Availableonlineatwww.sciencedirect.com Transactions of ScienceDirect Nonferrous Metals x)e Science Society of China ELSEVIER Press Trans. Nonferrous Met. Soc. China 19(2009)61-64 www.tnmsc.cn Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1 500 C YAN Zhi-qiao(闫志巧2, XIONG Xiang(熊翔), XIAO Peng(肖鹏) CHEN Feng(陈峰}, ZHANG Hong-bo(张红波), HUANG Bai-yun(黄伯云 1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China; 2. Guangzhou Research Institute of Nonferrous Metals, Quangzhou 510650, China Received 16 January 2008; accepted 28 April 2008 Abstract: Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and Sic/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD)alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 C was compared. The results show that the C/C-sic substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of nany bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense Sic layer and porous Si-Mo layer. Dense Sic layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating. Key words: C/C-SiC composites; oxidation protective coating, slurry painting; chemical vapor deposition composites in particular(4-7. Unfortunately, the process 1 Introduction has to be carried out above 1 800 C and residual silicon may escape from the C/C-SiC substrate causing substrate Molten silicon infiltration(MSI) is a major to become loose and the mechanical strength to decrease manufacturing process of C/C-Sic composites. In this regard, pack cementation is not suitable for MSI Compared with chemical vapor infiltration(CVI) and C/C-SiC composites polymer impregnation and pyrolysis(PIP), MsI has many In this work, three-layer Si-Mo coating prepared by advantages such as lower component fabrication time to slurry painting, and SiC/Si-Mo multilayer coating reduce costs significantly[l]. It is generally thought that prepared by CVd alternated with slurry painting, were C/C-SiC composites exhibit better oxidation resistance applied on MSI C/C-SiC composites, respectively. The than C/C composites because SiC and residual Si are oxidation of substrate and two coated samples at 1 500 contained. However. our research found that the initial C was investigated oxidation temperature of C/C-SiC composites is about 100 C lower than that of C/C composites. The mass loss 2 Experiment rate below 1 000 C was much higher[2]. Consequently MSI C/C-SiC composites need oxidation protection 2.1 Specimen preparation when exposed to oxidizing environment at hig The 2.5D bulk needled carbon fiber felts(bulk density of 0.56 g/cm) from Tianniao Yixing High Currently, three primary methods are used to apply Technology Co, Ltd, China, were used as preforms. A oxidation protection coating on the surface of carbon CVd process was used to deposit pyrocarbon in the felts materials:pack cementation, chemical vapor deposition and MSI was adopted to infiltrate the porous carbon/ (CVD)and slurry method(3]. Among these methods, carbon preforms to form C/C-SiC composites. The pack cementation is widely used to form coatings on C/c details were reported in Ref[2] oject(2006CB600908)supported by the National Basic Research Program of China Corresponding author: XIONG Xiang, Tel: +86-731-8836079; E-mail: xiong228@sinac DOI:10.10l6S1003-6326(0860229-0
Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1 500 ć YAN Zhi-qiao(䮿ᖫᎻ) 1, 2, XIONG Xiang(❞ 㖨) 1 , XIAO Peng(㙪 吣) 1 , CHEN Feng(䰜 ዄ) 1 , ZHANG Hong-bo(ᓴ㑶⊶) 1 , HUANG Bai-yun(咘ԃѥ) 1 1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China; 2. Guangzhou Research Institute of Nonferrous Metals, Quangzhou 510650, China Received 16 January 2008; accepted 28 April 2008 Abstract: Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and SiC/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD) alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 ć was compared. The results show that the C/C-SiC substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of many bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense SiC layer and porous Si-Mo layer. Dense SiC layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating. Key words: C/C-SiC composites; oxidation protective coating; slurry painting; chemical vapor deposition 1 Introduction Molten silicon infiltration(MSI) is a major manufacturing process of C/C-SiC composites. Compared with chemical vapor infiltration(CVI) and polymer impregnation and