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Sep 6, 2004 - Two types of large diameter SiC CVD filaments have been investigated on both chemical and mechanical standpoints: a 100#m filament with a ...
JOURNAL

OF MATERIALS

SCIENCE

19 ( 1 9 8 4 )

2731-2748

SiC filament/titanium matrix composites regarded as model composites Part 1 Filament microanalysis and strength characterization P. M A R T I N E A U , M. L A H A Y E , R. P A I L L E R , R. N A S L A I N Laboratoire de Chimie du Sol/de du CNRS, UniversitO de Bordeaux-I 351, Cours de la LibOration, 33405 Talence, France M. C O U Z I , F. CRUEGE Laboratoire de Spectroscopie Infrarouge associ~ au CNRS, LA 124, Universit~ de Bordeaux-l, 351, Cours de la Libkration, 33405 Talence, France Two types of large diameter SiC CVD filaments have been investigated on both chemical and mechanical standpoints: a 100#m filament with a tungsten core (from SNPE) and three 140#m filaments with carbon cores and surface coatings (from AVCO). On the basis of microprobe (X-ray, Auger and Raman), X-ray diffraction and SEM analyses, it appears that the former is made of a single homogeneous stoichiometric SiC deposit while the latter are mainly made of two concentric shells (the inner being a SiC+C mixture and the outer almost pure SIC). All the C-core filaments had received a surface coating (either pure pyrocarbon or SiC+C mixture) presumably to protect the brittle SiC deposit against abrasion due to handling in opposition to the W-core filament which seems to have no surface coating at all. The W-core filament, although smaller in diameter, is weaker than the C-core filaments (average UTS of 3 and 4 GPa respectively for a 40 mm gauge length). However, its strength distribution is much narrower (We/bull moduli of 7 - 8 and 2 - 5 respectively). Failures of most filaments appear to have a multimodal character.

1. Introduction The mechanical properties of metal matrix composites are closely related to the fibre-matrix (FM) interfacial phenomena occurring at high (or even medium) temperatures during their synthesis or utilization. This is mainly due to the fact that brittle fibres (e.g. carbon, silicon carbide, boron nitride and to a lesser extent alumina) or filaments (coated or uncoated boron, silicon carbide) used for the reinforcement of metals are generally very reactive. As a result, when such fibres or filaments are heated within a metal matrix, chemical reactions, mainly controlled by diffusion, take place at each fibre-matrix interface, giving rise to a FM reaction zone, usually brittle, whose thickness increases against time at a given temperature. When the FM reaction is very limited (reaction 0022-2461/84 $03.00 + .12

zone of the order of a few thousands of nanometres), it may have a beneficial effect since it results in strong interfaces and good adhesion allowing an efficient load transfer from matrix to fibres. On the contrary, when the FM reaction becomes too important, it may generate a new population of stress concentrators at the brittle fibre surface with a strong weakening of the reinforcing fibrous material. Therefore, the knowledge of the FM interfacial chemistry is a necessary and fundamental step in the development of any metal-matrix composite. This had been already well illustrated in the case of boron-titanium composites [1-5]. One of the main drawbacks of boron filaments lies in the fact that their strength significantly decreases with increasing temperature. Further-

9 1984 Chapman and Hail Ltd.

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more, boron reacts very rapidly with most light metals used as matrices. Silicon carbide, either as tows of small diameter fibres or large size monafflaments, appears to be a more suitable material for the reinforcement of metals. It has a higher melting point and a much better chemical inertia towards chemical reactants (including air and metals). SiC fibres or f'llaments are characterized by high strength and stiffness and keep a high fraction of their room temperature mechanical performances over a wide range of temperature [6-11]. Titanium matrix composites up to now were of limited practical interest. Nevertheless, they could be used in the future as structural materials for medium temperature applications. On the other hand, they can be regarded as very convenient model composites, from a fundamental point of view and as far as interfacial FM chemistry is concerned. Titanium has a high melting point (0F = 1668 ~ C, with respect to 650~ for magnesium and 660~ for aluminium), a property which makes possible diffusion experiments in a wide range of temperature. It reacts at medium temperature with carbon or silicon and the corresponding binary or ternary phase diagrams are known [ 12]. Finally, titanium matrix composites can be rather easily prepared by hot pressing [13]. For these reasons, a long term research programme was started several years ago in one of the laboratories of the authors (LCS) to illustrate, from the solid-state chemistry point of view, the relationships that exist between FM interfacial chemistry and mechanical behaviour, in metalmatrix composites [13-15]. It is also thought that the results of this research will help to better the understanding of the behaviour or the related materials of much more practical interest. The present work deals with the microanalysis and strength characterization of large diameter silicon carbide based CVD-filaments. 2. T h e silicon carbide C V D - f i l a m e n t s

SiC-based filaments of large diameter (100 to 150/zm) are obtained by CVD, on a wire substrate (tungsten or carbon) heated at 1200 to 1400 ~ C, from different gas mixtures whose main components are: a chlorosilane (e.g. CH3SiCls, (CH3)2SIC12 or CH3SiHC12), hydrogen and eventually an alkane (e.g. CH4 or better CsH~ or CaHto) and/or argon, as schematically shown in Fig. 1 [6-9, 16-18]. The CVD-process does have a few important 2732

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SiC based filament, after [17]. drawbacks which have been already analysed elsewhere. Its main advantage lies in the fact that it can be used to obtain in one single operation filaments with an optimized composition profile from the core to the external surface resulting in much higher strength and stiffness than those reported, up to now, for the fibres of small diameter obtained by spinning and pyrolysis of a liquid organosilane precursor. Strength and Young's modulus, respectively, as high as 4000 MPa and 420 GPa are currently achieved for the 140#m CVD carbon core filaments as compared to only 2500MPa and 180GPa for the 10 to 20/1m fibres derived from polycarbosilane liquid precursors. Moreover, surface coatings can be easily applied on filaments at the end of the CVD-process, in order to promote wetting by liquid metal matrices and/ or to decrease their chemical reactivity with metals. Therefore, SiC-CVD filaments do appear as very high performance fibrous materials for the reinforcement of metals. The CVD of SiC-based materials from chiorosilanes has been recently the subject of several thermodynamic and experimental studies. Christin et al. [19, 20] have shown, from thermodynamic calculations, that the nature of the deposit which is formed at equilibrium from CH3SiCls(MTS)-

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H2 gas mixtures, depends on the initial or= [H2m.]/[MTSi~.] ratio, at a given temperature and total pressure. The deposit is made of pure SiC for o q < o t < a s (with a i " 14, a s " 8 0 0 0 at 1600K and 1 atm) and of a mixture of SiC and elemental carbon for a < oti. Gas mixtures very rich in hydrogen (a > as) lead to SiC+Si and even to pure silicon deposits (Fig. 2). Increasing the deposition temperature does not change significantly the ai value, but strongly modifies the ~sic and r/c thermodynamic yields (Fig. 3). When hydrogen is partly replaced by argon, SiC+C deposits are obtained in

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2733

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100 important role played by hydrogen added to "qx(%) chlorosilanes [23-32]. Four samples of SiC-CVD filaments have been 80

analysed and used for the synthesis of SiC/Ti composites. All of them are experimental products. One of them, referred to as SIC(3), is a filament, 100tam in diameter and with a tungsten core, developed a few years ago by SNPE (Paris). The others, from AVCO (Lowell, Mass.), referred to as SiC(I), SIC(2) or SIC(4) are more recent materials but are different, in microstructure, from the SCSfilament presently produced on an industrial level. They are 140jum in diameter and have a carbon core.

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