CVD-SiC fibers

Properties. Properties of SiC fibers are shown in Table 19.2. They are similar to those of CVD boron fibers except that SiC is more refractory and less reactive than boron. CVD-SiC fibers retain much of their mechanical properties when exposed to high temperature in air up to 800°C for as long as one hour as shown in Fig. 19.3. [ 1 SiC reacts with some metals such as titanium in which case a diffusion barrier is applied to the fiber (see Sec. 2.5 below). 

Applications. Most applications of CVD SiC fibers are still in the development stage. They include the followingP k b... 

Figure 6.27 Comparison of the creep behavior of Nicalon, Hi-Nicalon and CVD SiC fiber. Note the superior performance of the CVD fiber, which is attributable to stoichiometric /3-SiC (after Dicarlo, 1994)...

N. P. Bansal, Influence of Fiber Volume Fraction on Mechanical Behavior of CVD SiC Fiber/SrAl2Si20g Glass-Ceramic Matrix Composites, SAMPEJ. Advanced Mater., 28 [1] 48-58 (1996). 

N. P. Bansal, CVD SiC Fiber-Reiirforced Barium Aluminosilicate Glass-Ceramic Matrix Composites, Mater. Sci. Eng. A, 220 [1-2] 129-139 (1996). 

The strength level of fibers obtained from the vapor phase is generally higher than that of equivalent fiber made from the melt or precursor fibers, reflecting the fact that they are made directly from the vapor phase and/or by a containerless process. For example, single crystal CVD-SiC whiskers (7.5 GPa) are stronger than polycrystalline CVD-SiC fibers (7.5 vs. 3.5 GPa) and both are stronger than polycrystalline SiC fibers which are made from solid precursor fibers (1.1-3.0 GPa). 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

Lara-Curzio, E. and Stemstein, S. (1993) Thermoelastic analysis of composite CVD SiC fibers. Comp. Sci. Technol., 46 265-275. 

Fiber Diameter. Since a core is required, it is impractical to produce small-diameter fibers (150 pm vs 8-15 pm for sol-gel derived fibers). Being stiff with a high modulus and large diameter, CVD-SiC fibers cannot readily be bent to a small radius or along compound shapes. They are difficult to weave into fabrics. Their use is limited to parts having a simple geometry such as plates, rods or tubes. A fiber cross-section is shown in Fig. 14.7.1501... 

Silicon CarbidG Fibers. Silicon carbide (SiC) filaments are produced by a CVD technique. The y3-SiC is obtained by the reaction of silane and hydrogen gases with the carbon filament being the substrate for deposition. The SiC fibers have mechanical and physical properties equal to those of boron, and can be used at higher temperatures than the present boron fiber when available in production quantities. CVD SiC fibers are primarily used for reinforcing metal and ceramic matrices. Alternatively, SiC fibers can be made from a polycarbosilane precursor which is meltspun at 350°C. The final form is obtained by pyrolyzing the fiber at 1200°C in an inert environment. 

Nicalon and Tyranno ceramic fibers, two well-known preceramic derived commercial products, are marketed for structural applications. Nicalon is a SiC based ceramic fiber processed using chemistry and techniques first developed by Yajima and coworkers [6-14]. Tyranno fibers are SiC/TiC based fibers produced via novel modifications to the original Yajima work [15-17]. Elastic moduli and tensile strengths for both fibers are of the order of 250-300 GPa and 2-3 GPa respectively. Textron s CVD SiC fibers (not preceramic) offer tensile strengths of up to 4 GPa [18]. The elastic modulus of sintered, hot pressed SiC is in the range of 400-450 GPa [19]. These compare with tensile strengths of =< 8 GPa and an elastic modulus of= 580 GPa for single crystal, SiC whiskers [18]. 

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

Thistable(and Table 19.2below) shows that the major competitor to CVD SiC is carbon as both fibers have similar properties and are in the same cost bracket. Another competitor is boron but it is expensive and may eventually be replaced by silicon carbide. 

