gasification did not fibers were used. Above 1300.C, the poor corrosion lead to the cavity formation. resistance of the CVD Sic caused the composite to have a Because the three mechanisms for the weight change poor corrosion resistance to the Na, sO4 vapor. Although would also lead to strength change, they could be identified the active oxidation of the cvd sic coating did not by the strength change of the composite determined by the decrease the strength of the composite largely before the weight change of the composite. Fig. 5 shows the flexural coating was corroded, further corrosion would lead to a strength change of the composite with temperature after large strength loss due to the direct oxidation of the carbon corrosion for 10 h. It should be noticed that the strength fibers. However, the composite would have a good resist- loss reached its minimum value when the weight gain of ance even up to 1300C if the concentration of the Na, SOa the composite reached its maximum value at 1200.C. This vapor was low enough. Actually, the concentration of the was considered to be related to the damage of the carbon salt vapor in combustion gas environments was much fibers caused by the oxidation. The strength of the compo- lower than 300 ppm. Consequently, it was the partial site was very sensitive to the oxidation of the fibers. Below pressure of oxygen in a combustion gas but not the 1200C, the uniform oxidation controlled by the reaction concentration of the salt vapor that controlled the degra- ate of So, with the carbon phases led to more strength dation process of the C/Sic composite. For structural oss. Above 1200.C, the superficial oxidation controlled by pplications in combustion gas environments, it was more the diffusing rate of SO, in the defects led to less strength important to increase the oxidation resistance of the C/Sic loss. Above 1300C, although the weight loss increased composite than ase the corrosion resistand rapidly with increasing temperature, the strength loss decreased slowly because the corrosion took place on the surface of the CVd Sic coating. On the one hand, the 4. Conclusions surface corrosion did not damage the carbon fibers serious- ly, on the other hand it decreased the thickness of the CVd Sic coating which was not related to the strength. As a 1. The degradation mechanisms for the C/Sic composite result, the surface corrosion did not have a large effect or a Na, SO, vapor environment at temperatures from the strength of the composite 1000 to 1500.C could be determined by dividing 与200 100 14 16 Temperature(x100℃) Fig. 5. Flexural strength change of the C/SiC composite with temperature after corrosion for 10 h
L. Cheng et al. / Carbon 40 (2002) 877 –882 881 with SO . Above 9008C, the oxidation in air was con- Below 12008C, the C/SiC composite had a higher 3 trolled by oxygen diffusion which leads to cavity forma- corrosion resistance to the Na SO vapor because not only 2 4 tion beneath the coating in air [14]. In the temperature the oxidation of the CVD SiC was passive, but also the range from 1100 to 13008C, the oxidation in the salt vapor activation energy for the oxidation of the carbon phases was controlled by the reaction of carbon with SO which was as high as that for the oxidation of the CVD SiC. It 3 leads to uniform oxidation of carbon phases. Even though could be concluded that the SiC/SiC composite would not no cracking and no spalling on the coating, SO can get in have a higher corrosion resistance than the C/SiC compo- 3 the composite by diffusion in the coating defects. There- site did to the Na SO vapor, even though Hi-Nicalon 2 4 fore, the weight loss due to the carbon gasification did not fibers were used. Above 13008C, the poor corrosion lead to the cavity formation. resistance of the CVD SiC caused the composite to have a Because the three mechanisms for the weight change poor corrosion resistance to the Na SO vapor. Although 2 4 would also lead to strength change, they could be identified the active oxidation of the CVD SiC coating did not by the strength change of the composite determined by the decrease the strength of the composite largely before the weight change of the composite. Fig. 5 shows the flexural coating was corroded, further corrosion would lead to a strength