pyrolysis(PIP), MSI has many advantages such as lower component fabrication time to reduce costs significantly[1]. It is generally thought that C/C-SiC composites exhibit better oxidation resistance than C/C composites because SiC and residual Si are contained. However, our research found that the initial oxidation temperature of C/C-SiC composites is about 100 ć lower than that of C/C composites. The mass loss rate below 1 000 ć was much higher[2]. Consequently, MSI C/C-SiC composites need oxidation protection when exposed to oxidizing environment at high temperatures. Currently, three primary methods are used to apply oxidation protection coating on the surface of carbon materials: pack cementation, chemical vapor deposition (CVD) and slurry method[3]. Among these methods, pack cementation is widely used to form coatings on C/C composites in particular[4í7]. Unfortunately, the process has to be carried out above 1 800 ć and residual silicon may escape from the C/C-SiC substrate causing substrate to become loose and the mechanical strength to decrease. In this regard, pack cementation is not suitable for MSI C/C-SiC composites. In this work, three-layer Si-Mo coating prepared by slurry painting, and SiC/Si-Mo multilayer coating prepared by CVD alternated with slurry painting, were applied on MSI C/C-SiC composites, respectively. The oxidation of substrate and two coated samples at 1 500 ć was investigated. 2 Experimental 2.1 Specimen preparation The 2.5D bulk needled carbon fiber felts (bulk density of 0.56 g/cm3 ) from Tianniao Yixing High Technology Co., Ltd., China, were used as preforms. A CVD process was used to deposit pyrocarbon in the felts and MSI was adopted to infiltrate the porous carbon/ carbon preforms to form C/C-SiC composites. The details were reported in Ref.[2]. Foundation item: Project(2006CB600908) supported by the National Basic Research Program of China Corresponding author: XIONG Xiang; Tel: +86-731-8836079; E-mail: xiong228@sina.com DOI: 10.1016/S1003-6326(08)60229-0
YAN Z hi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009)61-64 A Si-Mo layer was prepared by painting the mixture spectroscopy (EDS) slurry of Si and Mo powders and silica sol on the surface of substrate then drying and sintering at 1 500 C for 10 3 Results and discussion min. The above process was repeated for 3 times to form three-layer Si-Mo coating. The coating was about 1 3.1 Oxidation of c/c-sic substrate thick and the details were reported in Ref[8] morphologies of C/C-SiC composites before and For SiC/Si-Mo multilayer coating, a SiC layer was after oxidation are shown in Fig. 1. Pores in the origin first deposited on the substrate for 6 h at 1 100 C in C/C porous preforms are well infiltrated by the reaction- MrS(methyltrichlorosilane )-H2-Ar system. Then, a formed SiC and residual Si. C/C-SiC composites have Si-Mo layer was made by slurry painting. Thes se high density(Fig. 1(a)) and the open porosity is less than processes were repeated until the coating structure, form 3%(2], which indicates that MSI is a rapid process to inside to outside, was SiC-Si-Mo-SiC--Si-Mo-SiC. fabricate dense C/C-SiC composites, and to retain high This coating was about 100 um thick too, and the detail residual Si content(about 14.2%( mass fraction)[2D) information was reported in Ref.191 The C/C-Sic substrate has a network structure and the fiber bundles are distributed in the continuous 2.2 Oxidation tests network framework consisting of SiC and Si(Fig. 1(b)) The substrate and two coated samples atl500℃ corundum tube furnace to Therefore, when being oxidized at or below 1 400 C C/C-SiC composites al ways exhibit good shape retention investigate the isothermal thermal cycling behavior. The though carbon in the substrate is utterly burnt out. It is samples were put inside or taken out of the furnace found that the substrate does not distort after 10 h directly to air within several seconds. Cumulative mass oxidation at 1"C. While after oxidation at 1 500 C changes of the samples were measured at room for I h, it is badly distorted and many lumps appear on temperature by an electronic balance with a sensitivity of ±0.1mg. The mass change(△m) of the samples is calculated by the following equation the substrate becomes very loose and many microcracks -mo)/mo×100% (1) emerge By EDS analysis, these lumps contain mainly (1) and O, suggesting that they should be the molten Sio where mo and mn are the mass of the samples before and glass agglomeration after oxidation, respectively Above the Si melting point(1 410 C), residual Si in sapa t he morphologies and crystalline structures of the the substrate melts evaporates into Si(g) and diffuses ples were analyzed by scanning electron microscopy toward the surface of sample. Si(g) is rapidly oxidized (SEM, Jeol-6300LV)equipped with energy dispersive into SiOz and deposits on the surface. It is concluded that d Fig 1 Morphologies of C/C-SiC composites:(a),(b)Before oxidation; (c),(d)Oxidation at 1 500 C for I h