Properties. CVD boron fibers have high strength, high modulus, and low density. Their properties are summarized and compared with SiC fibers and other inorganic fibers in Table 19.2 (data supplied by the manufacturers). 

The reinforcing fibers are usually CVD SiC or modified aluminum oxide. A common matrix material is SiC deposited by chemical-vapor infiltration (CVI) (see Ch. 5). The CVD reaction is based on the decomposition of methyl-trichlorosilane at 1200°C. Densities approaching 90% are reported.b l Another common matrix material is Si3N4 which is deposited by isothermal CVI using the reaction of ammonia and silicon tetrachloride in hydrogen at 1100-1300°C and a total pressure of 5 torr.l" " ] The energy of fracture of such a composite is considerably higher than that of unreinforced hot-pressed silicon nitride. 

These high demands are not yet fulfilled by any available fiber coating. Only a C-coated SiC-fiber (NL 607) from Nippon Carbon is commercially available. To meet the above demands, a multilayer fiber coating is necessary. In a joint effort with ABB Heidelberg and TU Chemnitz, a CVD C-coating on C- and SiC-fibers and a C/SiC double CVD coating on C-fibers was developed and tested (Fig. 3). 

SiC monofilaments produced by the CVD process is generally superior to Nicalon SiC fibers in mechanical properties because of its almost 100% 6-SiC purity while Nicalon is a mixture of SiC, Si02 and free carbon. Representative properties of SiC monofilaments and Nicalon fibers are given in Table 5.15. 

Prouhet, S.. Camus, G, Labrugere, C., Gette, A. and Martin, E, (1994). Mechanical characterization of SiC fiber/SiC (CVD) matrix composites with a BN-interphase. J. Am. Ceram. Soc. 77, 649-659. Riess. G., Bourdeux, M., Brie, M.,. louquet, G. (1974). Carbon Eibers - Their Place in Modern Technology, Plast. Inst., London, p, 52. 

As noted earlier, CVl is nsed primarily to form ceramic-fiber-reinforced ceramic matrix composites. The most common of these combinations is SiC fiber/SiC matrix composites. One commercially available product has a two-dimensional 0/90 layup of plain weave fabric and fiber volume fraction of about 40%. This same composite can be fabricated with unidirectional fibers and with 45° architectures. The most commonly used SiC fiber for the preforms is Nicalon , the mechanical properties for which were provided earlier in Section 5.4.2.7. A number of other carbide and nitride fibers are also available, including Si3N4, BN, and TiC. Preform geometries can be tailored to the application in order to maximize strength and toughness in the direction of maximnm stresses. The reactions used to form the matrix are similar to those used in CVD processes (cf. Section 7.2.4) and those described previously in Eq. (3.105). 

The need to develop fibers with better microstructural stability at elevated temperatures and ability to retain their properties between 1000-2000°C. The requirements of fiber properties for strong and tough ceramic composites have been discussed by DiCarlo.83 A small diameter, stoichiometric SiC fiber fabricated by either CVD or polymer pyrolysis, and a microstructur-ally stable, creep-resistant oxide fiber appear to be the most promising reinforcements. 

Fibers that retain their high strength to temperatures in the range 1200-1500°C are needed for use as the reinforcement in ceramic and metal matrix composites. This portion of this chapter deals with the fabrication of SiC fibers using CVD. It is divided into sections on current status, hot- and cold-wall reactor designs, stresses in coated fibers, processing results, and economics. 

An engineering economic analysis was performed in order to estimate the costs associated with the manufacture of SiC fiber by the process just described. The basis for the analysis was an assumed plant consisting of ten or more CVD reactors each 10 feet in length. It was assumed that a carbon tow substrate such asT-300 was being coated with... 

Chemical vapor deposition is one of the most important deposition techniques for forming ceramic films and coatings. We described two examples in Chapter 20 in which CVD is used in composites. It is used to form SiC fibers by the reaction between CHsSiCls and H2 on a tungsten wire. It is also used to form the matrix phase in a ceramic matrix composite (CMC) by a process known as chemical-vapor infiltration. In later chapters we describe the CVD technique again, e.g., it is used in the formation of optical fibers. 

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