change of the composite with temperature after large strength loss due to the direct oxidation of the carbon corrosion for 10 h. It should be noticed that the strength fibers. However, the composite would have a good resistloss reached its minimum value when the weight gain of ance even up to 13008C if the concentration of the Na SO 2 4 the composite reached its maximum value at 12008C. This vapor was low enough. Actually, the concentration of the was considered to be related to the damage of the carbon salt vapor in combustion gas environments was much fibers caused by the oxidation. The strength of the compo- lower than 300 ppm. Consequently, it was the partial site was very sensitive to the oxidation of the fibers. Below pressure of oxygen in a combustion gas but not the 12008C, the uniform oxidation controlled by the reaction concentration of the salt vapor that controlled the degrarate of SO with the carbon phases led to more strength dation process of the C/SiC composite. For structural 3 loss. Above 12008C, the superficial oxidation controlled by applications in combustion gas environments, it was more the diffusing rate of SO in the defects led to less strength important to increase the oxidation resistance of the C/SiC 3 loss. Above 13008C, although the weight loss increased composite than to increase the corrosion resistance. rapidly with increasing temperature, the strength loss decreased slowly because the corrosion took place on the surface of the CVD SiC coating. On the one hand, the 4. Conclusions surface corrosion did not damage the carbon fibers seriously, on the other hand it decreased the thickness of the CVD SiC coating which was not related to the strength. As a 1. The degradation mechanisms for the C/SiC composite result, the surface corrosion did not have a large effect on in a Na SO vapor environment at temperatures from 2 4 the strength of the composite. 1000 to 15008C could be determined by dividing a Fig. 5. Flexural strength change of the C/SiC composite with temperature after corrosion for 10 h
L. Cheng et al./ Carbon 40(2002)877-882 complicated function describing the change of References functions describing the mechanisms [1] Graziani T, Baxter D, Nannetti CA. Degrad tified by the strength change of the composite with arbide- based material temperature environment. Key Eng Mater 1996: 113: 153-64 2. There were three reactions between the C/Sic compo- [2] Jacobson NS. Corrosion of silicon-based ceramics in com- site and the Na, SO vapor. The first was passive bustion environments. J Am Ceram Soc 1990, 76(10): 3-28 oxidation of the CVD SiC, leading a small weight gain. 3]Smialek JL, Jacbson NS Mechanism of strength degradation for hot corrosion of c-SiC.J Am Ceram Soc The second was oxidation of the carbon phases, leadin 19866910:74l-52 to a small weight loss. The third was active oxidation of 4 Graziani T, Baxter V, Bellosi A Corrosive degradation of a the CVD SiC, leading to a large weight loss. The dense Si, N, in a burner rig. J Mater Sci 1997; 32(6): 1631-7 threshold temperatures for these reactions were, respec 5]Jacbson NS, Smialek JL. Hot corrosion of a-SiC at 1000C tively, 1080, 1 100 and 1300.C. The transition tempera- J Am Ceram Soc 1985: 68(8): 432-9 ture from passive to active was 1200.C. The activation 6 Pareek V, Shores DA. Oxidation of silicon carbide in energies for these reactions were, respectively, calcu- environments containing potassium salt vapor. J Am Ceram lated by the weight change with temperature to be 1 14, Socl991;74(3):556-63 105 and 112 kcal/ mol [7 Federer JI. Corrosion of SiC Ceramics by Na, SO4. Ad 3. The flexural strength loss of the composite reached its Ceram mater1988;3(1):56-61 minimum value when the weight gain of the composite [18 Jacbson NS Kinetics and reached its maximum value at 1200C. Below 1200C molten salts. J Am Ceram Soc 1986; 69(11): 74-82 the C/SiC composite had a higher corrosion resistance [9] Jacbson NS. Corrosion of silicon-based ceramics in combus- tion environments. J Am Ceram Soc 1993 76(1): 3-28. to the Na2 SO, vapor because the carbon phases had a very high activation energy with the so, gas. Above [10] Song DY, Kitaoka S, Kawamoto H. Hot corrosion of hemical vapor deposited SiC and Si, N, in molten Na, SO 1300C, the poor corrosion