SHORT COMMUNICATION Effects of SiC coating on ablation resistance of carbon fibre reinforced bn-sin matrix composite B. Li", C.-R. Zhang, F. Cao, S.-Q. Wang, H.-F. Hu and Y -B Cao SiC coating was prepared on the surface of a carbon fibre reinforced BN-Si3Na composite by chemical vapour deposition. The coating was characterised by SEM and XRD, and the ablation behaviours of the coated and uncoated composites were investigated and compared. The coating is mainly amorphous Sic and quite compact; the ablated area of the composite is reduced considerably by the coating and the coated composite presents a lower linear ablation rate of 21- 4%and a lower mass ablation rate of 51- 6%. The Sic coating covers over the pores on the surface of the ablative composite, which prevents the flame from spreading to other regions and from penetrating the inside of the composite. As a result, both the chemical erosion and the mechanical denudation are restrained and the ablation resistance of the composite is improved Keywords: Silicon carbide, Coatings, Chemical vapour deposition, Ablation, Nitride, Composite materials Introduction reinforced nitride matrix composite(CFRN) by chemi cal vapour deposition(CVD)and the ablation beha The physicochemical properties of carbon fibre rein- viour of the coated composite was investigated forced carbon(CFRC) composites make them impor- nose cap or leading edges of a space shuttle. Beyond low Experimental density, thermal stability, thermal shock resistance and a Raw materials low thermal expansion coefficient, it is their ability to The starting preceramic hybrid precursor for the retain high mechanical strength at high temperatures composite is synthesised by mixing perhydropolysila a severe drawback to carbon matrix composites is their more than 1: 10. borazine was synthesised by thermo- poor chemical stability in an oxidising atmosphere. lysis of H,NBH3 in a pressure vessel and PhPS by the Therefore, new high temperature structural materials ammonolysis of dichlorosilane pyridine. I-13 need to be explored Polyacrylonitrile(PAn) based carbon fibres(toray In the authors previous study, a new ablative Co., Japan)were woven into three-dimensional four omposite reinforced by carbon fibres with a hybrid directional fabric(Fig. 1)with x45 vol- %of the fibre matrix comprising BN and Si3 N4 (CFRN) was prepared by Nanjing Fiberglass Research and Design Institute, by the precursor infiltration and pyrolysis process, and it China exhibited excellent ablation resistance. However when Liquid carbosilane used as the precursor of chemical the composite is used in the condition with a high speed vapour deposition were synthesised from halide silane airstream. the mechanical denudation would be serious because of the pores in the matrix derived from the Preparation of CFRN composite fabricating process and the inherent cork-like character CFRN cOI were prepared by precursor infiltra of hexagonal BN. Therefore, a rigid and compact tion and pyrolysis. First, the preform was infiltrated coating on the surface of the composite is desired. The with the above hybrid precursor in vacuum. Th excellent properties including extreme hardness, high preform filled with the precursor was cured at 100C in mechanical toughness and chemical resistance against nitrogen at 8 MPa. Finally, the cured preform was corrosion by oxygen at high temperatures make Sic the pyrolysed in ammonia at 900C. The infiltration and referred coating material.8-10 In the present paper, a pyrolysis cycles were repeated four times to densify SiC coating was prepared on the surface of carbon fibre composites. Deposition of Sic coating Key Laboratory of Advanced Ceramic Fibers and C The deposition experiments were carried out in a hot of Aerospace and Materials Engineering, National University of wall horizontal quartz tube reactor. The diameter of the Defense Technology, Changsha 410073, China tube was 45 mm and the length was l m. Hydrogen corRespondingauthoremailnudt_libin@163.com was used as carrier gas, which delivered the precursor o 2007 Institute of Materials, Minerals and Mining 1132Do1o.1179/174328407X192688 Materials Science and Technology 2007 VOL 23 No 9
