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examples of frequently used infiltrants. In order to fill the smallest pores, impregnation can be carried out using a low pressure (50 psi; 3445 Pa).

Since the density of resin char is relatively low, resin densification requires more cycles than those necessary using other precursors. Up to five cycles of resin impregnation are required to obtain a composite with a density of about 1.5 g/cm3 to 1.8 g/cm3 (ref. 4).

Gas Phase Infiltration Process

The CVI process for carbon deposition uses volatile hydrocarbon compounds such as methane, propane, or benzene as precursor gases. Thermal decomposition of any of these gases is achieved on the hot surfaces of the substrate, resulting in a deposit of pyrolytic carbon and the emission of volatile by-products, which consist mainly of hydrogen.

One of the problems associated with CVI is that under isothermal conditions, pyrolytic carbon deposits preferentially on the surface of the substrate; such deposits change open porosity to closed porosity and make the filling of internal pores difficult. Surface deposits always tend to build up rapidly if diffusion of the reacting gases to the deposition surface is allowed to control the overall reaction rate. This effect can be reduced, however, if chemical reaction on the external surface and inner surfaces is the rate controlling step. Kinetic studies (ref. 39) indicate that low temperatures tend to limit reaction while having relatively little effect on diffusional transport. Reaction rate control of the deposition process and hence more complete densification is therefore favored by low temperatures. Factors that influence the structure, uniformity, and rate of deposition of a CVI matrix include the nature of the substrate, the carrier gas temperature, the composition, the pressure, and the geometry (particularly the thickness) of the final structure (refs. 39 through 42).

Typical effects, summarized in figure 5 (ref. 43) illustrate how the microstructure of the CVI carbon deposit can be made to vary from columnar through laminar to isotropic by altering the gaseous composition and deposition temperature. Essentially, high-temperature, low-propane concentrations favor isotropic deposits, and low-temperature, high-propane concentrations favor a columnar structure. In separate work, Jachlewski and Diefendorf (ref. 41) confirmed the findings of these authors, but also showed that isotropic deposits can be formed at temperatures less than those indicated (<1000°C). Figure 6 (ref. 43) also illustrates how the elastic modulus of the CVI deposit varies with processing conditions. A smooth laminar deposit seems to be the preferred deposit if the modulus is maximized.

The effect of pressure has been studied by Kotlensky (ref. 39) who produced the plot shown in figure 7. In this case, the required laminar deposit can be maintained at higher temperatures if deposition is carried out at higher pressure. There are

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Figure 5. Effect of deposition condition on microstructure of pyrolytic carbon matrix deposition from propane (ref. 43). Reprinted with permission.

three methods of forming CVI carbon: isothermal, thermal gradient, and pressure gradient.

Isothermal Chemical Vapor Deposition

In this process, a woven structure is placed within a furnace susceptor and is heated uniformly. The pressure and the temperature of the furnace are kept constant at typical values of 1 psi (6 KPa) and 1100°C, respectively. The flow rate of hydrocarbon gas is predetermined depending on the surface area of the substrate. Intermittent machining of the surface is required because the chemical vapor deposition (CVD) technique leaves a crust on the outer surface of the substrate. The machining cycle is repeated until the desired density is achieved (ref. 4).

Thermal-Gradient Chemical Vapor Deposition

In this technique, a carbon preform is supported on a mandrel. Inductive coils heat the surface of the mandrel to a temperature of about 1100°C. The hottest portion of the substrate is in contact with the mandrel, while the other side,

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Figure 6. Variation of elastic modulus with deposition conditions from propane (ref. 43). Reprinted with permission.

in contact with the reacting gas, is cooler; thus, a thermal gradient through the substrate thickness is created. As the hydrocarbon gas passes through the furnace at atmospheric pressure, carbon is deposited on the hottest region of the woven structure. This hot section migrates through the thickness of the structure as the densified region grows toward the colder surface. This technique prevents the formation of a crust on the outer surface of the preform; thus, the machining step is eliminated. Unfortunately, the process tends to be restricted to large individual parts.

