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Figure l0. CVD SiC outer coating spallation due to sealant glass moisture sensitivity. 2 X.

exposure because of the degradation of borate glass that had formed beneath the SiC coating. Boric acid can be seen in the outer coating cracks.

The relatively high rate at which oxygen permeates borate glasses is not only the cause of outer coating corrosion and the generally negative effects associated with excessive glass formation, but also represents an inherent limitation of the current approach of using borate glasses to seal cracks in outer coatings. The oxygen permeability of B2O3 as a function of temperature is compared with the permeabilities of other materials in figure 11 (refs. 81 to 87). Using this type of information in conjunction with a model of a cracked outer coating with B2O3 filling the cracks, an analysis showed that under ideal conditions even the smallest cracks can allow significant oxidation of the CC substrate over the hundreds of hours of operation required for many important applications (ref. 79).

It is known that chemical modifications of the borate glasses can improve moisture resistance (refs. 74 and 80). Furthermore, it may be possible to vary the physical arrangement and chemistry of the coatings to prevent excessive glass formation. Limiting the glass formation would benefit both the moisture and

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corrosion problems and could allow thinner outer coatings for better adherence. The inherent limitation of high oxygen permeability of the cracked coatings combined with the realization that ideal coating protection is never established or maintained has led to the approach of using coatings within the CC composites to enhance oxidation resistance. Internal coatings aid the overall oxidation resistance of the CC when the external coating is intact and prevent rapid catastrophic oxidation if the external coating develops a major flaw.

Internal Coatings

The oxidation of porous carbon materials can be inhibited at relatively low temperatures by very thin discontinuous layers of oxygenated boron and phosphorus compounds that block the most active oxidation sites (refs. 88 and 89). However, coating internal surfaces and filling pores with coherent diffusion-limiting physical barriers are more effective methods for providing oxidation resistance over a wider temperature range. These internal coatings can be metals, hard ceramics, or glasses. Certain glasses are high in B2O3 or P2O5 wet carbon, and they are able to flow at low temperatures. These characteristics promote the formation of adherent and coherent coatings that can accommodate large strains and remain continuous during thermal cycling and the application of external stresses. Flow and wetting also produce external glass layers when the glasses are present in high concentrations near the surface of the CC.

Coatings have been made on the internal surfaces of CC composites by three techniques: liquid precursor impregnation, chemical modification of the carbon matrix, and CVD. Liquid precursor impregnation is the most chemically comprehensive of the three because with this method a wide range of inorganic coatings can be made. Chemical modifications, however, result in coatings only when a mechanism is present for producing glass layers. Finally, the CVD process is most appropriate for producing nonoxides in the presence of carbon; it can be used to make internal coatings, chemically modify the carbon matrix, or completely replace the matrix.

Liquid Precursor Method

The liquid precursor method involves impregnating the porous CC substrate with organometallics. metal alkoxides, metal salt solutions, or colloidal suspensions. Drying, gelation, or chemical conversion produces solid layers on internal surfaces that can then be decomposed or reacted by heating to form ceramic coatings. Multiple impregnation and heat treatment cycles are needed to produce continuous coatings of significant thickness because of the normally low yield of the precursors. This method has been the subject of numerous patents for improving the oxidation resistance of porous carbon bodies in general and recently was applied to CC composites (refs. 77 and 90 to 94). The recent CC work has shown that by virtue of B2O3 glass formation, internal coatings rich in boron are particularly effective in enhancing oxidation performance (ref. 77). As described earlier, the liquid precursor method is used to coat carbon fibers and can be effective either before or after the fibers have been configured into a composite.

Matrix Chemical Modifications

Chemical modifications are made to the carbon matrix either by adding the elements or compounds as a powder to the resin or pitch matrix precursor, or by altering the chemistry of the matrix precursor on the molecular level (refs. 68 to 70 and 95 to 100). The powder method was originally developed to improve the oxidation resistance of synthetic graphites (refs. 101 to 105). Chemical modifications are most effective and produce coherent coatings only when a lowviscosity, wetting glass is formed initially or when exposed to oxygen. This modification is illustrated in figure 12. which shows a region of borate glass formed by the oxidation of powders that were added to the matrix of a CC composite. The glass was produced near the surface of the composite because of oxygen permeation through a crack in the external CVD SiC coating. The glass forms at the expense of matrix oxidation, then coats fibers, fills pores, and partitions off regions of attack to prevent rapid catastrophic oxidation. Glass that originates in the composite directly below the external coating can also migrate to fill the CTE mismatch cracks in the coating. Chemical modification by glass-former powder additions is currently the

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Figure 12. Borate glass produced within CC composite by oxidation of particulate glass formers. 100 X.

most widely used method for forming internal coatings (refs. 68 to 70, 96, 98, and 99).

Chemical Vapor Deposition

The CVD process within a porous body is termed chemical vapor infiltration (CVI). This process involves the diffusion of gases into the body and the decomposition or reaction of the gases in order to produce a solid product that is deposited on the pore walls and fibers. Chemical vapor infiltration is one of the principal densification techniques used in the fabrication of CC composites (ref. 45). It can also be used to chemically modify the carbon matrix and to produce discrete internal coatings on the carbon constituents (refs. 45, 106, and 107). Unless the carbon fibers are previously coated for protection, the CVI deposit must be a nonoxide to prevent fiber degradation during processing.

Improvements in the oxidation resistance of carbon fiber composites were demonstrated when the carbon matrix was replaced almost entirely with CVI matrix materials such as SiC, TiC, and BN (refs. 108 to 111). From the standpoint of oxidation resistance, it clearly is logical to fully densify with the ceramic if the composite alterations resulting from the replacement of the carbon matrix with a ceramic can be accommodated. The microstructure of a fabric laminate composite with carbon fibers in a CVI SiC matrix that was fabricated at General Atomics is shown in figure 13. Chemical vapor deposition is previously described as a , 80 M" ,

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Figure 13. Microstructure of carbon fiber -SiC matrix composite fabricated by CVI. 250 X.

useful fiber coating method; figure 13 shows that CVI densification actually occurs by the buildup of coatings on the fibers. This type of composite has exhibited excellent short-term oxidation resistance at temperatures up to 1500°C (ref. 108). Over longer periods of time, oxygen permeation through pores and cracks in the matrix results in significant oxidation of the fibers, which demonstrates the need for effective external coatings.

Advantages and Limitations

The advantage of the liquid precursor method for producing internal coatings is the direct application of the coating to areas that are most susceptible to oxidative attack. Disadvantages of this method include the need for multiple processing cycles because of the low yield of most precursors, the need for compatibility between the carbon surfaces and liquid for thorough wetting to yield continuous and adherent coatings, and the need to control the high, potentially disruptive shrinkage that accompanies conversion of the liquid to a solid.

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