62 YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009) 61í64 A Si-Mo layer was prepared by painting the mixture slurry of Si and Mo powders and silica sol on the surface of substrate then drying and sintering at 1 500 ć for 10 min. The above process was repeated for 3 times to form three-layer Si-Mo coating. The coating was about 100 μm thick and the details were reported in Ref.[8]. For SiC/Si-Mo multilayer coating, a SiC layer was first deposited on the substrate for 6 h at 1 100 ć in MTS (methyltrichlorosilane)-H2-Ar system. Then, a Si-Mo layer was made by slurry painting. These processes were repeated until the coating structure, form inside to outside, was SiCėSi-MoėSiCėSi-MoėSiC. This coating was about 100 μm thick too, and the detail information was reported in Ref.[9]. 2.2 Oxidation tests The substrate and two coated samples were heated at 1 500 ć in air in a corundum tube furnace to investigate the isothermal thermal cycling behavior. The samples were put inside or taken out of the furnace directly to air within several seconds. Cumulative mass changes of the samples were measured at room temperature by an electronic balance with a sensitivity of ±0.1 mg. The mass change (ǻm) of the samples is calculated by the following equation: ǻm=(m1ím0)/m0h100% (1) where m0 and m1 are the mass of the samples before and after oxidation, respectively. The morphologies and crystalline structures of the samples were analyzed by scanning electron microscopy (SEM, Jeolí6300LV) equipped with energy dispersive spectroscopy(EDS). 3 Results and discussion 3.1 Oxidation of C/C-SiC substrate Morphologies of C/C-SiC composites before and after oxidation are shown in Fig.1. Pores in the original C/C porous preforms are well infiltrated by the reactionformed SiC and residual Si. C/C-SiC composites have high density (Fig.1(a)) and the open porosity is less than 3%[2], which indicates that MSI is a rapid process to fabricate dense C/C-SiC composites, and to retain high residual Si content (about 14.2% (mass fraction) [2]). The C/C-SiC substrate has a network structure and the fiber bundles are distributed in the continuous network framework consisting of SiC and Si (Fig.1(b)). Therefore, when being oxidized at or below 1 400 ć, C/C-SiC composites always exhibit good shape retention though carbon in the substrate is utterly burnt out. It is found that the substrate does not distort after 10 h oxidation at 1 400 ć. While after oxidation at 1 500 ć for 1 h, it is badly distorted and many lumps appear on the surface (Fig.1(c)). SEM image (Fig.1(d)) shows that the substrate becomes very loose and many microcracks emerge. By EDS analysis, these lumps contain mainly Si and O, suggesting that they should be the molten SiO2 glass agglomeration. Above the Si melting point (1 410 ć), residual Si in the substrate melts evaporates into Si(g) and diffuses toward the surface of sample. Si(g) is rapidly oxidized into SiO2 and deposits on the surface. It is concluded that Fig.1 Morphologies of C/C-SiC composites: (a), (b) Before oxidation; (c), (d) Oxidation at 1 500 ćfor 1 h
YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009)61-64 C/C-SiC composites with residual Si cannot be directly FANG[II] reported a Si-Mo slurry coating on C/C used at1500℃ composites. The coating consisted of Si and MoSi2. It worked well at 1 370 C. but soon became invalid at 3.2 Oxidation of C/C-SiC substrate with three-layer 1 450 C In a conclusion, coatings with residual Si can Si-Mo coating not be used at 1 500 C directly SiC, Si, MoSi2 and Sio2 are found in the three-layer Si-Mo coating. After 140 h oxidation at 1 400 C, the 3.3 Oxidation of C/C-SiC substrate with SiC/Si-Mo mass loss of the coated sample is 1. 