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order of magnitude greater when moisture is present. This same enhanced diffusion of oxygen in moist environments was noted by Singhal [7.66] and Narushima et al. [7.67].

Other Gases. Hydrogen can react with SiC-forming silicon or with carbon-forming methane [7.37]. This reaction is negligible below 1100°C in essentially dry environments (containing =100 ppm of moisture). With increasing amounts of moisture, this temperature limit increases reaching about 1300°C at moisture contents as high as 10%. The dissociation of molecular hydrogen occurs at temperatures above 1100°C or at lower temperatures by heterogeneous surface reactions. Once dissociated, hydrogen can become extremely reactive. The dissociation of hydrogen by heterogeneous surface reactions is much easier on metals than on carbon or ceramics. Hallum and Herbell [7.68] reported a weight loss at 1000°C, grain boundary corrosion at 1100°C, and both grain and grain boundary corrosion at 1300°C for samples of SiC exposed to pure hydrogen. The effects of weight loss and corrosion were noted at times as low as 50 hr. After 500 hr at 1100°C and 1300°C, the room temperature MOR decreased by one-third.

Carbon-Carbon Composites

Carbon-carbon (i.e., carbon fiber reinforcement and carbon matrix) composites are probably the only materials that possess a combination of high strength/weight ratio, very low thermal expansion, excellent thermal shock resistance, and strength retention over a wide temperature range. This combination of properties makes them highly desirable in the aerospace industry. The major drawback for widespread use of C/C composites, however, is their poor oxidation resistance above 500°C. Only through the use of oxygen barrier coatings can C/C composites be useful at elevated temperatures in oxidative environments. Silicon nitride applied by CVD has proven to work well as an oxygen barrier for applications in rapid thermal cycles up to 1800°C [7.69]. For less rapid cycling to lower temperatures (<1500°C) and thermal soaking at temperatures between 600°C and 1000°C, multilayer coatings containing

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boron with CVD Si3N4 or SiC overlays have been tested. Although the boron can form a low-temperature liquid that will seal microcracks, its use at high temperatures may be limited due to volatility and the high fluidity of the glass formed. Labruquere et al. [7.70] reported that silicon-rich coatings on the carbon fibers enhanced their oxidation resistance. Although the coating did not stop oxidation, it oxidized at a slower rate than the carbon fibers and was confined to the region between the coating and the fiber and between the coating and the matrix. After testing silicon-rich and boron-rich Si–B–C coatings, Labruquere et al. concluded that to protect the carbon/carbon composite efficiently, the following parameters were required:

1.The coating must have a Pilling and Bedworth* coefficient greater than 1.

2.The coating must oxidize at a faster rate than the carbon.

3.The coating must have a minimum thickness (~100 nm).

4.The coating must not react with the carbon fibers.

5.The coating must form a stable oxide.

In another study, Labruquere et al. [7.71] found that wet air (500 L/hr of 3 vol.% H2O) increased the oxidation rate by a factor of 3.

Cawley et al. [7.28] reported that the crystallographic orientation of well-crystallized carbon determined the rate of oxidation. Thomas [7.72] has shown that oxidation parallel to the c-axis was lower by 30 times compared to the basal plane and that the rate exhibited about 10% anisotropy in the basal plane at 800°C.

The reaction of carbon with hydrogen forms methane at low temperatures. Other products begin to form as the

* The Pilling and Bedworth coefficient is defined as the ratio of the volume of silica formed to the volume of the ceramic consumed. See N.G. Pilling and R.E. Bedworth, J. Int. Met., 29, 529 (1923).

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temperature is raised above 1400°C. The severity of the reaction is dependent upon the amount of atomic hydrogen present. Molecular hydrogen is relatively inert.

Other Nonoxide Matrix Composites

The ternary compound Ti3SiC2 has been reported by Li et al. [7.73] to have excellent properties. When reinforced with SiC (in situ formed), a weight gain of 7.9 mg/cm2 was reported for heat treatment in air at 1200°C for 21 hr. At all temperatures between 1000°C and 1500°C, TiO2 formed on the surface. SiO2 was present on the surface only at 1000°C. At higher temperatures, the outer surface layer was porous allowing oxygen diffusion to the interior, which allowed the formation of silica along with TiO2 in the interior layer. The porous outer layer caused the weight gain at 1300°C to be approximately 2.5 times greater than at 1200°C.

Nicalon™ fiber reinforced TiB2 matrix composites have been tested for use in the production of aluminum. Exposure to molten aluminum for 24 hr caused heavy attack of the TiB2 and delamination of the composite. This deterioration was attributed by Lowden et al. [7.74] to retain chlorine that caused premature dissolution of TiB2 into aluminum. The retained chlorine was from the TiCl4 precursor material and an infiltration temperature of 900° C. When an infiltration temperature of 1200°C was used, the TiB2 was unaffected even after 10 weeks of exposure to molten aluminum. These longer exposure times, however, resulted in attack of the reinforcement fibers.

Various ceramic matrix materials have been manufactured from the pyrolysis of polymers, called preceramic polymer precursors. Although these materials are supposedly nonoxides, one must be careful as to the actual production route and source. According to French [7.23], the polysilazane materials are moisture-sensitive and therefore yield a ceramic high in oxygen. Decomposition reactions of the type:

(7.11)

Copyright © 2004 by Marcel Dekker, Inc.