Fabric-reinforced ceramic matrix composites

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). 

FIGURE 12.12 Examples of FGM CMCs (left) the combination of NLM Nicalon SiC fiber reinforced zirconia (in white) and mullite (in black) matrices (right) a Nasicon matrix reinforced with mullite (Nextel) and SiC (NLMTM) fibers (see the sketch). (Reprinted from Colomban, R, Process for fabricating a ceramic matrix composite incorporating woven fibers and materials with different compositions and properties in the same composite, Mater. TechnoL, 10, 89, 1995. With permission.)... 

For each phenomenon, there are also many elements involved which determine the behaviour of each phenomenon. These phenomena are described by a wide range of characteristic time and length values. For the case of CVI fabrication of fibre-reinforced ceramic-matrix composites, the diameter of a molecule and the thickness of the interfacial phase are about 10 1 run and 102nm respectively, whilst the sizes of the substrate/component and the reaction are around 1 m. In addition, elementary chemical reactions occur in a time range of 10 " to 10 4 s, the time for heat transfer and mass transfer is around 1 s to 10 min. By contrast, the total densification time for one CVI run is as long as approximately 102 h. In such cases, it is necessary to establish multiscale models to understand and optimise a CVD process. 

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. 

These fibers have high tensile strength and heat resistance. In recent years, SiC fibers or their fabrics have been used to prepare fiber-reinforced ceramic-matrix composites. Such composites have high strength and high fracture toughness, even at elevated temperatures [5,6], and are a promising class of... 

One approach for fabricating fiber reinforced ceramic matrix composites is the directed oxidation of metals, a process first introduced by Lanxide Corporation [1, 2] and later used successfully to produce turbine engine and aerospace components. Rights to the DIMOX technology, as it was identified, were ultimately acquired by Power Systems Composites, L.L.C., a subsidiary of the Power Systems business of the General Electric Company. 

A.S. Fareed, Ceramic Matrix Composite Fabrication and Processing Directed Metal Oxidation, Handbook on Continuous Fiber Reinforced Ceramic Matrix Composites, ed. R.L. Lehman, S.K. El-Rahaiby Jr and J.B. Wachtman, Jr., ClAC/ACers, 301-324 (1995). 

T. N. Tiegs and K. J. Bowman, Fabrication of Particulate-, Platelet- and Whisker-Reinforced Ceramic Matrix Composites, pp. 91-138 in Handbook on Discontinuously Reinforced Ceramic Matrix Composites, Am. Ceram. Soc., Westerville, OH (1995). 

Fiber reinforced ceramic matrix composites (CMCs) are under active consideration for large, complex high temperature structural components in aerospace and automotive applications. The Blackglas resin system (a low cost polymer-derived ceramic [PDC] technology) was combined with the Nextel 312 ceramic fiber (with a boron nitride interface layer) to produce a sihcon oxycarbide CMC system that was extensively characterized for mechanical, thermal, and electronic properties and oxidation, creep mpture, and fatigue. A gas turbine tailcone was fabricated and showed excellent performance in a 1500-hour engine test. 

Dong et al proposed a facile route to fabricate carbon fiber reinforced ceramic matrix composites (Cf/SiC-BN) by an active-filler-controlled polymer pyrolysis (AFCOP) process. In the proposed process, boron was introduced into the carbon fibers as active filler to form some boron-bearing species by in-situ reactions during the subsequent heat-treatment process. The composites were prepared by PIP using PCS as the polymer precursor. XRD patterns of the obtained composites confirmed the presence of H-BN. With the presence of BN, the oxidation of the composites was greatly improved. The weight losses of Cf/SiC and Cf/SiC-BN after being oxidized at 800°C for lOh were 36% and -16% respectively and most of the carbon fibers in... 

FABRICATION OF CARBON FIBER REINFORCED CERAMIC MATRIX COMPOSITES POTENTIAL FOR ULTRA-HIGH-TEMPERATURE APPLICATIONS... 

Fabrication of Carbon Fiber Reinforced Ceramic Matrix Composites... 

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. 

Chemical vapour infiltration (CVI) is an extension of CVD processes only when a CVD process occurs on an internal surface of a porous substrate (especially for the fibre preform). As compared with CVD, the CVI process for ceramics is much more effective and important because it is the optimal technique to fabricate fibre reinforced ceramics and particularly carbon fibre reinforced carbon and advanced ceramic matrix composites. Both CVI and CVD techniques share some common features in overall chemistry, however, the CVI is much more complex than the CVD process in mass transport and chemical reactions. 

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. 

This section centers on fiber coatings for non-oxide eomposites in which either the fiber or the matrix is a non-oxide ceramic. Although oxide fiber-reinforced composites have been studied, most of the research available in the literature has focused on SiC fiber-reinforced composites. For example, mullite (3Al203-2Si02) fiber-reinforced SiC matrix composites have been fabricated by CVI (chemical vapor infiltration). However, SiC fiber-reinforced SiC matrix (SiC/SiC) composites are superior for the following reasons (1) mullite fiber-reinforced composites do not improve resistance to oxidation, one of the major factors limiting the use of non-oxide composites and (2) SiC fibers have mechanical properties superior to those of mullite fibers. This section will be concerned primarily with SiC fiber-reinforced ceramic composites, which offer the best oxidation resistance of any non-oxide fiber at high temperatures (particularly above 1,100°C [2012°F]). 

Both oxide and non-oxide matrices have been used with SiC reinforcements. Examples include alumina matrix composites fabricated by oxidation of an aluminum melt (DIMOX process) (Newkirk et al., 1986), glass-ceramic matrix composites fabricated by hot pressing (Prewo and Brennan, 1980), SiC matrix composites made by CVI (Stinton et al., 1986), SiC or SiC/Si3N4 matrix composites made by polymer pyrolysis, and SiC-Si matrix composites produced by silicon melt infiltration (Luthra et al., 1993). 

But CMCs will be commercially successful only when they are produced cost-effectively. Polymer-derived ceramic (PDC) technology is one of the most promising low cost fabrication methods for ceramic matrix composites, particularly for large, complex shapes. In PDC technology, a silicon-based polymer (siloxane, carbosilane, silazane, etc) with fiber or particle reinforcement is shaped and cured in the polymer condition and then pyrolyzed in a controlled atmosphere to form a stable silicon-based ceramic, such as silicon carbide, sihcon nitride, silicon oxycarbide, or silicon oxynitride. 

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