34%, showing that multilayer coating the coating can provide longtime oxidation protection for Based on the above analysis it can be inferred that C/C-SiC composites at that temperature[ 8]. While after the evaporation of residual Si and higher chemical I h oxidation at 1 500 C, the mass loss reaches 0. 76% activity of Si-Mo layer lead to the failure of the C/C-SiC (Fig. 2(a)) and many bubbles appear on the surface. The substrate with and without three-layer Si-Mo coating surface SEM image, shown in Fig. 2(b), reveals that Our purpose is thus to develop the coating system with continuous SiO2 glass has been formed accompanied dense barriers. The designed SiC/Si-Mo multilayer with bubbles. The phase formation Mot2Si-MoSi2 is coating, from inside to outside, was Sic-Si-Mo-SiC accompanied by a volume reduction of about 27%[10].-Si-Mo-SiC Even after 150 h oxidation at 1 400 C, This reduction produces certain pores and interface in the the mass loss of the coated sample was 0.25%[9].When Si-Mo layer and leads to higher chemical activity. At being oxidized at 1 500 C, the coated sample kept m I 500 C, SiC, Si and MoSi are rapidly oxidized into gaining during 105 h, which suggests that the coating can SiO2 glass and CO. Simultaneously, residual Si in the protect the C/C-SiC substrate from oxidation at that coating melts and evaporates. Gases of Si(g) and Co temperature. Surface SEM image(Fig 3(b) shows a gather in the coating and bubbles are formed in the Sio2 glassy SiO2 layer covering the surface well except the glass when the pressure exceeds the critical value. Then existence of several microcracks. During the oxidation he coating is out of service. Therefore, three-layer test, the sample was taken out of the furnace directly into Si-Mo coating cannot provide oxidation protection for air within several seconds for weighing. The cooling C-SiC composites at 1 500C rate from 1 500C to room temperature was very quick. 0.10 0.20.40.60.81.0 80100120 Oxidation time/h ep 500m Fig 2 Oxidation of C/C-SiC composites with three-layer Si-Mo Fig 3 Oxidation of C/c-Sic composites with SiC/Si-Mo coating at 1 500 C:(a) Isothermal oxidation curve;(b) sEm multilayer coating at 1 500 C: (a) Isothermal oxidation curve image after I h oxidation (b) SEM image after 105 h oxidation
YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009) 61í64 63 C/C-SiC composites with residual Si cannot be directly used at 1 500 ć. 3.2 Oxidation of C/C-SiC substrate with three-layer Si-Mo coating SiC, Si, MoSi2 and SiO2 are found in the three-layer Si-Mo coating. After 140 h oxidation at 1 400 ć, the mass loss of the coated sample is 1.34%, showing that the coating can provide longtime oxidation protection for C/C-SiC composites at that temperature[8]. While after 1 h oxidation at 1 500 ć, the mass loss reaches 0.76% (Fig.2(a)) and many bubbles appear on the surface. The surface SEM image, shown in Fig.2(b), reveals that continuous SiO2 glass has been formed accompanied with bubbles. The phase formation Mo+2SiėMoSi2 is accompanied by a volume reduction of about 27%[10]. This reduction produces certain pores and interface in the Si-Mo layer and leads to higher chemical activity. At 1 500 ć, SiC, Si and MoSi2 are rapidly oxidized into SiO2 glass and CO. Simultaneously, residual Si in the coating melts and evaporates. Gases of Si(g) and CO gather in the coating and bubbles are formed in the SiO2 glass when the pressure exceeds the critical value. Then the coating is out of service. Therefore, three-layer Si-Mo coating cannot provide oxidation protection for C/C-SiC composites at 1 500 ć. Fig.2 Oxidation of C/C-SiC composites with three-layer Si-Mo coating at 1 500 ć: (a) Isothermal oxidation curve; (b) SEM image after 1 h oxidation FANG[11] reported a Si-Mo slurry coating on C/C composites. The coating consisted of Si and MoSi2. It worked well at 1 370 ć, but soon became invalid at 1 450 ć. In a conclusion, coatings with residual Si can not be used at 1 500 ć directly. 