Differential Pressure Chemical Vapor Deposition

Differential pressure CVD is a variation of the isothermal technique in which the inner portion of the fiber preform is sealed off from the furnace chamber at the base. Hydrocarbon gases are fed into the inner cavity at a positive pressure with respect to the furnace chamber. A pressure difference that forces the hydrocarbon to flow through the pores depositing carbon and exiting as hydrogen (refs. 4 and 39) is created across the wall of the structure. This technique also prevents the formation of an outer crust on the surface of the preform and facilitates densification uniformity.

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Figure 7. Relationship of density and structure with temperature and pressure (ref. 39). Reprinted by permission of the Society for the Advancement of Material and Process Engineering. Kotlensky, W. V.: A Review of CVD Carbon Infiltration of Porous Substrates. SAMPE J. 16, 1971, p. 257.

Matrix Inhibition

The oxidation of carbon becomes quite rapid at temperatures above approximately 500°C (ref. 44). The exact rate of oxidation has been shown to depend on the active surface area, the porosity, the degree of crystallinity, the purity, the internal stress, and the absolute temperature (refs. 45 and 46). It has been shown (refs. 6, 7, and 47) that levels of 2 to 5 percent weight loss during oxidation leads to a substantial degradation (40 to 50 percent) of mechanical properties. Although short-term high-temperature applications (i.e., rocket nozzles and exhausts) may only require limited oxidation protection, long-term exposure normally requires that a surface barrier coating be present (ref. 48).

Surface coatings are generally composed of refractory layers designed to separate the oxidizing gases from the composite. Unfortunately, most refractory coatings exhibit coefficients of thermal expansion that differ significantly from the CC substrate. Temperature changes produce stresses in the coating that are sufficient to cause cracking; thus, a glassy layer designed to flow and seal such

cracks (refs. 18, 44, and 48) is also incorporated. Oxidation protection using barrier layer techniques is the subject of a different chapter in this publication and will not be considered here. However, it is recognized that oxidation of the basic carbon structure should be prevented. Appropriate treatments are therefore applied to the matrix (and fiber) to achieve a minimum oxidation rate. Unfortunately, the details of most of these treatments are proprietary.

The density of a composite is particularly important because gaseous penetration of any coating can produce gasification at both internal and external surfaces. A highly porous structure will gasify at a high rate, and it is for this reason that the rate of oxidation of any carbon tends to increase initially because of the expansion of internal surface area. Eventually, however, a decrease in reaction rate tends to occur as the less ordered structure (less graphitic) is gasified. Donnet, in a series of articles (refs. 49 and 50), has emphasized the importance of crystalline order; "...the oxidation rate (of carbon) is strongly dependent on the structure of the carbon layers, less organized parts appearing more reactive than better organized parts...." In these experiments, it was stated that less graphitic structures always oxidize first, leaving the more graphitized material.

Gasification is extremely sensitive to purity; many metallic impurities, even in very small amounts, are known to be very aggressive oxidation catalysts (refs. 31 and 51). Elements such as iron, calcium, lead, copper, vanadium, chromium, manganese, nickel, and cobalt have been shown to increase the rate of gasification of carbons. Baker (ref. 52) has described a series of electron microscope experiments which emphasize the catalytic effect of various metals (and some oxides) on the gasification of pure graphite. Attempts have been made to improve the oxidation resistance by removing impurities. Acid washing, for instance, has been shown to be beneficial (ref. 8), as has purification of the original feedstock and purification of the carbon by high-temperature (3000°C) halogen treatments (ref. 53). These high-temperature treatments have the additional effect of graphitizing suitable matrices, as well as reducing porosity (ref. 54).

As stated previously, a maximum oxidation resistance is obtained by coating the outer surfaces of a structure with a layer of material specifically designed to stop oxygen from contacting the carbon substrate. A viscous glass, frequently silicon based, is usually incorporated into this coating. This material is designed to flow at high temperatures and fill any cracks that might develop. An alternative approach is particularly useful at lower temperatures or for conditions in which oxygen has diffused through the outer coating or along cracks. Low-temperature protection can be achieved by adding a boron containing compound to the matrix precursor which on oxidation forms a viscous borate glass that covers the internal surfaces (ref. 18). This type of protection is similar but involves a material less viscous than the silicate glass formed at higher temperatures (refs. 18, 44, and 48). Other inhibition treatments involve the addition of materials to the matrix that can also produce glass sealants. A JTA quality nuclear graphite contains

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