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 9SHORT 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 rein￾forced carbon (CFRC) composites make them impor￾tant 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 chemi￾cal vapour deposition (CVD) and the ablation beha￾viour of the coated composite was investigated. Experimental Raw materials The starting preceramic hybrid precursor for the composite is synthesised by mixing perhydropolysila￾zane (PHPS) and borazine; the ratio between them is no more than 1 : 10. Borazine was synthesised by thermo￾lysis 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 infiltra￾tion 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