Composites chemical vapor infiltration

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

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. 

Chemical Vapor Infiltration) [9]. As the carbon fiber itself is the main material, a low cost production process for the composite itself and for coating the composite might be advantageously investigated. 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

Chemical vapor infiltration (CVl) is similar to CVD in that gaseous reactants are used to form solid products on a substrate, but it is more specialized in that the substrate is generally porous, instead of a more uniform, nominally flat surface, as in CVD. The porous substrate introduces an additional complexity with regard to transport of the reactants to the surface, which can play an important role in the reaction as illustrated earlier with CVD reactions. The reactants can be introduced into the porous substrate by either a diffusive or convective process prior to the deposition step. As infiltration proceeds, the deposit (matrix) becomes thicker, eventually (in the ideal situation) filling the pores and producing a dense composite. 

Chemical Vapor Infiltration (CVI). Recall from Section 3.4.2 that CVI is primarily nsed to create ceramic matrix composites, CMCs. Fabrication of CMCs by CVI involves a sequence of steps, the first of which is to prepare a preform of the desired shape and fiber architecture. This is commonly accomplished by layup onto a shaped form of layers from multifilament fibers using some of the techniques previously described, such as filament winding. 

Chung, G. Y. and Benjamin, J.M., Modeling of chemical vapor infiltration for ceramic composites reinforced with layered, woven fabrics , J. Am. Ceram. Soc., 74, 746 (1991). 

C. V. Burkland and J.-M. Yang, Chemical Vapor Infiltration of Fiber-Reinforced SiC Matrix Composites , SAM PE Journal, 25[5], 29-33 (1989). 

A combined analytical and numerical method is employed to optimize process conditions for composites fiber coating by chemical vapor infiltration (CVI). For a first-order deposition reaction, the optimum pressure yielding the maximum deposition rate at a preform center is obtained in closed form and is found to depend only on the activation energy of the deposition reaction, the characteristic pore size, and properties of the reactant and product gases. It does not depend on the preform specific surface area, effective diffusivity or preform thickness, nor on the gas-phase yield of the deposition reaction. Further, this optimum pressure is unaltered by the additional constraint of prescribed deposition uniformity. Optimum temperatures are obtained using an analytical expression for the optimum value along with numerical... 

Chemical vapor infiltration (CVI) is widely used in advanced composites manufacturing to deposit carbon, silicon carbide, boron nitride and other refractory materials within porous fiber preforms. " Because vapor phase reactants are deposited on solid fiber surfaces, CVI is clearly a special case of chemical vapor deposition (CVD). The distinguishing feature of CVI is that reactant gases are intended to infiltrate a permeable medium, in part at least, prior to... 

For preforms of fiber reinforcements, a thin coating is applied to the fibers using chemical vapor infiltration (CVI). This coating step is essential both to protect the fiber from chemical attack by the strongly reducing aluminum alloy and to provide for a weak fiber/matrix interface in the composite. Because the coating is thin, the CVI step requires only a few hours, unlike CVI matrix formation processes, where long times are necessary to achieve sufficient densification. 

D. P. Stinton, Ceramic composites by chemical vapor infiltration. Proc.—Electrochem. Soc. 87-8, pp. 1028-40 (1987). 

Fabrication of fibre-reinforced ceramic composites by chemical vapor infiltration, Caputo and Lackey, 1984 [127]... 

Tai NH, Chou TW (1990) Modelling of an improved chemical vapor infiltration process for ceramic composites fabrication. J Am Ceram Soc 73 1489-1498... 

Rovillain D, Trinquecoste M, Bruneton E, Derre A, David P, Delhaes P (2001) Film boiling chemical vapor infiltration an experimental study on carbon/carbon composites materials. Carbon 39 1355-1365... 

Golecki I (2003) Industrial carbon chemical vapor infiltration (CVI) processes. In Delhaes P (ed) Fibre and composites. Taylor Francis, London, ppl 12-138... 

Tang SF, Deng JY, Wang SJ, Liu WC (2007) Fabrication and characterization of C/SiC composites with large thickness, high density and near-stoichiometric matrix by heaterless chemical vapor infiltration. Mater Sci Eng A465 290-294... 

Tang SF, Deng JJ, Du HF, Liu WC, Yang K (2005) Fabrication and micro structure of C/SiC composites using a novel heaterless chemical vapor infiltration technique. J Am Ceram Soc 88 3253-3255... 

Figure 5.18 is reprinted from Ceramic Engineering and Science Proceedings, Vol. 5, A J Caputo and W J Lackey, Fabrication of fiber-reinforced ceramic composites by chemical vapor infiltration, pp. 654-667,1984, with permission from Wiley. 

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. 

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