3.3 Oxidation of C/C-SiC substrate with SiC/Si-Mo multilayer coating Based on the above analysis, it can be inferred that the evaporation of residual Si and higher chemical activity of Si-Mo layer lead to the failure of the C/C-SiC substrate with and without three-layer Si-Mo coating. Our purpose is thus to develop the coating system with dense barriers. The designed SiC/Si-Mo multilayer coating, from inside to outside, was SiCėSi-MoėSiC ėSi-MoėSiC. Even after 150 h oxidation at 1 400 ć, the mass loss of the coated sample was 0.25%[9]. When being oxidized at 1 500 ć, the coated sample kept mass gaining during 105 h, which suggests that the coating can protect the C/C-SiC substrate from oxidation at that temperature. Surface SEM image (Fig.3(b)) shows a glassy SiO2 layer covering the surface well except the existence of several microcracks. During the oxidation test, the sample was taken out of the furnace directly into air within several seconds for weighing. The cooling rate from 1 500 ć to room temperature was very quick. Fig.3 Oxidation of C/C-SiC composites with SiC/Si-Mo multilayer coating at 1 500 ć: (a) Isothermal oxidation curve; (b) SEM image after 105 h oxidation
YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009)61-64 Owing to this quick cooling, the coating would suffer composites at 1 500 C. The porous Si-Mo inner layer from tensile stress. which ed the formation of ensures the coating has excellent thermal shock microcracks. These microcracks could be self-sealed resistance. Coatings with residual Si should be avoided quickly when the coated sample was heated to 1 500c to use at 1 500C solely, but they can act as inner layers again, such that they have little effect on the oxidation when dense outer barriers exist resistance of the coating. Moreover, the coating remained intact during the whole test for 21 cycles of thermal References shock between 1 500 C and room temperature. The oxidation curve is changed steadily. These verify that the [] SCHULTE-FISCHEDICK J, SCHMIDT J, TAMME R, KRONER U, Sic/ Si-Mo multilayer coating has excellent oxidation ARNOLD J. ZEIFFER B. Oxidation behavior of C/C-SiC coated resistance at 1 500C with good thermal shock resistance with SiC-B,C-SiC-cordierite oxidation protection system []. Mater Sci Eng a,2004,386(1/2)428-434. Compared with the results in Ref[12](SiC-MoStz [2) YAN Zhi-qiao, XIONG Xiang, XIAO Peng, HUANG Bai-yun. (Tio.8 Moo. 2)Si2 multi-composition coating, 2.18%mass Oxidation kinetics and mechanism of C/SiC composites fabricated by loss at 1 500 C for 49 h for C/C composites )and Ref [131 MSI process [J]. Journal of Inorganic Materials, 2007, 22(6): (a dense C/SiC gradient oxidation protective coating, 2.46% mass loss at 1 500 C for 35 h for C/C [3] SNELL L, NELSON A, MOLIAN P. A novel laser technique for composites), the oxidation resistance is obviously oxidation-resistant coating of carbon-carbon composites J]. Carbon, improved, and the protective temperature is higher than (4) ZHANG YL, LI HI. FUQGLIKZ, HOU D S, FEI J. A Si-Mo that of SiC/Si-MoSi coating for carbon material oxidation protective coating for C/SiC coated carbon/carbon (reported to be 1 400 C[14D) composites J]. Carbon, 2007, 45(5): 1130-1133 Compared with three-layer Si-Mo coating, the [5] LI H J, XUE H, WANG Y J, FU Q Gi YAO D J. A MoSin-SiC-Si excellent oxidation resistance of Sic/Si-Mo multilayer oxidation protective coating for carbon/carbon composites U). Surf coating should be attributed to the alternated structure Coat Technol,2007,201(24):9444-9447 pecially the outer Sic layer. CVD SiC coati 16 FU Q G LI H J, LI K Z, SHI X H, HU Z B, HUANG M. Si deposited at 1 100 C is usually quite dense, and even a hisker-toughened MoSis-SiC-Si coating to protect carbon/carbon composites against oxidation [J]. Carbon, 2006, 44(9): 1866-1869 very thin layer could play the dual role of physical and [7 HUANG J F, ZENG XR, LI HJ, LI KZ, XIONG X B Oxidation chemical barrier. On the one hand, the SiC layer hinders behavior of SiC-Al-O3-mullite multi-coating coated carbon/carbon the diffusion of oxygen in and Si(g) out. On the other composites at high temperature [J). Carbon, 2005, 43(7): 1580-1583 hand, it is oxidized into dense SiO2 glass and resists the [8] YAN Zhi-qiao, XIONG Xiang, XIAO Peng, CHEN Feng, HUANG oxidation of Si-Mo layer. These prolong the coating Bai-yun, Oxidation behavior of Mo-Si coated C/SiC composites PJ lifespan [9] YAN Z Q, XIONG X, XIAO P, CHEN F, ZHANG H B, HUANG B The excellent thermal shock resistance of the Y. A multilayer coating of dense SiC alternated with porous Si-Mo SiC/Si-Mo multilayer coating is attributed to the Si-Mo for the oxidation protection of carbon/carbon silicon carbide layer. The porous Si-Mo layer has lower elastic modulus, which helps reducing thermal stress