Ceramic matrix composites reinforcements used

A reinforcing fiber with high strength and modulus with 2.7 density. Primary purpose for this development was for the reinforcement of metal matrix and ceramic matrix composite structures used in advanced aerospace applications by the military. SiC fibers were developed to replace boron fibers in these RPs, where boron had its drawbacks principally degradation of mechanical properties at temperatures greater than 540C (lOOOF) and very high cost. 

R. L. Callender and A. R. Barron, Formation and evaluation of highly uniform aluminate interfece coatings for sapphire fiber reinforced ceramic matrix composites (FRCMCs) using carboxy late-alumoxane nanoparticles, J. Mater. Sci., 36 4977-A987 (2001). 

In an earlier study, Ducheyne and Hench fabricated composite materials with a bioactive glass (Bioglass ) matrix and stainless steel fibre reinforcement by an immersion technique [25]. Donald et al. have reported on the fabrication of glass and glass-ceramic matrix composites reinforced by stainless steel and Ni-based alloy filaments with diameters in the range 4 to 22 [jum [100, 101]. Glass-encapsulated metal filaments prepared by the Taylor-wire process were used for the fabrication of the composites [100]. 

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). 

Access to phase pure silicon nitride materials via processable precursors is limited to just three approaches. The first, shown in reaction 6, provides one of the first oligomers exploited as a preceramic polymer24,253. This simple polysilazane, containing only Si, N and H, is known to be relatively unstable and will crosslink on its own to give intractable gels. Furthermore, it does not offer the 3Si I4N stoichiometry required for Si3N4. Nonetheless, it is useful as a binder and for fiber-reinforced ceramic matrix composites (CMCs)31. 

Although few applications have so far been found for ceramic matrix composites, they have shown considerable promise for certain military applications, especially in the manufacture of armor for personnel protection and military vehicles. Historically, monolithic ("pure") ceramics such as aluminum oxide (Al203), boron carbide (B4C), silicon carbide (SiC), tungsten carbide (WC), and titanium diboride (TiB2) have been used as basic components of armor systems. Research has now shown that embedding some type of reinforcement, such as silicon boride (SiBg) or silicon carbide (SiC), can improve the mechanical properties of any of these ceramics. 

The model proposed by Rouby and Reynaud46 represents the first systematic approach for understanding how the microstructural damage governs the fatigue life of continuous fiber-reinforced ceramic matrix composites. This model will be used to explain various aspects of fatigue failure in the remaining portion of this chapter. 

High stiffness ceramic fibers such as alumina, alumina-silica, silicon carbide, boron, etc. are used as reinforcement fibers for polymeric, metallic, and ceramic matrix composites (Chawla, 1987). Silicon carbide whisker reinforced alumina composites are used as high speed cutting tools (Chawla, 1993). 

These advanced SiC-based fibers have been widely investigated and used for reinforcement in ceramic matrix composites because of their high tensile strength, high heat resistance, and thin diameters, appropriate for being shaped into fabrics. 

In some applications the lack of toughness of ceramics or CMCs prohibits their use. In cases where enhanced stiffness, wear resistance, or elevated temperature capabilities greater than those provided by metals are necessary, metal matrix composites (MMCs) offer a reasonable compromise between ceramics or CMCs and metals. Typically, MMCs have discrete ceramic particulate or fiber reinforcement contained within a metal matrix. In comparison to CMCs, MMCs tend to be more workable and more easily formed, less brittle, and more flaw tolerant. These gains come primarily at the expense of a loss of high-temperature mechanical properties and chemical stability offered by CMCs. These materials thus offer an intermediate set of properties between metals and ceramics, though somewhat closer to metals than ceramics or CMCs. Nonetheless, like ceramic matrix composites, they involve physical mixtures of different materials that are exposed to elevated temperature processes, and therefore evoke similar thermodyamic considerations for reinforcement stability. 

The market needs for more advanced materials provide the driving force of the CVD processes. For example, the development of high performance aero-engines requires the strong and tough fibre reinforced ceramic matrix composites with low density which can potentially be used in ultra-high temperatures. 

The above CVD techniques have been widely applied and used to fabricate superconductor wires and interphase layers for fibre-reinforced metal- and ceramic- matrix composite materials. 

Silicon carbide has attracted considerable interest because of its good mechanical and physical properties and chemical inertness. One of the most important applications of SiC is to produce a matrix reinforced by fibres, forming ceramic-matrix composites. These composite materials exhibit much better fracture toughness than monolithic ceramics. Compared with carbon/carbon composites, fibre-reinforced SiC matrix composites possess superior oxidation resistance and mechanical properties. The Si-C-H-Cl system (e.g. methyltrichlorosilane, CH3SiCl3) has been used for SiC deposition because it is easy to produce stoichiometric SiC deposits. 

An understanding of the above characteristics and requirements for materials science and engineering forms the basis of the structure of this book, as it summarises precisely the essential knowledge requirements for CVD technology. Whilst the authors tackle a wide range of theoretical topics, the focus of the book is on the fibre-reinforced ceramic matrix composites used by the CVD or chemical vapour infiltration (CVI) processes. Based on the requirement of a systematic understanding of CVD processes, the related materials by some special CVD techniques and their potential applications, the book is structured as follows. 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

---