Silicon Carbide SiC Fibers

The major deficiency of carbon fibers is their sensitivity to oxidation even at relatively low temperatures. Although silicon carbide (SiC) fibers are also sensitive to oxidation, their oxidation starts at higher temperatures and yields a protective silica coating. In an oxidative environment, SiC and Si-C-0 fibers are generally more useful than carbon fibers [1-3]. Large diameter silicon carbide fibers are obtained by chemical vapor deposition (Chapter 4). Small diameter silicon carbide and oxycarbide fibers are derived from solid polydimethylsilazane precursor fibers (this chapter). 

Boron and Silicon-Carbide Fibers. Boron and silicon-carbide (SiC) fibers have high strength and modulus, but have low oxidation resistance and high cost. They are produced by chemical-vapor deposition (CVD) and sol-gel methods. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Silicon carbide (SiC) matrix composites have been fabricated by chemical vapor infiltration (CVl), polymer impregnation and pyrolysis (PIP), and reaction sintering (RS). The RS process can be recognized as an attractive technique, because it offers a high density and good thermal conductivity, compared to those of CVl and PIP process. In general, the fabrication of fiber reinforced SiC matrix composites by reaction sintering involves melt infiltration (Ml) or liquid silicon infiltration (LSI). However, the fabrication of continuous fiber reinforced SiC matrix composites by RS focused in melt infiltration (Ml) such as liquid silicon infiltration (LSl) Vapor silicon infiltration was rarely used for SiC matrix composites. 

The CMC market is divided into two classes, oxide and non-oxide materials. Oxide composites consist of oxide fibers (e.g., alumina [AI2O3]), interfacial coatings, and matrices. If any one of these three components consists of a non-oxide material (e.g., silicon carbide [SiC]), the composite is classified as a non-oxide composite. These classes have different properties, different levels of development, and different potential applications. 

The fibers typically consist of carbon (C), silicon carbide (SiC), alumina (AI2O3), or mullite (Al203-Si02). For the matrix components, alumina, zirconium oxide, and silicon carbide are most commonly used. The terminology of CMC usually follows the principle t3q>e of fiber/t3q>e of matrix. C/SiC stands for a carbon-fiber-remforced silicon carbide. Today, the most important CMCs are C/C, C/C-SiC, C/SiC, and SiC/SiC. In some cases, the term is preceded with the abbreviation of the manufacturing process. 

Silicon carbide (SiC) is a hard semiconductor, and is an inert and inexpensive ceramic used in sandpaper and in grindstones. It is made in bulk on an industrial scale by heating flint with coal tar up to very high temperatures or by means of chemical vapor deposition. SiC fibers are used in technical ceramics as a toughening aid. Like many other III-V semiconductors, it occurs in two modifications, the sphalerite (cubic) and the Wiirtzite (hexagonal) structures. In both structures carbon and silicon are tetrahedrally surrounded by silicon and carbon. The two structures do not differ much in material properties. 

Chemical vapor infiltration (CVI) is a CVD variant capable of internally coating porous objects, e.g., an object made out of carbon fibers, with a ceramic material. Silicon carbide (SiC) or boron carbide (B4C) are examples of ceramic matrix materials that are used in combination with carbon fibers. Strong, light, durable, wear-resistant, and biocompatible joint prostheses made of ceramic-ceramic composites are manufactured by means of CVI. Figure 6.18 shows how the degree of penetration is affected by temperature and pressure. Clearly, to get deposit deep in the interior of the porous object low temperatures are necessary for reaction limitation and low pressures for helping the diffusion. Under these conditions growth rates are low. 

Silicon carbide (SiC) monofilaments are usually made by chemical vapor deposition (CVD) by decomposing a silane such as methyltrichlorosilane (CHjSiCy in a hydrogen atmosphere onto a hot and fast-moving tungsten wire or pyrolitic carbon monofilament at a temperature of 1300°C. The equipment and process is the same as that used for making boron fibers (see Section 18.4.2). The chemical reaction occurring at the surface of the hot substrate is ... 

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. 

The formation of ceramic fibers is illustrated by silicon carbide (SiC), or carborundum. The first step in the production of SiC fibers is the synthesis of a polymer, polydimethylsilane. 

Metal matrix composites (MMCs) are metals that are reinforced with fibers or particles that usually are stiff, strong, and lightweight. The fibers and particles can be metal (e.g., tungsten), nonmetal (e.g., carbon or boron), or ceramic (e.g., silicon carbide (SiC) or (alumina) AljOj). The purpose for reinforcing metals with fibers or particles is to create composites that have properties more useful than that of the individual constituents. For example, fibers and particles are used in MMCs to increase stiffness [/], strength [f ], and thermal conductivity [2], and to reduce weight [f], thermal expansion [3], fiiction [4], and wear [5]. 

As opposed to oxides, carbides such as silicon carbide (SiC) and boron carbide (B4C) are compatible with carbon fibers, and satisfactory composites are produced with these matrices and PAN-based yarn by chemical vapor infiltration (CVI).l l A boron-carbon intermediate coating provides optimum strength and toughness as it prevents fiber degradation. 

The most mature CMCs consist of silicon carbide matrices reinforced with silicon carbide-based fibers (SiC/SiC) and silicon carbide reinforced with carbon fibers (C/SiC). 

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