SHORT COMMUNICATION Effects of SiC coating on ablation resistance of carbon fibre reinforced BN–Si3N4 matrix composite B. Li*, C.-R. Zhang, F. Cao, S.-Q. Wang, H.-F. Hu and Y.-B. Cao A SiC coating was prepared on the surface of a carbon fibre reinforced BN–Si3N4 composite by chemical vapour deposition. The coating was characterised by SEM and XRD, and the ablation behaviours of the coated and uncoated composites were investigated and compared. The coating is mainly amorphous SiC and quite compact; the ablated area of the composite is reduced considerably by the coating and the coated composite presents a lower linear ablation rate of 21?4% and a lower mass ablation rate of 51?6%. The SiC coating covers over the pores on the surface of the ablative composite, which prevents the flame from spreading to other regions and from penetrating the inside of the composite. As a result, both the chemical erosion and the mechanical denudation are restrained and the ablation resistance of the composite is improved. Keywords: Silicon carbide, Coatings, Chemical vapour deposition, Ablation, Nitride, Composite materials Introduction The physicochemical properties of carbon fibre reinforced carbon (CFRC) composites make them important materials for aerospace applications, such as the nose cap or leading edges of a space shuttle. Beyond low density, thermal stability, thermal shock resistance and a low thermal expansion coefficient, it is their ability to retain high mechanical strength at high temperatures that constitutes their very unusual behaviour. However, a severe drawback to carbon matrix composites is their poor chemical stability in an oxidising atmosphere.1–5 Therefore, new high temperature structural materials need to be explored. In the authors’ previous study,6,7 a new ablative composite reinforced by carbon fibres with a hybrid matrix comprising BN and Si3N4 (CFRN) was prepared by the precursor infiltration and pyrolysis process, and it exhibited excellent ablation resistance. However, when the composite is used in the condition with a high speed airstream, the mechanical denudation would be serious because of the pores in the matrix derived from the fabricating process and the inherent cork-like character of hexagonal BN. Therefore, a rigid and compact coating on the surface of the composite is desired. The excellent properties including extreme hardness, high mechanical toughness and chemical resistance against corrosion by oxygen at high temperatures make SiC the preferred coating material.8–10 In the present paper, a SiC coating was prepared on the surface of carbon fibre reinforced nitride matrix composite (CFRN) by chemical vapour deposition (CVD) and the ablation behaviour of the coated composite was investigated. Experimental Raw materials The starting preceramic hybrid precursor for the composite is synthesised by mixing perhydropolysilazane (PHPS) and borazine; the ratio between them is no more than 1 : 10. Borazine was synthesised by thermolysis of H3N?BH3 in a pressure vessel and PHPS by the ammonolysis of dichlorosilane pyridine.11–13 Polyacrylonitrile (PAN) based carbon fibres (Toray Co., Japan) were woven into three-dimensional four directional fabric (Fig. 1) with y45 vol.-% of the fibre by Nanjing Fiberglass Research and Design Institute, China. Liquid carbosilanes used as the precursor of chemical vapour deposition were synthesised from halide silane.14 Preparation of CFRN composite CFRN composites were prepared by precursor infiltration and pyrolysis. First, the preform was infiltrated with the above hybrid precursor in vacuum. Then, the preform filled with the precursor was cured at 100uC in nitrogen at y8 MPa. Finally, the cured preform was pyrolysed in ammonia at 900uC. The infiltration and pyrolysis cycles were repeated four times to densify the composites. Deposition of SiC coating The deposition experiments were carried out in a hot wall horizontal quartz tube reactor. The diameter of the tube was 45 mm and the length was y1 m. Hydrogen was used as carrier gas, which delivered the precursor State Key Laboratory of Advanced Ceramic Fibers and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author, email nudt_libin@163.com 1132 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 1 March 2007; accepted 16 March 2007 DOI 10.1179/174328407X192688 Materials Science and Technology 2007 VOL 23 NO 9
