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. Coatings with residual Si should be avoided to use at 1 500 ć solely, but they can act as inner layers when dense outer barriers exist. References [1] SCHULTE-FISCHEDICK J, SCHMIDT J, TAMME R, KRONER U, ARNOLD J, ZEIFFER B. Oxidation behavior of C/C-SiC coated with SiC-B4C-SiC-cordierite oxidation protection system [J]. Mater Sci Eng A, 2004, 386(1/2): 428í434. [2] YAN Zhi-qiao, XIONG Xiang, XIAO Peng, HUANG Bai-yun. Oxidation kinetics and mechanism of C/SiC composites fabricated by MSI process [J]. Journal of Inorganic Materials, 2007, 22(6): 1151í1158. (in Chinese) [3] SNELL L, NELSON A, MOLIAN P. A novel laser technique for oxidation-resistant coating of carbon-carbon composites [J]. Carbon, 2001, 39(7): 991í999. [4] ZHANG Y L, LI H J, FU Q G, LI K Z, HOU D S, FEI J. A Si-Mo oxidation protective coating for C/SiC coated carbon/carbon composites [J]. Carbon, 2007, 45(5): 1130í1133. [5] LI H J, XUE H, WANG Y J, FU Q G, YAO D J. A MoSi2-SiC-Si oxidation protective coating for carbon/carbon composites [J]. Surf Coat Technol, 2007, 201(24): 9444í9447. [6] FU Q G, LI H J, LI K Z, SHI X H, HU Z B, HUANG M. SiC whisker-toughened MoSi2-SiC-Si coating to protect carbon/carbon composites against oxidation [J]. Carbon, 2006, 44(9): 1866í1869. [7] HUANG J F, ZENG X R, LI H J, LI K Z, XIONG X B. Oxidation behavior of SiC-Al2O3-mullite multi-coating coated carbon/carbon composites at high temperature [J]. Carbon, 2005, 43(7): 1580í1583. [8] YAN Zhi-qiao, XIONG Xiang, XIAO Peng, CHEN Feng, HUANG Bai-yun. Oxidation behavior of Mo-Si coated C/SiC composites [J]. Aerospace Materials & Technology, 2007(6): 39í43. (in Chinese) [9] YAN Z Q, XIONG X, XIAO P, CHEN F, ZHANG H B, HUANG B Y. A multilayer coating of dense SiC alternated with porous Si-Mo for the oxidation protection of carbon/carbon silicon carbide composites [J]. Carbon, 2008, 46(1):149í153. [10] SCHUBERT T, BOHM A, KIEBACK B, ACHTERMANN M, SCHOLL R. Effects of high energy milling on densification behavior of Si-Mo powder mixtures during pressureless sintering [J]. Intermetallics, 2002, 10(9): 873í878. [11] FANG Hai-tao. A Si-Mo fused slurry coating of C/C composites and the oxidation resistance [D]. Harbin: Harbin Institute of Technology, 2001. 81í83. (in Chinese) [12] JIAO G S, LI H J, LI K Z, WANG C, HOU D S. SiC-MoSi2-(Ti0.8Mo0.2)Si2 multi-composition coating for carbon/ carbon composites [J]. Surf Coat Technol, 2006, 201(6): 3452í3456. [13] ZHANG Y L, LI H J, FU Q G, LI K Z, WEI J, WANG P Y. A C/SiC gradient oxidation protective coating for carbon/carbon composites [J]. Surf Coat Technol, 2006, 201(6): 3491í3495. [14] ZHAO J, GUO Q G, SHI J L, ZHAI G T, LIU L. SiC/Si-MoSi2 oxidation protective coatings for carbon materials [J]. Surf Coat Technol, 2006, 201(3/4): 1861í1865. [15] DING S Q, ZENG Y P, JIANG D L. Thermal shock resistance of in situ reaction bonded porous silicon carbide ceramics [J]. Mater Sci Eng A, 2006, 425(1/2): 326í329. (Edited by YUAN Sai-qian)