resistance of the CVD SiC J Mater Sci1998;33:1031-6. caused the composite to have a poor corrosion resist- [11] Natesan K, Yanez-Herrero M, Fornasieri C. Corrosion per- ance to the Na, SO, vapor dvanced combustion svstems Argonne National Lab, DE94-006726, 1993 [12 Lowden RA, James RD. High-temperature corrosion of Nicalon( tm)/SiC composites. Oak Ridge National Lab Acknowledgements DE92-000281,1991 [13] Cheng L, Xu Y, Yin X, Gao R. Effect of glass sealing on the The authors acknowledge the of the chinese oxidation behavior of three dimensional C/SiC composit National Foundation for Natural under Contract in air. Carbon 2001: 39: 1 127-33 No. 59772023 and the Chinese [14] Cheng L, Xu Y, Yin X, Zhang L. Preparation of an oxidation Foundation for protection coating for c/c composites by low pressure Sciences under Contract No. 99J12.5.2 chemical vapor deposition. Carbon 38:1493-8
882 L. Cheng et al. / Carbon 40 (2002) 877 –882 complicated function describing the weight change of References the composite with temperature into several monotone functions describing the mechanisms, and then iden- [1] Graziani T, Baxter D, Nannetti CA. Degradation of silicon tified by the strength change of the composite with carbide-based materials in a high temperature combustion environment. Key Eng Mater 1996;113:153–64. temperature. [2] Jacobson NS. Corrosion of silicon-based ceramics in com- 2. There were three reactions between the C/SiC compo- bustion environments. J Am Ceram Soc 1990;76(10):3–28. site and the Na SO vapor. The first was passive 2 4 [3] Smialek JL, Jacbson NS. Mechanism of strength degradation oxidation of the CVD SiC, leading a small weight gain. for hot corrosion of a-SiC. J Am Ceram Soc The second was oxidation of the carbon phases, leading 1986;69(10):741–52. to a small weight loss. The third was active oxidation of [4] Graziani T, Baxter V, Bellosi A. Corrosive degradation of a the CVD SiC, leading to a large weight loss. The dense Si N in a burner rig. J Mater Sci 1997;32(6):1631–7. 3 4 threshold temperatures for these reactions were, respec- [5] Jacbson NS, Smialek JL. Hot corrosion of a-SiC at 10008C. tively, 1080, 1100 and 13008C. The transition tempera- J Am Ceram Soc 1985;68(8):432–9. ture from passive to active was 12008C. The activation [6] Pareek V, Shores DA. Oxidation of silicon carbide in energies for these reactions were, respectively, calcu- environments containing potassium salt vapor. J Am Ceram Soc 1991;74(3):556–63. lated by the weight change with temperature to be 114, 105 and 112 kcal/mol. [7] Federer JI. Corrosion of SiC Ceramics by Na SO . Adv 2 4 Ceram Mater 1988;3(1):56–61. 3. The flexural strength loss of the composite reached its [8] Jacbson NS. Kinetics and mechanism of corrosion of SiC by minimum value when the weight gain of the composite molten salts. J Am Ceram Soc 1986;69(11):74–82. reached its maximum value at 12008C. Below 12008C, [9] Jacbson NS. Corrosion of silicon-based ceramics in combus- the C/SiC composite had a higher corrosion resistance tion environments. J Am Ceram Soc 1993;76(1):3–28. to the Na SO vapor because the carbon phases had a 2 4 [10] Song DY, Kitaoka S, Kawamoto H. Hot corrosion of very high activation energy with the SO gas. Above 3 chemical vapor deposited SiC and Si N in molten Na SO . 34 2 4 13008C, the poor corrosion resistance of the CVD SiC J Mater Sci 1998;33:1031–6. caused the composite to have a poor corrosion resist- [11] Natesan K, Yanez-Herrero M, Fornasieri C. Corrosion perance to the Na SO vapor. formance of materials for advanced combustion systems. 2 4 Argonne National Lab., DE94-006726, 1993. [12] Lowden RA, James RD. High-temperature corrosion of Nicalon(tm)/SiC composites. Oak Ridge National Lab., DE92-000281, 1991. Acknowledgements [13] Cheng L, Xu Y, Yin X, Gao R. Effect of glass sealing on the oxidation behavior of three dimensional C/SiC composites The authors acknowledge the support of the Chinese in air. Carbon 2001;39:1127–33. National Foundation for Natural Sciences under Contract [14] Cheng L, Xu Y, Yin X, Zhang L. Preparation of an oxidation No. 59772023 and the Chinese Defense Foundation for protection coating for c/c composites by low pressure Sciences under Contract No. 99J12.5.2. chemical vapor deposition. Carbon 2000;38:1493–8