Li et al. Ablation resistance of carbon fibre reinforced bn-siN, matrix ≥v三 1 Planform (left) and diorama(night) of three-dimensional four directional fabric through the bubbler to the reactor; the Cfrn com- posite was machined into samples of 30 mm diameter 3 X-ray diffraction pattern of Sic coating deposited on and 10 mm thickness, and polished in advance with surface of CFRN composite: deposited coating is particle size of 10 um as deposition substrate. The almost amorphous flowrate of hydrogen was about 5-10 mL min. The deposition experiments were performed at 800oC under The XRD pattern of the coating is shot a total pressure of 1.0 kPa. The deposition period was No speculate peak can be observed except two quite 5 h with a growth rate of <2 um h broad bands: one is from 25 to 45 and the other is from 55 to 70.. Moreover. the curve is thick and coarse. All of Characterisation the above evidences indicate that the deposited coating The ablation test was performed using an oxyacetylene is almost amorphous torch. The pressures of oxygen and acetylene are 0. 4 and Ablation resistance of sic coated cErn 0.1 MPa respectively. The ablation period was 10 s and omposite 3000oC. The linear ablation rates were calculated by Figure 4 shows the photos of ablated CFRN composite six points average thickness change and the mass with a SiC coating and the composite without any ablation rates were calculated by the weight change of coating, and the ablation results of both composites. It samples before and after the test. can be seen that the CFrn composite coated by Sic The crystalline phase and its preferred orientation of exhibits a much smaller ablated area (encircled with a Sic coating were characterised by X-ray diffractometry white ring), only -584% of the area of the uncoated (XRD) at a wavelength of 1 5418 A(Cu K, radiation). compo Dosite. The co ompact SiC coating Important The microstructures of the CVD SiC coating and the part in the ablation process, which effectively prevents ablated surfaces of the composites were observed using a the spread of the flame and the heat. As a result,the scanning electron microscope(JSM-5600LV) ablation rates of the coated cfrn composite are educed considerably. The coated composite experiences Results and discussion a lower average linear ablation rate of 21-4% and a lower average mass ablation rate of 51- 6%. Characterisation of SiC coating deposited by CVD Because the nitride matrix of CFRn comp Figure 2 shows the SEM image of deposited SiC coating the matrix are inevitable. Whe of precursor,pores inside ks pores are easily attacked by the strong airstream and diffusion of heat and the friction on the surfaces are picked up, and the ablation, including thermochemical erosion and mechanical denudation. will be accelerated A heat resistant compact SiC coating covers over the pores on the surface of the ablative composite, which prevents the flame spreading to other regions nearby Moreover, the high hardness and excellent wear resistance of CVD SiC coating can slow the mechanical denudation caused by high speed airstream Microstructures of ablated surface Scanning electron microscopy images of the surface of ablated composite are shown in Fig. 5. The chosen regions are marked as A and B in Fig 4b. The region A is in the centre and the region B is on the edge of the ablated area. Their morphologies are quite different. In he centre of the ablated area, the Sic coating has bee ablated off, and the matrices and fibres are exposed on 2 Scanning electron microscopy image of Sic coating the surface. The ablated carbon fibres display a needle sited on surface of CFRN composite: coating shape. In the region B, the coating still exists with some ppears homogeneous and compact, without any open pores of 1-5 um diameters. In such region, the Sic pores or cracks on it coating resists to the flame for certain time and is still Materials Science and Technology 2007 VOL 23 No 9 1133