and thermal [0 SCHUBERT T, BOHM A, KIEBACK B, ACHTERMANN M, expansion mismatch between the coating and the SCHOLL R. Effects of high energy milling on densification behavior substrate[ 15]. These pores provide sites for volume of Si-Mo powder mixtures during pressureless sintering J). intermetallics,2002,10(9):873-878. expansion of Sioz formation. These factors ensure the [11 FANG Hai-tao. A Si-Mo fused slurry coating of C/C composites and excellent thermal shock resistance of the multilayer the oxidation resistance [ D]. Harbin: Harbin Institute of Technology, coating 2001.8l-83 [12] JIAO G S, LI H J, LI K Z, WANG C, HOU D S 4 Conclusions SiC-MoSiy-(TiosMoo?)Sig mul arbon composites J). Surf Coat Technol, 2006, 201(6): 3452-3456 Owing to the evaporation of Si and higher chemical [13] ZHANG Y L, LI H J, FU G, LI KZ, WEl J, WANG PY. A C/SiC gradient oxidation protective coating for carbon/carbon composites activity of Si-Mo layer, the MsI C/C-SiC substrate with UJ]. Surf Coat Technol, 2006, 201(6): 3491-3495 and without three-layer Si-Mo coating cannot be used at [14] ZHAO J, GUO Q G, SHI J L, ZHAI G T, LIU L.SiC/Si-MoSi2 1 500 C. For SiC/Si-Mo multilayer coating, the outer oxidation protective coatings for carbon materials J). Surf Coat Sic layer plays the dual role of physical and chemical Technol,2006,201(34):186l-1865 barrier, which hinders the diffusion of oxygen in and Si(g) [I5] DING S Q, ZENG Y P, JIANG D L Thermal shock resistance of in situ reaction bonded porous silicon carbide ceramics [ J]. Mater S out, and resists the oxidation of Si-Mo layer. The coating EngA,2006,425(12)326-329 could provide longtime protection for C/C-SiC (Edited by YUAN Sai-qian)
64 YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009) 61í64 Owing to this quick cooling, the coating would suffer from tensile stress, which induced the formation of microcracks. These microcracks could be self-sealed quickly when the coated sample was heated to 1 500 ć again, such that they have little effect on the oxidation resistance of the coating. Moreover, the coating remained intact during the whole test for 21 cycles of thermal shock between 1 500 ć and room temperature. The oxidation curve is changed steadily. These verify that the SiC/ Si-Mo multilayer coating has excellent oxidation resistance at 1 500ć with good thermal shock resistance. Compared with the results in Ref.[12] (SiC-MoSi2- (Ti0.8Mo0.2)Si2 multi-composition coating, 2.18% mass loss at 1500ć for 49h for C/C composites) and Ref.[13] (a dense C/SiC gradient oxidation protective coating, 2.46% mass loss at 1 500 ć for 35 h for C/C composites), the oxidation resistance is obviously improved, and the protective temperature is higher than that of SiC/Si-MoSi2 coating for carbon materials (reported to be 1 400 ć[14]). Compared with three-layer Si-Mo coating, the excellent oxidation resistance of SiC/Si-Mo multilayer coating should be attributed to the alternated structure, especially the outer SiC layer. CVD SiC coating deposited at 1 100 ć is usually quite dense, and even a very thin layer could play the dual role of physical and chemical barrier. On the one hand, the SiC layer hinders the diffusion of oxygen in and Si(g) out. On the other hand, it is oxidized into dense SiO2 glass and resists the oxidation of Si-Mo layer. These prolong the coating lifespan. The excellent thermal shock resistance of the SiC/Si-Mo multilayer coating is attributed to the Si-Mo layer. The porous Si-Mo layer has lower elastic modulus, which helps reducing thermal stress and thermal expansion mismatch between the coating and the substrate[15]. These pores provide sites for volume expansion of SiO2 formation. These factors ensure the excellent thermal shock resistance of the multilayer coating. 4 Conclusions Owing to the evaporation of Si and higher chemical activity of Si-Mo layer, the MSI C/C-SiC substrate with and without three-layer Si-Mo coating cannot be used at 1 500 ć. For SiC/Si-Mo multilayer coating, the outer SiC layer plays the dual role of physical and chemical barrier, which hinders the diffusion of oxygen in and Si(g) out, and resists the oxidation of Si-Mo layer. The coating could provide longtime protection for C/C-SiC composites at 1 500 ć. The porous Si-Mo inner layer ensures the coating has excellent thermal shock resistance. 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