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performed for periods of 25 h to 100 h with no weight loss. However, consistently poor performance was shown in static oxidation tests below 760° C because of obviously inadequate sealing by the glaze overlay coating and the boronated SiC conversion.

The initial tests of the ACC structural CC with the boron-modified RCC coating were encouraging for limited-life applications in which the material experiences rapid heating and is required to perform for several hours at temperatures in the 1000°C to 1400°C range. On the other hand, extended-life applications require hundreds of hours of exposure from 649°C to 1371°C. Oxidation at lower temperatures was a problem with the modified RCC coating system, and a major development effort was clearly needed. Furthermore, it was recognized that the basic RCC coating approach had features that posed serious problems for many of the new structural applications. First, the outer glaze coating was susceptible to flow and particulate erosion. The glaze was also susceptible to alkali loss at high temperatures and acted as an adhesive that prevented movement of mating parts without damage to the coating. A second problem was that the proposed structural parts were often 2.5 mm to 5 mm thick and the airfoil trailing edges could be as thin as 1 mm. Very often the coating approached 1 mm in thickness and the mechanically poor SiC conversion layer constituted a large portion of the part.

The outer glaze and coating thickness problems associated with the RCC type coatings have led to the current concept of using a hard, dense outer coating for both limited-life and extended-life applications. The CVD process is used to produce the outer coatings that act as the primary barriers to oxygen ingress. The materials SiC and Si3N4 have received the most attention for outer coatings because of their relatively low CTE values and excellent oxidation resistance up to at least 1700°C. Recent work for limited-life applications has shown that under conditions of rapid heating to above the coating temperature, in which thermal expansion mismatch cracks in the coating are closed, CVD outer layers can provide excellent CC protection for several hours up to 1750°C (ref. 65).

Extended-life applications and even limited-life use at temperatures below that in which the coating cracks are open require a glass sealant to fill the cracks (refs. 65 and 66). Figure 6 shows the current concept of using a boron-rich inner layer to provide the glass for sealing the cracks in the outer coating. Elemental boron, boron carbide, and several configurations of mixed boron compounds with and without SiC and elemental silicon are now under evaluation as inner layers (refs. 65 to 70). Inner layers are being made by particulate slurry coating, CVD, and carbide conversion of the CC surface (refs. 61 to 70). The main purpose of the inner layer is to form a borate glass by oxidation through the cracks in the outer coating. Oxidation of the portion of the inner layer beneath the cracks to form a glass produces a 200-percent to 250-percent volume increase that forces the glass into the cracks. In addition to the outer coating and inner glass-forming layer, a

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base SiC conversion layer that improves bonding to the CC surface will be used often if the glass-forming layer is not a conversion layer.

In addition to acting as the primary oxygen barrier, outer SiC and SisN^ coatings provide hard erosion-resistant bearing surfaces that cover the inner layers and inhibit vaporization of the borate glass sealants. The inner layers provide the sealant glasses but they also must establish and maintain strong bonding with the outer coating and CC to inhibit coating spallation. Conversion layers that are integral to the CC surface and have been densihed by some pore-filling technique may be optimum for bonding.

The use of borate and phosphate glasses to protect carbon bodies from oxidation has a long history (refs. 71 to 73). Boric oxide is particularly attractive because it melts at approximately 450°C and has viscosity, wetting, and thermal stability characteristics that make it an effective sealant over a wide range of temperatures. In figure 7, the viscosity of B2O3 as a function of temperature is compared with the viscosities of other glasses; the figure shows that the behavior of B2O3 is unique in that the viscosity is low and does not decrease or increase rapidly with changes in temperature (ref. 74).

Figure 8 shows that the surface tension of B2O3 is also low compared with that of other glasses, and this translates into excellent wetting on most materials (ref. 74). The ability of B2O3 to wet and flow over a broad temperature range results in the formation and maintenance of protective glass films within coating cracks and surface pores of the CC substrate. Phosphate glasses have wetting

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Figure 7. Glass viscosities (ref. 74).

and viscosity characteristics similar to those of borate glasses, but the high vapor pressure and reactivity of P2O5 limit the use of these glasses to temperatures below 700°C.

Boric oxide has relatively low vapor pressures at high temperatures and is thermodynamically stable in contact with many materials. For example, B2O3 has a vapor pressure of approximately 10-7 MPa at 1000°C and 10 MPa at 1400°C under dry conditions, and it is stable in contact with carbon up to 1575°C (refs. 74 and 75). Moisture-induced volatility and low viscosity limit the utility of borate glasses to about 1000°C unless the glasses are protected by overlay coatings (refs. 76 to 79). Experience has shown that borate glasses can seal cracks in outer coatings during hundreds of hours of thermal cycling in which maximum temperatures of 1400°C are maintained for significant fractions of the time (ref. 65). This is due to the outer coating that protects the glass and the presence of inner layers that oxidize to renew glass that is lost by flow or evaporation.

Performance Issues

Performance issues associated with the present generation of CC coating systems are coating spallation due to CTE mismatch, borate glass corrosion of

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Figure 8. Glass surface tensions (ref. 74).

the outer coating, moisture sensitivity of the borate glasses, and the high oxygen permeability of the borate glasses.

Coating spallation is often a problem because very few coatings can match the ultralow CTE of CC composites made with high-performance fibers. The traditional ways of avoiding spallation are to maximize coating adherence and minimize coating thickness. Often, pure CTE mismatch failures are caused by shear failure of the CC rather than by a lack of coating adhesion. Regardless of whether spallation is caused by a lack of coating adhesion or cohesive failure of the CC substrate, converting a thin layer of the CC surface to a carbide is beneficial. The rough, hard surface promotes adhesion, and a gradual transition from carbon to carbide distributes the strain mismatch rather than isolating it at a discrete interface.

Glass-forming inner layers are generally thin, no more than approximately 50 [im. Thin outer coatings have been tried, but because of rapid degradation by the borate sealant glasses, the outer coatings are usually at least 200 nm thick (refs. 65 and 66). Corrosion of the outer SiC and Si3N4 coatings is a result of destructive oxidation caused by dissolution of the protective SiO2 that is normally maintained on the silicon-based ceramics (ref. 65). This corrosion reflects the high oxygen permeability of the borate glasses both in terms of excessive glass formation . 10 mm ■

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Figure 9. Cavitation of CVD SiC outer coating due to sealant glass corrosion. 2 X.

by rapid oxidation of the inner layer and outer coating corrosion that again results from the nonprotective nature of the borate glasses. Figure 9 shows an example of outer coating corrosive failure. Thick outer coatings eventually develop cavities that allow gross oxidation of the inner layer and the attack of the CC substrate. Thin outer coatings fail rapidly by massive coating dissolution.

The moisture sensitivity of B2O3 and most borate glasses is well-known (refs. 74 and 80). Hydrolysis under ambient conditions in moist air produces swelling and converts coherent and adherent glass layers into loosely bonded boric acid particulate. The moisture attack of the glass that eventually forms beneath an outer coating during long-term oxidation can result in a coating spallation due to a lack of adherence. Subsequent heating to rapidly release the moisture can also be disruptive, and exposure of the glass to moisture at high temperatures makes the glass susceptible to vaporization by the formation of volatile HBO2 (ref. 77). Figure 10 shows a CC specimen with a CVD SiC outer coating that spalled after several hundred hours of cyclic oxidation testing and a number of intermittent roomtemperature moisture exposures. The spallation occurred during the final moisture

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