through the bubbler to the reactor; the CFRN composite was machined into samples of 30 mm diameter and 10 mm thickness, and polished in advance with a particle size of 10 mm as deposition substrate. The flowrate of hydrogen was about 5–10 mL min21 . The deposition experiments were performed at 800uC under a total pressure of 1?0 kPa. The deposition period was y5 h with a growth rate of y2 mm h21 . Characterisation The ablation test was performed using an oxyacetylene torch. The pressures of oxygen and acetylene are 0?4 and 0?1 MPa respectively. The ablation period was 10 s and the ablation surface temperature was estimated to be y3000uC. The linear ablation rates were calculated by six points average thickness change and the mass ablation rates were calculated by the weight change of samples before and after the test. The crystalline phase and its preferred orientation of SiC coating were characterised by X-ray diffractometry (XRD) at a wavelength of 1?5418 A˚ (Cu Ka radiation). The microstructures of the CVD SiC coating and the ablated surfaces of the composites were observed using a scanning electron microscope (JSM-5600LV). Results and discussion Characterisation of SiC coating deposited by CVD Figure 2 shows the SEM image of deposited SiC coating on the surface of CFRN composite. The coating appears homogeneous and compact, without any pores or cracks on it. The XRD pattern of the coating is shown in Fig. 3. No speculate peak can be observed except two quite broad bands: one is from 25 to 45u and the other is from 55 to 70u. Moreover, the curve is thick and coarse. All of the above evidences indicate that the deposited coating is almost amorphous. Ablation resistance of SiC coated CFRN composite Figure 4 shows the photos of ablated CFRN composite with a SiC coating and the composite without any coating, and the ablation results of both composites. It can be seen that the CFRN composite coated by SiC exhibits a much smaller ablated area (encircled with a white ring), only y58?4% of the area of the uncoated composite. The compact SiC coating plays an important part in the ablation process, which effectively prevents the spread of the flame and the heat. As a result, the ablation rates of the coated CFRN composite are reduced considerably. The coated composite experiences a lower average linear ablation rate of 21?4% and a lower average mass ablation rate of 51?6%. Because the nitride matrix of CFRN composite is converted from the pyrolysis of precursor, pores inside the matrix are inevitable. When the composite is ablated, pores are easily attacked by the strong airstream and become the accesses of the flame. As a result, the diffusion of heat and the friction on the surfaces are picked up, and the ablation, including thermochemical erosion and mechanical denudation, will be accelerated. A heat resistant compact SiC coating covers over the pores on the surface of the ablative composite, which prevents the flame spreading to other regions nearby. Moreover, the high hardness and excellent wear resistance of CVD SiC coating can slow the mechanical denudation caused by high speed airstream. Microstructures of ablated surface Scanning electron microscopy images of the surface of ablated composite are shown in Fig. 5. The chosen regions are marked as A and B in Fig. 4b. The region A is in the centre and the region B is on the edge of the ablated area. Their morphologies are quite different. In the centre of the ablated area, the SiC coating has been ablated off, and the matrices and fibres are exposed on the surface. The ablated carbon fibres display a needle shape. In the region B, the coating still exists with some open pores of 1–5 mm diameters. In such region, the SiC coating resists to the flame for certain time and is still 1 Planform (left) and diorama (right) of three-dimensional four directional fabric 2 Scanning electron microscopy image of SiC coating deposited on surface of CFRN composite: coating appears homogeneous and compact, without any pores or cracks on it 3 X-ray diffraction pattern of SiC coating deposited on surface of CFRN composite: deposited coating is almost amorphous Li et al. Ablation resistance of carbon fibre reinforced BN–Si3N4 matrix Materials Science and Technology 2007 VOL 23 NO 9 1133
Li et al. Ablation resistance of carbon fibre reinforced BN-SiaN, matrix (a) A a uncoated, linear ablation rate 0056 mm s, mass ablation rate 5 10gs; b Sic coated, linear ablation rate 0-044 mm s, mass ablation rate 2. 65 x 10gs 4 Ablation appearances of CFRN composites (a) (b) lum a in centre of ablated b on edge of ablated area 5 Scanning electron microscopy images of ablated surfaces of Sic coated CFRN composite with protection for the composite under the surface. Just References because of the resistance of SiC coating to the flame the strained and the ablation I.O. Paccaud and A Derre: Chem. Vapor. Depos, 2000,6(1), resistance of CFRN composite is improved 2. O. Paccaud and A. Derre: Chen. Vapor. Depas, 2000, 6, (1), Conclusions 3. Y.J. Lee and H. J. Joo: Composites A, 2004. 35A, 1285 4. D. P. Kim and J Economy: Chem. Mater, 1993. 5, 1216 A coating was prepared on the surface of CFRN 5. C G. Cofer, J Economy, YXu,A.Zangvil,ELCurzio,M.K composite by chemical vapour deposition using liqui Ferber and K. L More: Compos. Sci. TechnoL. 1996, 56, 967. anes as recursor and the effects of the coating 6. B. Li, C.R. Zhang F. Cao, S.Q. Wang, Y. B Cao and Y. G on the ablation behaviour of the composite were primarily Jiang: New Carbon Mater, to be published. investigated. The coating deposited was mainly amor- 7. B. Li, C.R. Zhang, F. Cao, S Q. Wang, Y. B. Cao and Y.G. ublished hous Sic and quite compact. The Sic coating played an 8.Y. J Lee and D J. Choi:J. Mater.Sci,2000,35,4519 important part in improving the ablation resistance of the 9. Y. Gogotsi, S. Welz, D. A. Ersoy and M. J. McNallan: Nature, 2001,411,(17),283 reduced obviously and the coated composite presented a 10. JnX, seo, s, x. Yoon, K Nhara and K H. Kim: Thin Solid Films lower linear ablation rate of 21 4% and a lower mass 11.k. Su E.E. remsen ablation rate of 51-6%. The presence of Sic coating ater,1993,5,547 prevented the flame from spreading to other regions and 12. w.v. Hough, C.R. Guibert and G. T. Hefferan: US Patent chemical erosion and the mechanical denudation of the 13. T Isoda, H. Kaya and H. Nishi: J Inorg. Organomet. Polym composite were restrained. As a result, the composite 14. B Li, C R. Zhang H F Hu and G.J. Qi: J. Finct Mater Device coated by SiC exhibited a much better ablation resi 2006,12,(5),447. 1134 Materials Science and Technology 2007 VOL 23 No 9
with protection for the composite under the surface. Just because of the resistance of SiC coating to the flame, the spread of the heat is restrained and the ablation resistance of CFRN composite is improved. Conclusions A coating was prepared on the surface of CFRN composite by chemical vapour deposition using liquid carbosilanes as the precursor and the effects of the coating on the ablation behaviour of the composite were primarily investigated. The coating deposited was mainly amorphous SiC and quite compact. The SiC coating played an important part in improving the ablation resistance of the composite. The ablated area of the coated composite was reduced obviously and the coated composite presented a lower linear ablation rate of 21?4% and a lower mass ablation rate of 51?6%. The presence of SiC coating prevented the flame from spreading to other regions and from penetrating into the composite, and both the chemical erosion and the mechanical denudation of the composite were restrained. As a result, the composite coated by SiC exhibited a much better ablation resistance. References 1. O. Paccaud and A. Derre´: Chem. Vapor. Depos., 2000, 6, (1), 33. 2. O. Paccaud and A. Derre´: Chem. Vapor. Depos., 2000, 6, (1), 41. 3. Y. J. Lee and H. J. Joo: Composites A, 2004, 35A, 1285. 4. D. P. Kim and J. Economy: Chem. Mater., 1993, 5, 1216. 5. C. G. Cofer, J. Economy, Y. Xu, A. Zangvil, E. L. Curzio, M. K. Ferber and K. L. More: Compos. Sci. Technol., 1996, 56, 967. 6. B. Li, C. R. Zhang, F. Cao, S. Q. Wang, Y. B. Cao and Y. G. Jiang: New Carbon Mater., to be published. 7. B. Li, C. R. Zhang, F. Cao, S. Q. Wang, Y. B. Cao and Y. G. Jiang: Carbon, to be published. 8. Y. J. Lee and D. J. Choi: J. Mater. Sci., 2000, 35, 4519. 9. Y. Gogotsi, S. Welz, D. A. Ersoy and M. J. McNallan: Nature, 2001, 411, (17), 283. 10. J. Y. Seo, S. Y. Yoon, K. Niihara and K. H. Kim: Thin Solid Films, 2002, 406, 138. 11. K. Su, E. E. Remsen, G. A. Zank and L. G. Sneddon: Chem. Mater., 1993, 5, 547. 12. W. V. Hough, C. R. Guibert and G. T. Hefferan: US Patent 4150097, 1979. 13. T. Isoda, H. Kaya and H. Nishii: J. Inorg. Organomet. Polym., 1992, 2, 151. 14. B. Li, C. R. Zhang, H. F. Hu and G. J. Qi: J. Funct. Mater. Device, 2006, 12, (5), 447. a uncoated, linear ablation rate 0?056 mm s21 , mass ablation rate 5?4761023 g s21 ; b SiC coated, linear ablation rate 0?044 mm s21 , mass ablation rate 2?6561023 g s21 4 Ablation appearances of CFRN composites a in centre of ablated area; b on edge of ablated area 5 Scanning electron microscopy images of ablated surfaces of SiC coated CFRN composite Li et al. Ablation resistance of carbon fibre reinforced BN–Si3N4 matrix 1134 Materials Science and Technology 2007 VOL 23 NO 9
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